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Postharvest Diseases of Fruits and Vegetables 

Development and Control 



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POSTHARVEST DISEASES OF FRUITS 

AND VEGETABLES 

DEVELOPMENT AND CONTROL 

RIVKA BARKAI-GOLAN 

Department of Postharvest Science of Fresh Produce 

Institute of Technology and Storage of Agricultural Products 

The Volcani Center, Bet-Dagan, Israel 

2001 

ELSEVIER 

Amsterdam - London - New York - Oxford - Paris - Shannon - Tokyo 



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made. 

First edition 2001 

Britisch Library Cataloguing in Publication data 

Barkai-Golan, Rivka 

Postharvest diseases of fruits and vegetables : development 

and control 

1.Fruit - Diseases and pests 2.Vegetables - Diseases and 

pest 

I.Title 

634' .046 

ISBN 0444505849 

Library of Congress Cataloging in Publication Data 

Barkai-Golan, Rivka 

Postharvest diseases of fruits and vegetables : development and control / Rivka 

Barkai-Golan—1" ed. 

p.cm. 

ISBN 0-444-50584-9 (hardcover) 

1. Fruit-Postharvest diseases and injuries. 2. Vegetables-Postharvest diseases and 

injuries. I. Title. 

SB608.F8 B27 2001 

634'.0493-dc21 

ISBN: 0-444-50584-9 

2001023788 

® The paper used in this publication meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper). 

Printed in The Netherlands. 



Dedicated to my husband, Eli, without whose loving 

support this book would not have been completed; 

and to my colleagues and students, without whose 

encouragement it would not have been started. 



This Page Intentionally Left Blank 



CONTENTS 

PREFACE xi 

Chapter 1. INTRODUCTION 1 

Chapter 2. POSTHARVEST DISEASE INITIATION 

A. The Pathogens 3 

B. The Origin of the Pathogens 3 

C. Spore Germination 4 

D. Pathogen Penetration into the Host 11 

(1) Infield Penetration and Quiescent Infections 11 

(2) Penetration through Naturallnlets 20 

(3) Penetration During and After Harvest 21 

Chapter 3. EACH FRUIT OR VEGETABLE AND ITS 

CHARACTERISTIC PATHOGENS 

A. Host-Pathogen Combinations in 

Postharvest Diseases 25 

B. The Main Pathogens of Harvested Fruits 

and Vegetables 27 

Chapter 4. FACTORS AFFECTING DISEASE DEVELOPMENT 

A. Preharvest Factors, Harvesting and Handhng 33 

B. Inoculum Level 35 

C. Storage Conditions 37 

(1) Temperature 37 

(2) Relative Humidity and Moisture 40 

(3) The Storeroom Atmosphere 42 

D. Conditions Pertaining to the Host Tissues 

(1) Acidity Level (pH) 42 

(2) Growth Stimuh 43 

(3) The Fruit Ripening Stage 45 

(4) Effects of Ethylene 47 

E. Host-Pathogen Interactions 51 



Vlll 

Contents 

Chapter 5. ATTACK MECHANISMS OF THE PATHOGEN 

A. Enzymatic Activity 54 

B. Toxin Production 59 

C. Detoxification of Host-Defense Compounds 

by Pathogens 64 

Chapter 6. HOST PROTECTION AND DEFENSE MECHANISMS 

A. The Cuticle as a Barrier Against Invasion 66 

B. Inhibitors of Cell-Wall Degrading Enzymes 66 

C. Preformed Inhibitory Compounds 69 

D.Phytoalexins-Induced Inhibitory Compounds 76 

E. Wound Healing and Host Barriers 84 

F. Active Oxygen 90 

G. Pathogenesis-Related Proteins 92 

Chapter 7. PHYSIOLOGICAL AND BIOCHEMICAL CHANGES 

FOLLOWING INFECTION 

A. Changes in Fruit Respiration and 

Ethylene Evolution 94 

B. Ethylene Source in Infected Tissue 100 

C. Pectolytic Activity and its Source in 

Infected Tissue 102 

D. Stimulation of Fruit Softening and Changes 

in the Pectic Compound Contents 103 

E. Changes in Biochemical Constituents 

of Infected Tissues 105 

Chapter 8. MEANS FOR MAINTAINING HOST RESISTANCE 

A. Cold Storage 108 

B. Modified and Controlled Atmospheres 121 

(1) Controlled Atmosphere 122 

(2) Controlled Atmosphere with 

Carbon Monoxide 130 

(3) Modified Atmosphere Packaging 131 

(4) Hypobaric Pressure 135 

C. Growth Regulators 137 

D. Calcium Application 142 



Contents ix 

Chapter 9. CHEMICAL CONTROL 

A. Preharvest Chemical Treatments 147 

B. Sanitation 150 

C. Postharvest Chemical Treatments 154 

D.Generally Recognized As Safe (GRAS) Compounds ... 170 

E. Natural Chemical Compounds 177 

F. Lectins 184 

Chapter 10. PHYSICAL MEANS 

A. Heat Treatments 189 

B. Ionizing Radiation 205 

C. Ultraviolet Illumination 217 

Chapter 11. BIOLOGICAL CONTROL 

A. Isolation and Selection of Antagonists 222 

B. Introduction of Antagonists for Disease Control 226 

C.Modeof Action of the Antagonist 233 

D. Antagonist Mixtures to Improve 

Disease Biocontrol 242 

E. Combined Treatments to Improve 

Disease Control 244 

F. Integration into Postharvest Strategies 246 

Chapter 12. NOVEL APPROACHES FOR ENHANCING HOST 

RESISTANCE 

A. Induced Resistance 253 

(1) Physical Elicitors 253 

(2) Chemical Elicitors 257 

(3) Biological Elicitors 260 

B. Genetic Modification of Plants 262 

(1) Disease-Resistant Transgenic Plants 262 

(2) Sources of Genes for Bioengineering Plants 263 

C. Manipulation of Ethylene Biosynthesis 

and Genetic Resistance in Tomatoes 265 



Contents 

POSTHARVEST DISEASE SUMMARY 

FOUR FRUIT GROUPS 

I. SUBTROPICAL AND TROPICAL FRUITS 

Citrus Fruits 268 

Banana 279 

Mango 281 

Papaya 283 

Avocado 286 

Pineapple 288 

Persimmon 290 

Guava 291 

Litchi 292 

II. POME AND STONE FRUITS 

Pome Fruits 293 

Stone Fruits 303 

III. SOFT FRUITS AND BERRIES 

Strawberries and Raspberries 311 

Blueberries and Gooseberries 313 

Grapes 315 

Kiwifruit 318 

IV. SOLANACEOUS FRUIT VEGETABLES 

Tomato, Pepper and Eggplant 320 

SUBJECT INDEX 395 



PREFACE 

This book brings together various topics concerning the diseases of 

harvested fruits and vegetables. A considerable part of the material on 

which this book is based has been gathered from my lectures given to 

students at the Hebrew University of Jerusalem, at the Faculty of 

Agriculture in Rehovot. The material has generally been based on 

original articles and reviews written by scientists from all over the world. 

Throughout the book I have tried to impart to the reader an awareness 

of the great variety of research that has been constantly done in the field 

of postharvest pathology. Among the studies done in the field, some 

research has made a significant and exciting contribution to the 

understanding of the inter-relationships between the pathogen and the 

harvested fruits or vegetables. Throughout the years, with the 

advancement in this research, the challenge to integrate the ultimate 

aim - the lengthening of the postharvest life of fruits and vegetables - 

with an understanding of the biochemical processes that enable us to do 

just that, has become most prominent. 

Although each chapter in the book constitutes a separate unit, the 

order and continuity of the chapters are relevant to a better 

understanding of the whole subject. 

The first four chapters describe the causal agents of postharvest 

diseases of fruits and vegetables, their sources and their ways of 

penetration into the host, factors that may accelerate the development of 

the pathogen in the host - and those that suppress them. A list of the 

main pathogens of fruits and vegetables, their hosts and the diseases 

elicited by them are given in a separate chapter, while a detailed 

description of the major diseases of selected groups of fruits - subtropical 

and tropical fruits, pome and stone fruits, soft fruits and berries, and 

solanaceous vegetable fruits - is given at the end of the book, in the 

Summary of Postharvest Diseases of Four Groups of Fruits. 

Chapters five and six focus on the attack mechanisms that the pathogen 

may use to invade the plant tissues and develop within them, and the 

defense mechanisms with which the host is equipped to stop invaders or to 

suppress their development. Physiological and biochemical changes 

following infection are described in Chapter seven. The following chapters, 

8 to 12, all deal with treatments aimed at suppressing postharvest diseases. 

While chapter 8 describes treatments for maintaining the natural 



xii Preface 

resistance of the young fruit or vegetable, chapters 9, 10 and 11 describe 

chemical, physical and biological means for preventing disease initiation, 

inhibiting its development or reducing its severity. Increased official and 

public concern about the chemical residues left on the fresh produce and 

the resistance some pathogens have developed toward major fungicides 

have stimulated the search for alternative technologies for postharvest 

disease control. This section emphasizes the search for natural and safe 

chemical compounds, and the variety of alternative physical and biological 

control methods. It also points to the possibility of exploiting the natural 

defensive substances of fruits and vegetables, both constitutive and 

induced, to enhance host resistance during storage. This leads us to the 

last chapter, which introduces new approaches to the prevention of 

postharvest diseases, by inducing the natural host resistance by means of 

chemical, physical and biological elicitors. At present, with the 

developments in molecular genetics, we are standing at the dawn of a new 

era in which genetic crop modification may be of central importance 

among the variety of methods used to increase host resistance against 

postharvest diseases. Approaches to enhancing fruit resistance by creating 

disease-resistant transgenic plants have been discussed. 

This book is intended for teachers and students who focus on 

postharvest pathology and plant pathology programs, for scientists in 

research institutes, companies and organizations dealing with fruit and 

vegetable preservation technologies and with extension and training in 

the areas of harvesting, handling, packaging and storage, and for all 

those striving to improve the quality of harvested fruits and vegetables 

and to lengthen their postharvest life. 

I would like to thank the many students who expressed the desire to 

put the material of the lectures on paper and, therefore, encouraged me 

to write this book. My thanks also go to the members of the Department 

of Postharvest Science of Fresh Produce at the Volcani Center, who 

advised me and aided by reading of the text or parts of it, and to all the 

scientists who approved the inclusion of the illustrations of their 

research in this book. 

A special thanks to colleagues who provided me with several photos 

which appear in this book: Dr. Nehemia Aharoni (Photos 1, 2) and Dr. 

Samir Droby (Photos 7, 8), and to the Atomic Energy Board, Pelindaba, 

South Africa for Photos 3, 4, 5, 6. 

Rivka Barkai-Golan 

Beit Dagan - Rehovot 



CHAPTER 1 

INTRODUCTION 

We often witness the phenomenon that just harvested fresh fruits or 

vegetables ripen and are ready to be marketed, but the markets are 

overstocked and the demand is low. To achieve prolonged and constant 

marketing of produce that is gathered in large quantities over a 

relatively short period of time - the prime time for each fruit and 

vegetable - the produce must be stored for weeks or months prior to 

marketing. Fresh fruits and vegetables destined for export also require 

long periods of storage and shipment prior to arrival. In the course of 

storage and shipment part of the fresh produce, which is rich in moisture 

and nutrients and therefore may serve as a suitable substrate for the 

development of microorganisms, may rot and become unfit for sale. The 

evolution of decay as a result of microorganism attack during storage is 

one of the main causes for the deterioration of the fresh produce and can, 

in itself, become a limiting factor in the process of prolonging the life of 

the harvested fruits and vegetables. The economic loss incurred through 

storage diseases might exceed that caused by field diseases because of 

the large investments in the overall treatments and processes the 

product undergoes from harvest until it reaches the customer, and which 

include harvesting, sorting, packing, shipping and storing. 

Even in societies with access to the most advanced technologies for 

handling fresh fruits and vegetables, such as computerized sorting and 

grading, improved packing materials and methods, and advanced regular 

or modified atmosphere storage, losses caused by postharvest diseases 

remain substantial. In fact, international agencies that monitor world 

food resources have acknowledged that one of the most feasible options 

for meeting future needs is the reduction of postharvest losses (Eckert 

and Ogawa, 1988). 

Fresh fruits and vegetables supply essential nutrients, such as 

vitamins and minerals, and are a major source of complex carbohydrates, 

antioxidants and anticarcinogenic substances which are important to 

human health and well being (Arul, 1994). Being aware of the 

advantages of fresh fruits and vegetables, the consumer prefers the 

wholesomeness of the fresh produce over processed products. However, at 



2 Postharvest Diseases of Fruits and Vegetables 

the same time, the demand of the consumer for produce free of 

pathogens, and also free of chemical residues remaining from chemical 

treatments for disease control, has been growing during recent years. 

Furthermore, fungal rots have been associated with mycotoxin 

contamination of fruits (Paster et al., 1995) and their possible interaction 

with the human pathogen. Salmonella, has also been suggested (Wells 

and Butterfield, 1999). In other words, the safety of fresh produce has 

become a major public concern. 

The aim of adequate storage is, therefore, to help the harvested fruits 

and vegetables to arrive at their destination fresh, disease free and safe 

to the consumer - despite the complex of treatments they have to undergo 

prior to or during storage, and despite the long period between their 

harvest and their reaching the consumer. All the means and methods 

with the power to aid in preserving the quality of the harvested produce 

and in protecting it from decay agents during storage and shelf-life, are 

aimed at this objective. 

To understand how decay can be prevented or delayed we must, as 

stage one, be familiar with the decaying agents, their nature, their origin 

and their time and mode of penetrating into the host, as well as with the 

factors that might affect their postharvest development. At a later stage 

we should also be familiar with suitable means for preventing disease 

initiation or for arresting pathogen development within the host tissues 

and, on the other hand, with the possibility of maintaining or enhancing 

host resistance to infection. 



CHAPTER 2 

POSTHARVEST DISEASE INITIATION 

A. THE PATHOGENS 

Upon harvest, ripe fruits and vegetables become subject to attacks of 

various microorganisms incapable of attacking earlier in the course of 

growth in the field. These are largely weak pathogens, fungi and 

bacteria, typical of the harvested and stored fruits and vegetables. 

Disease resistance, which was embodied in the plant organ designated 

for storage during its developmental stages on the plant, weakens as a 

result of separation from the parent plant. In addition, picked fruits and 

vegetables are rich in moisture and nutrients, which suit the 

development of pathogens. Upon ripening the fruits and vegetables often 

become more susceptible to injury and, therefore, more susceptible to the 

attack of those microorganisms that require an injury or damaged tissue 

to facilitate their penetration (Eckert, 1975). Moreover, during the 

prolonged storage of fruits and vegetables a series of physiological 

processes occurs which leads to the senescencing of the vegetal tissues 

and, in parallel, to their increased susceptibility to weak pathogens that 

attack senescencing vegetal tissues. 

B. THE ORIGIN OF THE PATHOGENS 

Fungi and bacteria responsible for in-storage decay often originate in 

the field or the orchard. When penetration into the host takes place in 

the field, the pathogen, which is then in its early or quiescent stages of 

infection, will get to the storeroom within the host tissue without 

eliciting any symptoms of decay. Yet, even when preharvest infection has 

not taken place, there are always fungal spores and bacterial cells, which 

are typical components of the airborne microorganism population, on the 

fruit and the vegetable during their growth. This cargo of spores and 

cells is transferred to storage with the harvested fruits and vegetables. 

Examination of the fungal spore population on the surface of stems, 

leaves, flower parts, fruits and other plant organs, after harvest, reveals 




the presence of many important airborne fungi such as species of 

Cladosporium, Alternaria, Stemphylium, Penicillium, Aspergillus, 

Rhizopus, Mucor, Botrytis, Fusarium and others. Many of the airborne 

fungi are among the most important decay agents which affect harvested 

fruits and vegetables and, given the right conditions, develop and cause 

decay. 

Many postharvest pathogens perpetuate on crop debris in the field and 

can, under suitable conditions, develop and produce abundant new 

spores. These fungal spores are easily carried by air currents, winds and 

rain, or dispersed by insects, to the flowers and the young fruits, at 

various stages of development, and form a potential source of infection. 

Soil, irrigation water and plant debris form an important source of 

infection of various vegetables. Soil-residing fungi and bacteria can 

attack the bulb, tuber, root and other vegetal parts, while these are still 

attached to the parent plant, through a tight contact with the soil, by 

lifting of soil particles by winds, rains or irrigation, through growth, or by 

arriving in storage with soil residues attached to the vegetable. Some soil 

microorganisms, such as species of the fungi Botrytis, Sclerotinia and 

Fusarium or the bacterium Erwinia carotovora, are among the main 

decay agents in stored vegetables. Host infection can occur preharvest, 

during harvest or during any of the postharvest handling stages. 

Harvesting instruments, containers, packing houses with their 

installations, the hands of the packers or selectors, the atmosphere of the 

storage rooms - are all bountiful sources of fungal spores. 

Despite the diversity of microorganisms that the fruits and vegetables 

carry with them into storage, only a few species can naturally attack 

them, while other species that reside on the surface, at times in large 

quantities, will not penetrate and will not cause any decay. The 

development of disease during storage depends, primarily, on the 

existence of the appropriate microorganisms alongside a given host. 

However, in order for the fungal spores or bacterial cells that have 

reached the suitable host to be capable of infecting, they have to 

encounter the appropriate conditions for germination on the surface of 

the host, to penetrate into the host tissues and to develop there. 

C. SPORE GERMINATION 

Spore germination is a preliminary stage to fungal penetration into 

the host. The right environmental temperature, available water or 



Postharvest Disease Initiation 5 

moisture and, sometimes, the presence of nutrients transferred from the 

host into the water, are the most important environmental factors that 

aid spore germination. 

Temperature. Most of the storage decay fungi are mesophiles - 

preferring moderate temperatures. The optimal temperature for the 

germination of their spores is around 20-25°C, although the temperature 

range allowing germination can be much wider. The further from the 

optimal temperature, the longer the time until the initiation of spore 

germination - a phenomenon that affects the prolongation of the 

incubation period of the disease. Shifting the temperature away from the 

optimum reduces the rate of germination and retards germ-tube 

elongation. A sufficiently low temperature can inhibit spore germination 

altogether and thus constitute a disease-limiting factor. 

Water. Water or moisture is essential for fungal spore germination, 

although spores of some fungi are capable of germination if there is very 

high relative humidity of the surrounding air (Roberts and Boothroyd, 

1984). Spores of most fruit and vegetable pathogens can germinate in 

pure water or water with low nutrient concentrations, transferred from 

the host surface to the water by osmosis or supplied to the spores by 

injured and battered cells in the wound region, the typical court of 

infection for many postharvest pathogens. 

Nutrient additives. Spores of Penicillium digitatum, the green mold 

fungus which attacks only citrus fruit, germinate to a minor extent in 

pure water, whereas the addition of fruit juice greatly accelerates 

germination (Pelser and Eckert, 1977). Testing the effect of the juice 

components on spore germination revealed that of the sugars within the 

juice (glucose, sucrose and fructose), glucose is the best stimulant. A 

greatly enhanced germination is stimulated by the ascorbic acid, whereas 

the citric acid has no stimulating effect. The combination of glucose and 

ascorbic acid results in a germination rate quite close to that stimulated 

by the whole juice. 

Penicillium spore germination is also stimulated by the addition of oil 

derived from the rind of orange, lemon, grapefruit or other citrus fruits 

(French et al., 1978). Fig. 1 displays the stimulating effect of various 

concentrations of oil produced from an orange rind on the germination 

rate of P. digitatum conidia. In the presence of 250 ppm oil, 15% of the 

spore population had germinated after 24 hours at 19°C, while no 

germination occurred in the control spores (water only). Ten days later, 

the germination rate amounted to 70% with the oil, in comparison with 

approximately 10% in the control. A similar effect occurred on 



c 

o 

'+-' 

CO 

c 

1_ t 

(U 

CD 

80 

70 + 

60 

50 

40 

30 

20 4- 

10 

Day 1 (control = 0.00%) 



"Day 10 (control = 9.56%) 


\—I I mil I I iiiiiifi-Ti-'r-rtrtt 

10 50 100 500 1000 10,000 

Concentration (ppm) 

Fig. 1. Effect of orange oil suspension in 1% water agar on germination of 

conidia of Penicillium digitatum. (Reproduced from French et al., 1978 with 

permission of the American Phjôpathological Society). 

Penicillium italicum conidia also. Examination of the various oil 

component activities revealed that nonanal and citral were the most 

effective compounds. Moreover, a mixture of these two compounds had a 

synergetic effect, leading to high germination rates in the conidia 

population (French et al., 1978). It appeared that oils produced from 

different citrus fruits might differ in their compositions, hence the 

relative differences among the stimulating or suppressing activities of 

oils produced from different sources. 

Since little or no germination of P. digitatum occurred on water agar 

alone, it was assumed that substrate nutrition was the determinant of 




spore germination and an important factor in host specificity of the 

pathogen to citrus fruits. However, when P. digitatum spores on water 

agar were exposed to several wounded oranges in closed containers, 

germination did take place in the absence of substrate nutrients (Eckert 

and Ratnayake, 1994). Searching for the reason for this phenomenon, 

Eckert and Ratnayake (1994) found that a mixture of volatiles 

evaporating from the abrasions of wounded oranges were capable of 

accelerating or inducing germination of P. digitatum spores on water 

agar as well as within an injury of the rind. The major components of 

this mixture were the terpenes, limonene, oc-pinene, P-myrcene, and 

sabinene, accompanied by acetaldehyde, ethanol, ethylene and CO2, as 

identified by gas chromatography (Fig. 2). At a concentration typical of 

the natural mixture surrounding wounded oranges (1 x concentration), 

45% of the spores germinated on water agar. Germination was reduced 

by both higher and lower concentrations of the mixture of volatiles (Fig. 3). 

0 

(/) 

c 

o 

Q. 

(/) 

0) 

(D 

O 

o 

CD 

Acetaldehyde 

Ethanol 

Limonene X 16 

|3-Myrcene 

Sabinene 

Pinene 

2 4 

Minutes 

Fig. 2. Gas chromatogram of volatile organic compounds in the atmosphere 

surrounding wounded oranges. Limonene peak shown was attenuated 16 times. 

(Reproduced from Eckert and Ratnayake, 1994 with permission of the American 

Phytopathological Society). 




1x 2x 

Relative concentration of orange volatiles 

Fig. 3. Effect of concentration of the synthetic mixture of volatile compounds on 

the germination of Penicillium digitatum spores. Ix is the concentration typical 

of the natural mixture measured surrounding wounded oranges. Bars indicate 

standard errors. (Reproduced from Eckert and Ratnayake,1994 with permission 

of the American Phytopathological Society). 

The fact that the average quantity of volatile compounds emanating 

from wounded fruits was 75 times greater than that from non-injured 

fruit may explain the lower germination of P. digitatum spores on water 

agar exposed to sound citrus fruits. The terpenes alone, in several 

concentrations and combinations, failed to stimulate spore germination 

significantly above its level on water agar. On the other hand, a synthetic 

mixture of limonene, the major terpene in the wounded fruit atmosphere, 

with acetaldehyde, ethanol and CO2, at concentrations similar to those 

measured in the atmosphere around wounded oranges, stimulated spore 

germination on water agar to the same degree as the natural mixture of 

volatiles. 

It is interesting to note that volatile compounds evolved from diced 

peels of various Citrus spp. (oranges, lemons, grapefruits, tangerines, 

kumquats) have also induced stimulation of germination of P. digitatum 

spores. Furthermore, lemons infected with Phytophthora citrophthora, 

Geotrichum candidum or P. digitatum also emanated volatiles that 




induced germination of P. digitatum spores on water agar (Eckert et al., 

1992; Eckert and Ratnayake, 1994). 

Ripening apricots also release ethanol and acetaldehyde which, at low 

concentrations, stimulate germ tube growth of Monilinia fructicola, in 

vitro (Cruickshank and Wade, 1992). Volatile compounds that stimulate 

spore germination or fungal development were suggested by French 

(1985) to act by altering membrane permeability or regulating 

metabolism. At higher concentrations, however, some volatiles, such as 

acetaldehyde, were found to be fungitoxic (Prasad, 1975) and have been 

evaluated as fumigants to control postharvest diseases of various crops 

(see the chapter on Chemical Control - Natural Chemical Compounds). 

Germination of various fungal spores may be stimulated by solutes 

that diffuse from within the fruit or other plant organs into the water 

film over the infection site. Spore germination of Colletotrichum musae, 

the cause of banana anthracnose, is very poor in pure water. However, on 

the surface of banana fruits, both spore germination and the formation of 

appresoria (specific resting spores responsible for fruit infection) were 

accelerated (Swinburne, 1976). This stimulation was attributed to the 

permeation of anthranilic acid from the inner tissues to the fruit surface. 

Fungal spores can rapidly degrade this acid to 2,3-dihydroxybenzoic acid, 

which is responsible for the accelerated germination. This acid is 

involved in the active transport of iron in various microorganisms and 

reduces the iron level around the spores on the surface of the banana 

(Harper and Swinburne, 1979; Harper et al., 1980). This phenomenon 

explains why siderophores, which are chelating agents with a high 

affinity for this metal, that are formed on the banana surface by bacteria, 

under conditions of iron deficiency (Neilands, 1981), stimulate and 

accelerate germination of Colletotrichum spores and the formation of 

appresoria (McCracken and Swinburne, 1979). 

Spores of Rhizopus stolonifer, which causes a watery soft rot in many 

fruits and vegetables, also need nutrient additives in the water in order 

to germinate and infect carrots (Menke et al., 1964). Spores of Botrytis 

cinerea, the causal agent of gray mold, though capable of germination in 

pure water to some degree, cannot infect strawberries or cabbage leaves 

without external nutrients (Jarvis, 1962; Yoder and Whalen, 1975). Early 

studies with B. cinerea showed that distilled water on the surfaces of 

leaves and petals contains more electrolytes than distilled water on glass 

slides, and that spores of Botrytis usually germinate better in water 

containing electrolytes (Brown, W., 1922a). Several studies indicated 

that grape berry exudates stimulated B. cinerea spore germination and 




that stimulation increased during the last month of fruit ripening. This 

period corresponds with the large increase in sugar content of the 

exudates, along with the increased susceptibility of grapes to infection 

(Kosuge and Hewitt, 1964; Padgett and Morrison, 1990). 

Various fungi, which penetrate through wounds, encounter the 

moisture required for germination of their spores in fresh wounds in the 

cuticle and epidermis of the host. The fractured cells in the injured area 

also supply the nutrients required for the germination and infection 

phases. Pathogens which penetrate through the host lenticels can feed on 

the nutrients secreted from the cells adjoining the lenticels, especially 

after injury following tissue senescence (Eckert, 1978). 

Atmospheric gases. Additional environmental factors, such as 

oxygen (O2) and carbon dioxide (CO2) levels in the atmosphere in which 

the fruits or vegetables are stored, can also affect germination. Reducing 

the O2 level in the air to below 21% or increasing the CO2 level to more 

than 0.03% can inhibit fungal spore germination, although various 

species react differently to the depressing levels. Generally, an 

atmosphere of 15-20% CO2 directly inhibits spore germination of 

Rhizopus stolonifer, Botrytis cinerea, and Cladosporium herbarum, with 

inhibition rising as the gas level increases. However, such an atmosphere 

does not affect spores of Alternaria alternata; inhibition of these spores 

requires that the gas level be increased to more than 32%. On the other 

hand, spore germination of Fusarium roseum accelerated under the effect 

of high concentrations of CO2, but when the concentration was over 32% 

the germination of these spores, too, was inhibited (Wells and Uota, 

1970). (See Fig. 21a). 

Exposure of spores to low oxygen levels, too, might damage their 

germination ability, but a significant delay normally occurs only with the 

reduction of oxygen level to 1% and below (See Fig. 21b). The capacity of 

high CO2 levels and low O2 levels, or a combination of the two, to inhibit 

spore germination is used for creating a modified or controlled 

atmosphere suitable for the prolongation of the postharvest life of 

harvested fruits and vegetables (See the chapter on Means for 

Maintaining Host Resistance - Modified and Controlled Atmospheres). 

Spores encountering conditions suitable for germinating, including 

available water or moisture combined with the required nutrients, 

adequate temperature and other environmental conditions, will swell, 

develop germ-tubes and be ready for the next stage: penetrating into the 

host. 




D. PATHOGEN PENETRATION INTO THE HOST 

Postharvest pathogens can be divided, according to the timing of their 

penetration into the host, into those that penetrate the fruits or 

vegetables while still in the field but develop in their tissues only after 

harvest, during storage or marketing, and those that initiate penetration 

during or after harvest. 

1. INFIELD PENETRATION AND QUIESCENT INFECTIONS 

Harvesting and Picking after Pathogen Penetration 

Late blight of potatoes, caused by Phytophthora infestans, is an 

example of decay originating in tuber infection in the field. The infection 

is caused by the zoospores found in the soil or that fall onto the tubers 

from infected foliage during harvest. Following germination the 

zoospores penetrate into the tubers through the "eyes", lenticels, growth 

cracks, wounds, or via the point of attachment to the plant (the stolon) 

(Lapwood, 1977). Tubers that were infected a few days prior to harvest or 

during the harvest itself are brought to storage carrying the disease in its 

early developmental stages, with no visible symptoms of decay. These 

tubers will decay while stored under high humidity and at a temperature 

over 5°C. 

In a warm and damp climate, the blight fungus can also attack the 

tomato fruit at its various ripening stages, in the field. The attack 

usually takes place at the edge of the fruit stalk scar, although, given 

prolonged humidity, the fungus can also penetrate directly through the 

skin (Eggert, 1970). During epidemics the entire crop may rot in the 

field. However, when the disease is less severe, fruits with no visible 

symptoms or with slight blemishes might be picked, and the fungus will 

continue to develop during storage. 

The brown rot, which develops in citrus fruits during storage, 

originates in preliminary infections initiated in the orchard by 

Phytophthora citrophtora and other Phytophthora species. During the 

rainy season, the fungal zoospores descend on the lower fruits on the tree 

and penetrate them directly (Feld et al., 1979). The fungus can develop in 

the orchard, but when zoospore infection occurs a few days prior to 

picking, when the external symptoms are not yet visible, the fruit will 

later rot and might comprise a serious problem during storage or 

shipping. 




Penetration of the various Phytophthora species into the host, in the 

field or in the orchard, is not hnked with a typical quiescent infection 

wherein the pathogen development ceases following its penetration into 

the tissues or at an earlier stage. The principle concerning this fungus is 

that the harvest ends its development within the host while the latter is 

still on the parent plant, thus causing the decay to occur after harvest or 

during storage. 

In many vegetables, such as carrot, cucumber, lettuce, celery, cabbage, 

cauliflower and others, a soft watery decay caused by Sclerotinia 

sclerotiorum is common. This fungus, common in the soil, and especially 

in heavy soil, might attack the vegetable when still in the field or during 

harvest and continue to develop in its tissues throughout storage or 

marketing. 

Quiescent or Latent Infections 

The fungi that penetrate into the host in the field also include 

pathogens that cause latent or quiescent infection. These pathogens reach 

fruits or vegetables that are still on the parent plant. However, during 

one of the phases between their reaching the host and the development of 

progressive disease, their growth is arrested until after the harvest, 

when physiological and biochemical changes occurring within the host 

will enable their renewed growth. Verhoeff (1974) has described such 

arrested infections as "latent infections". However, since this term has 

been used for describing different phenomena in various areas, it was 

later suggested by Swinburne (1983) to leave the term "latent infection" 

for wider uses and adopt "quiescent infection" for cases in which the 

pathogen growth is temporarily inhibited. In some instances we find that 

the inactive state is termed 'latent' if not visible to the eye and 'quiescent' 

if visible (Smilanick, 1994), while in others the two terms are frequently 

used to describe the same phenomenon. Jarvis (1994) believes that the 

latent state of the pathogen, whether involving spores that have landed 

on the host surface, spores that have commenced germinating, or 

primary hyphal development within the host tissues, is linked to a 

dynamic balance among the host, the pathogen and the environment. 

The physiological and biochemical changes occurring in the host tissues 

after harvest and during storage might affect the pathogen, the host and 

their interrelationships and lead to the activation of the latent pathogen. 

The gray mold, which is the main cause of decay in harvested 

strawberries, is an example of disease originating in a quiescent infection 

with Botrytis cinerea that is acquired in the field. The Botrytis spores. 




with which the strawberry field air is filled during bloom, can germinate 

in a drop of water on the petal or other parts of the flower, and later 

penetrate through the senescenced parts of the flower, into the edge of 

the receptacle of the strawberry where they develop a dormant 

mycelium. During ripening and storage, as the resistance of the fruit to 

the pathogen decreases, the preliminary mycelium enters an active 

stage, and the decay develops (Powelson, 1960; Jarvis, 1962). In grapes, 

too, where the gray mold fungus is one of the most important decay 

agents during storage, the origin of the disease is usually in infections 

that occur in the vineyard at the time of bloom, and which remain 

dormant until the fruit is stored (McClellan and Hewitt, 1973). The 

development of the gray mold at the stem-end region of the harvested 

tomato is also the result of Botrytis penetration into the young fruit 

through the flower parts (Lavi-Meir et al., 1989). 

The brown rot, caused by Monilinia fructicola in stone fruits (Tate and 

Corbin, 1978; Wade and Cruickshank, 1992) and the decay caused by 

Nectria galligena in apples (Swinburne, 1983), also originate in the 

infection of young fruit in the orchard. The Nectria fungus forms a 

preliminary mycelium within the young tissues, which is the dormant 

stage of the disease, while the interrupted stage in the development of 

the brown rot caused by Monilinia can occur during spore penetration 

through the stomata or even at the pre-germination stage. 

The stem-end rots in citrus fruits, caused by Diplodia natalensis and 

Phomopsis citri, develop from quiescent infections located in the 

stem-end button (calyx and disc) (Brown and Wilson, 1968). In moist 

conditions a large quantity of Diplodia and Phomopsis picnidia can be 

found on the bark of citrus trees and on dry branches. The spores that 

break loose from the picnidia are spread by rain splash onto the flowers 

and young fruit, where a preliminary infection occurs at the stem-end 

region. A further development of the disease is hindered by healing of the 

wound and the formation of protective barriers of tightly packed cells by 

the host tissue (see the chapter on Host Protection and Defense 

Mechanisms - Wound Healing and Host Barriers). After the fruit is 

picked, the senescencing processes in the button region lead to the 

separation of the button from the fruit and the formation of an opening 

that enables the fungi to penetrate into the fruit and initiate the 

stem-end decay. 

Another quiescent infection is anthracnose, caused by Colletotrichum 

gloeosporioides in many tropical and subtropical fruits, such as mango, 

papaya, avocado and various citrus fruits (Baker, R.E.D., 1938; Brown, 




G.E., 1975; Dickman and Alvarez, 1983; Binyamini and Schiffmann- 

Nadel, 1972; Spalding and Reeder, 1986b), or by Colletotrichum musae in 

banana fruits (Simmonds, 1941). The fungal conidia are found in large 

quantities on the surface of the fruits during their development on the 

tree. In the presence of free water upon the fruit, the spores germinate 

and form an appressorium at the tip of the germ tube. The appressoria 

function as resting spores on the fruit, since they resist environmental 

conditions much better than the conidia; they remain vital for long 

periods while embedded in the natural wax of the fruit or bound to its 

surface. The binding can occur through a mucous secretion of the fruit, as 

with C musae (Simmonds, 1941). Yet, the appressoria are infection 

bodies capable of germinating, under suitable conditions, and forming 

germinating tubes which penetrate into the tissues; they may even 

develop fine "infecting hyphae" that penetrate under the cuticle or the 

external layers of the epidermis. The appressoria themselves, or the 

"infecting hyphae" which they form, comprise the quiescent stage of 

fungal infection (Muirhead, 1981b; Prusky et al., 1990). Following 

picking, when the fruit commences ripening, the quiescent infection 

transforms into an active stage and initiates the development of the 

typical anthracnose. In papaya, the penetration of the germ tubes occurs 

through the stomata of the young, unripe fruit, while the appressoria can 

remain dormant for a long period over the cuticle or in "caves" beneath 

the stomata (Stanghellini and Aragaki, 1966). 

Assumptions and explanations raised in connection with the 

establishment of quiescent infections in unripe fruits were summarized 

and classified into four categories by Simmonds (1963). Verhoeff (1974), 

in his review of latent infections, had grouped these theories into three 

groups (combining the nutritional and energetic requirements of the 

pathogen into one group): 

(1) The shortage in the young fruit of adequate substrate to meet with 

nutritional and energetic requirements of the pathogen; 

(2) The incapability of the pathogen to produce cell-wall degrading 

enzymes in the young fruit; 

(3) The presence of preformed antifungal compounds in the young fruit 

that inhibit pathogen growth or enzymatic activity. Swinburne 

(1983), in a later review, added the fourth category of theory: 

(4) The accumulation of phytoalexins - antifungal compounds induced 

by the host tissue as a result of actual or attempted fungal infection. 

Following the description by Prusky and Keen (1995) of preformed 

compounds that can be further induced in the host tissues, the third 




theory can be broadened to include: "the presence of preformed 

antifungal compounds in the young fruit and their further inducement in 

the host tissues". 

Nutritional and energetic requirements of the pathogen. The 

first theory claims that young unripe fruit does not provide the pathogen 

with the nutrition and energy required for its development. Since, during 

ripening, a conversion of insoluble carbohydrates to soluble sugars occurs, 

it is no wonder that the resistance to rotting of the young fruit has been 

attributed, in several cases at least, to the sugar content of the tissues. 

This theory complies with the results of experiments conducted on apples, 

in which an artificial increase of the sugar level, by addition of 

sugar-regulated compounds (such as 2,4-dinitrophenol) to the fruit, had 

accelerated the decay caused by Botryosphaeria ribis (Sitterly and Shay, 

1960). Yet, since these compounds also accelerated the onset of the 

climacteric peak of the fruit respiration, their stimulating effect cannot be 

attributed to the sugar level alone. In addition, the accelerated onset of 

susceptibility of apple fruits to the pathogen could also be attributed to the 

reduction in the toxicity of antifungal compounds in the presence of sugars 

(Sitterly and Shay, 1960). The fact that fungi which normally attack ripe 

fruit may also develop on extracts prepared from unripe fruit, attests to a 

lack of a clear link between resistance and the shortage of available 

nutrients in the young fruit (Swinburne, 1983). 

Activation of pathogen enzymes. The second theory suggests that 

the unripe fruit does not supply the pathogen with compounds that 

induce the formation and activity of cell-wall degrading pectolytic 

enzymes. In addition, in cases whereby the fungus produces cell-wall 

degrading enzymes, cross-linking of the cell-wall pectic compounds might 

block the access of these enzymes to the sites within the cell wall. 

Furthermore, enzymes may be inactivated by inhibitors present in higher 

quantities in immature fruits than in mature fruits (see the chapter on 

Host Protection and Defense Mechanisms - Inhibitors of Cell-Wall 

Degrading Enzymes). 

The presence or induction of antifungal compounds in the host. 

The third and fourth theories point at a relation between the presence or 

formation of anti-fungal compounds in the young tissues and the creation 

of quiescent infections. These antifungal compounds may be: (a) preformed 

compounds, the existence of which is not pathogen related; (b) preformed 

compounds further induced by the host, either in the same tissue it was 

originally formed or in a new tissue; (c) phytoalexins (postformed 

compounds) induced by the host as a counterattack against the pathogen. 




A correlation between preformed antifungal compounds and resistance 

to postharvest disease in the young fruit is found in various hostpathogen 

systems. The presence of phenolic compounds in the young 

banana fruits was reported as early as 90 years ago (Cook and 

Taubenhaus, 1911); particular significance was attributed to the tannins 

since they are often found in higher concentrations in unripe than in ripe 

fruits. Examples of such antifungal compounds are the phenolic compounds 

in apples, which inhibit pathogen development in the young fruit 

(Ndubizu, 1976) and the 3,4-dihydroxybenzaldehyde compound that has 

proven fungistatic activity in the green banana fruit (Mulvena et al., 1969). 

Another example of a preformed compound is the glycoalkaloid 

tomatine. Its presence at high concentrations in the young fruit peel has 

been related to the quiescent infection of 5. cinerea in the green tomato 

fruit (Verhoeff and Liem, 1975). In this case, however, the fungus would 

not actively grow as the fruit ripened when the tomatine level 

significantly decreases. 

In unripe avocado fruit, a link was established between the presence of 

a diene and of monoene antifungal compounds in the fruit rind and the 

quiescent infection of C. gloeosporioides in such a fruit (Prusky et al., 

1982). The diene, which is the major compound, inhibits spore 

germination and the growth of Colletotrichum mycelium in 

concentrations lower than those present in the young fruit rind (Fig. 4). 

2000 6000 10000 14000 18000 

Antifungal diene concentration(j.ig/mr^) 

Fig. 4. Effect of the purified antifungal diene from avocado peel on conidial 

germination and germ tube elongation on Colletotrichum gloeosporioides. 

(Reproduced from Prusky et al., 1982 with permission of the American 





1 2 3 4 5 6 7 

Days after harvest 

9 10 

+ 500 

400 

+ 300 - 

200 

+ 100 

Fig. 5. Fruit firmness (O), antifungal activity (•), and concentration of the 

antifungal diene (n) in crude extracts from peel of avocado cultivar Fuerte at 

different stages after harvest. The arrow denotes the first visible decay 

symptoms. (Reproduced from Prusky et al., 1982 with permission of the 

American Phytopathological Society). 

During ripening, the diene concentration decreases tenfold to 

subfungitoxic levels and enables the continuation of the mycelium 

growth and the development of the disease (Fig. 5). The reduction in the 

concentration of the diene results, probably, from lipoxygenase enzymatic 

activity that increases as ripening progresses and the fruit softens 

(Prusky et al., 1985b). 

The dormant state of Alternaria alternata in young mango fruits has 

been attributed to the presence of two antifungal resorcinols in the 

unripe fruit rind (Droby et al., 1986). 

The effect of the resorcinols on the germination rate of A. alternata 

spores and on the prolongation of the germ tubes is given in Fig. 6. The 

concentration of the resorcinols in the rind of various mango varieties 

decreases during the ripening process, with a parallel quiescent fungal 

infection (Fig. 7). 

The presence of the phenolic compound, chlorogenic acid in young apple 

fruits has been related to their resistance to Gloeosporium perennans. The 

weak point of this hypothesis is the fact that the concentrations needed for 

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100 -f- 

80 

I 40 

20 4- 

Germ tube elongation 

50 100 150 200 250 300 

Resorcinols' concentration (ing/mr^) 

Fig. 6. Effect of 5-substituted resorcinols on spore germination (o) and germtube 

elongation (•) oiAlternaria alternata. Vertical bars indicate standard error 

(Reproduced from Droby et al., 1986 with permission of Academic Press). 

fungal suppression are greater than those naturally present in the fruit 

(Swinburne, 1983). 

The resistance of Bramley seedling apples to N. galligena has been 

attributed to induced phytoalexin formation. Nectria invades wounds and 

lenticels of apple fruits before harvest, but fruit rotting does not become 

severe until after harvest. 

The quiescence of Nectria in apples has been attributed to the 

formation of benzoic acid in the necrotic tissue, following fungal 

penetration into the fruit (Swinburne, 1978). The fungus resumes active 

growth when the fruit ripens and the benzoic acid is decomposed 

(Swinburne, 1983). 

Some of the preformed compounds, such as the antifungal diene in 

unripe avocado fruits, can be further induced in the host tissues. 

Induction of higher concentrations of this compound was found in 

response to challenge inoculation of unripe, but not of ripe fruit with C. 

gloeosporioides. 

The preformed antifungal substances naturally residing within the 

host, the inducible preformed substances, and the phytoalexins formed as 

a result of infection, and their role in host resistance, will be discussed 

below in the chapter on Host Protection and Defense Mechanisms. 


500



(/> O 

CO 0) 

5 10 15 20 


Fig. 7. (A) - Time courses of change in the concentration of 5-substituted 

resorcinols in peel from Tommy Atkins (•) and Haden (A) mango fruits. Arrows 

denote symptom appearance of disease expressed on 50% of inoculated sites. 

(B) - Percentage of Alternaria alternata infection sites of over 2 mm in diameter 

in relation to time after harvest. Vertical bars indicate standard error. 

(Reproduced from Droby et al., 1986 with permission of Academic Press). 

Suggestions have been made of additional causes that might prolong 

the dormant phase of a fungus within the fruit. It was thus found that 

the quiescent period of M fructicola, the causal agent of brown rot in 

peach fruits, lengthened with increasing thickness of the fruit cuticle and 

cell walls (Adaskaveg et al., 1991). As a matter of fact, peach cultivars 

with a thick cuticle and denser epidermis were found to be more resistant 

to the brown mold than those with a thin epidermis. This resistance has 

been expressed both in delayed penetration of the pathogen into the fruit 

and in prolonged incubation phases within the host (Adaskaveg et al., 

1991). 




2. PENETRATION THROUGH NATURAL INLETS 

Some pathogenic fungi and bacteria that cannot normally penetrate 

the sound host directly, without the presence of a wound in its surface, 

can penetrate through natural openings such as stomata and lenticels. 

The penetration of germ tubes of Colletotrichum gloeosporioides spores 

into young papaya fruits (Stanghellini and Aragaki, 1966) and the 

penetration of Monilinia fructicola spore germ tubes into young stone 

fruits (Tate and Corbin, 1978) can take place through the stomata while 

the fruit is still in the orchard. 

The fungus Dothiorella gregaria (perfect state: Botryosphaeria ribis) 

penetrates the lenticels of avocado fruits and forms hyphae that remain 

dormant until the harvested fruit has aged (Home and Palmer, 1935). 

Penetration through lenticels has also been described for Alternaria 

alternata spores in mango and persimmon fruits (Prusky et al., 1981; 

Prusky et al., 1983). Penetration through lenticels is typical of 

Gloeosporium album and Gloeosporium perennans in apples that grow in 

humid areas. In the warm humid season the fungal development in the 

lenticels is limited, with their growth being halted until, during storage, 

the fruit reaches the required ripening stage (Moreau et al., 1966). 

Penicillium expansum, the cause of apple blue mold, and which is 

considered a typical "wound pathogen", can also penetrate the fruit 

through its lenticels (Baker and Heald, 1934). This phenomenon is most 

notable if the lenticels are bruised or when the fruit ages. 

The lenticels in potato tubers are liable to develop bacterial soft-rot 

during storage. During harvest, most of the lenticels are already infested 

with cells of Erwinia cartovora. The bacteria remain inactive within the 

lenticels until the development of conditions enhancing the tuber 

sensitivity to decay, such as mechanical pressure, the presence of free 

water, or a low oxygen pressure within the tuber (Lund and Wyatt, 1972; 

Perombelon and Lowe, 1975). Penetration via lenticels is also typical of 

Helminthosporium solani, the causal agent of silver scurf of potato tubers 

(Burke, 1938), although the fungus can penetrate directly through the 

skin of these tubers. 

An open calyx tube, typical of some fruits, can also become a natural 

penetration point for storage pathogens. This is what enables the 

penetration ot Alternaria alternata into Delicious apple fruits (Ceponis et 

al., 1969) and Na'ama tomato fruits (Barkai-Golan, unpublished), both of 

which are characterized by open calyces. In apple fruits, the fungus that 

has penetrated through an open sinus in the calyx region, can develop a 




spore-bearing mycelium within the fruit core, without causing the 

external rind to decay (Miller, P.M., 1959). Core rot of apples, that 

results from infection through the sinus between the calyx and the core 

cavity, may be attributed to various pathogens, including Aspergillus 

niger and Fusarium spp. (Miller, P.M., 1959); Botrytis cinerea, Mucor 

piriformis, Pleospora herbarum, Penicillium funiculosum and P. 

expansum (Combrink et al., 1985; Spotts et al., 1988); Phomopsis mali 

(Rosenberger and Burr, 1982) and Trichothecium roseum (Raina et al., 

1971). 

3. PENETRATION DURING AND AFTER HARVEST 

Penetration via Wounds 

In contrast to pathogens that attack the fruit and vegetable in the 

field, most of the storage pathogens are incapable of penetrating directly 

through the cuticle or epidermis of the host, but require a wound or an 

injury to facilitate their penetration. Therefore, the fungi and bacteria 

that develop during storage are often called "wound pathogens". The 

wound can vary in nature. Growth cracks present on harvested fruits 

and vegetables are natural avenues of infection. The actual harvesting is 

accompanied by mechanical injuries that enable the weak pathogens to 

penetrate. Careless separation of the fruit or vegetable from the parent 

plant might result in an injury liable to be attacked by the pathogen. The 

extent of injury caused by mechanical harvesting is far greater than that 

caused by a manual operation (Fuchs et al., 1984). Each scratch, incision, 

blow or other mechanical injury inflicted on the fruit or vegetable during 

each of the handling processes - harvesting, gathering, transporting, 

sorting, packing and storing - might present adequate penetration points 

for the storage pathogens. A likely penetration point is the stem-end 

separation area, where damage often occurs during fruit picking. In this 

regard, the separation area is no different from any other injury. It often 

happens that simultaneously with the injury, a large amount of fungal 

spores and bacterial cells arrive at the injured area, some of which will 

use the injury site to penetrate and infect the host. 

Penetration through wounds is characteristic of Penicillium 

digitatum and Penicillium italicum conidia in citrus fruits. Yet it has 

turned out that the depth of the injury, combined with the atmospheric 

humidity conditions during storage, can determine the fate and extent 

of the infection (Schiffmann-Nadel and Littauer, 1956; Kavanagh and 




Wood, 1967). Artificial inoculation into the fruit pulp always results in 

the fruit being infected, but inserting the fungus into a superficial 

scratch in the rind ends with infection only when the fruit is 

simultaneously placed under very high humidity. Maximal infection 

takes place when the relative humidity reaches 100%. In addition, the 

location of the injury is important. Inserting the fungus into the oil 

glands of the citrus fruit rind leads to higher infection rates than its 

insertion into the rind between the glands. 

Wound infection is also characteristic of Rhizopus stolonifer, that 

causes watery soft rot in many species of fruits and vegetables; of 

Alternaria alternata that causes a dark, rather dry decay in a large 

number of fruits and vegetables stored for a prolonged period; of 

Geotrichum candidum which causes the sour rot in citrus, melons and 

tomato fruits; and of various Aspergillus, Cladosporium and 

Trichotecium species, and other storage fungi, in various fruits and 

vegetables. 

Even pathogens that do not require an injury to facilitate their 

penetration - such as various Erwinia species that penetrate potato 

tubers through the lenticels, or Monilinia fructicola and Penicillium 

expansum, which are capable of penetrating deciduous fruits through 

the lenticels - will easily penetrate via wounds. In very much the same 

way the Botrytis cinerea fungus, which is capable of penetrating into 

young strawberry fruits through the flower parts, or into grape berries 

via the style-ends (McClellan and Hewitt, 1973), will penetrate into the 

host through an injury. Moreover, blows or pressure on apples or potato 

tubers increase the blue mold rot (P. expansum) or the bacterial soft rot 

(Erwinia carotovora), respectively, because of an injury inflicted on the 

cells around the lenticels - injury that eases the penetration of 

pathogens via the lenticels. 

Penetration Following Physiological Damage 

Physiological damage caused by low temperatures, heat, oxygen 

shortage or any other environmental stress, increases the fruit or 

vegetable sensitivity and exposes it to storage fungi. The physiological 

damage can be externally expressed through tissue browning and 

splitting, thus forming locations vulnerable to invasion of wound 

pathogens. Yet extreme environmental conditions might enhance 

sensitivity to an attack without any visible external signs of damage. A 

tomato fruit exposed to chilling temperatures or heat treatments is liable 

to be attacked by Botrytis cinerea even when there are no visible 




symptoms of damage (Lavy-Meir et al., 1989). Exposure to extreme 

temperatures also partially broke the resistance of previously resistant 

tomato genotypes {nor and rin fruits and their hybrids) both to B, cinerea 

(Lavy-Meir et al., 1989) and to Alternaria alternata (Barkai-Golan and 

Kopeliovitch, 1989). Alternaria rot develops typically also in zucchini, 

following chilling injury, whereas cucumbers and melons exposed to 

excessively low temperatures are sensitive to various Penicillium and 

Cladosporium species, respectively (Snowdon, 1992). A alternata and 

Stemphylium hotryosum also tend to attack apple fruits following the 

development of sun scald lesions, while Alternaria may also be associated 

with other physiological disorders on apples such as bitter pit or soft 

scald (Snowdon, 1990). Cladosporium herbarum is another weak 

pathogen that may be associated with scald and other physiological 

disorders in some cultivars of apples (Dennis, 1983a). 

Penetration Following a Primary Pathogen 

Several pathogens will enter the host following a primary pathogen 

"breakthrough". Sometimes, nature deploys a sequence of pathogens. 

Various bacteria enter potato tubers infected with Phytophthora 

infestans: with the development of the secondary bacteria, the primary 

decay, which is hard in nature, turns into a soft decay as a result of the 

bacterial enzymatic activity on the tuber cell walls. Penicillium 

expansum can enter the apple fruits following their infection by the fungi 

Mucor, Gloeosporium and Phytophthora (Snowdon, 1990). Soft rot 

bacteria can enter tomato fruits following the sour rot caused by 

Geotrichum candidum (Barkai-Golan, unpublished) and the penetration 

of various Fusarium species into the melon is also, in many cases, 

secondary and occurs following infection by primary pathogens. 

A typical sequence of primary and secondary pathogens can attack 

apples. The pink mold rot fungus (Trichothecium roseum) typically 

develops within the cracks caused to the fruit by the Scab fungus 

(Venturia inaequalis), Venturia penetrates the fruit while it is in the 

orchard, and can develop there while the fruit is still on the tree. 

However, although the fungus is still in its early stages of development 

within the fruit during harvest, the disease will develop during storage, 

and is liable to be followed by the invasion of Trichotecium. Similarly, the 

fungus may also gain entry at the infection site of the black rot fungus, 

Botryoshaeria obtusa (Snowdon, 1990). 

Studying the types of wounds associated with the development of 

Rhizopus soft rot on papaya fruits, Nishijima et al. (1990) showed that 




Rhizopus stolonifer was consistently capable of infecting the fruit 

through existing lesions caused by Colletotrichum and Phomopsis 

species. Furthermore, the reduction in the incidence of Rhizopus rot 

following fungicidal field sprays was related to the reduced incidence of 

field-initiated diseases whose lesions serve as routes of infection for 

R. stolonifer. 

Penetration due to Tissue Senescence 

Tissue senescence during prolonged storage also reduces disease 

resistance. Thus, at the end of the storage period, the sensitivity of melon 

to the blue-green mold caused by various species of Penicillium and to 

the pink mold caused by Trichothecium roseum is increased 

(Barkai-Golan, unpublished). A senescencing onion that has commenced 

sprouting often harbors base decay, caused by various species of 

Fusarium (Marlatt, 1958). 

Generally, the rate of decay during storage increases with the duration 

of storage as tissue senescence progresses. Increasing the tissue 

sensitivity to diseases during storage also contributes to contact-infection 

of a healthy product by an infected one covered with spore-bearing 

mycelium. 

Contact-Infection 

Fruits or vegetables that were spared a pathogen invasion via any of 

the means of penetration might still be infected during actual storage, 

through contact with infected produce. Contact-infection is a significant 

factor in the spreading of white watery rot (Sclerotinia spp.) and 

bacterial soft rot (Erwinia spp.) in lettuce, cabbage, celery, carrot or 

squash during storage. The development of Botrytis in stored 

strawberries, which it turns into "mummies" covered with a gray layer of 

spore-bearing mycelium, causes a "chain" contact-infection and 

jeopardizes the entire basketful of fruit. Similarly, one strawberry or 

tomato, or a single grape berry infected by Rhizopus constitutes a focus 

from which the decay can spread within the container when it is 

transferred from refrigeration to shelf conditions. In fact, 

contact-infection by Botrytis or Rhizopus is typical of many fruits and 

vegetables, and may account for the major losses caused by these 

pathogens during long-term storage. In citrus fruits, contact-infection by 

the green and blue mold rots {Penicillium digitatum and P. italicum) is 

very common; it often occurs during shipment and can, under certain 

conditions, disqualify the entire shipment. 



CHAPTER 3 

EACH FRUIT OR VEGETABLE AND ITS 

CHARACTERISTIC PATHOGENS 

A. HOST-PATHOGEN COMBINATIONS IN POSTHARVEST 

DISEASES 

The surface of the fruit or vegetable is covered with fungal spores that 

they have acquired from the air during their development on the parent 

plant, or with which they have come in contact during picking or any of 

the subsequent stages of handling the harvested produce. However, not 

every fungal spore or bacterial cell that reaches the harvested product 

can develop and cause decay, even when conditions suitable for 

penetration and development are present. 

Harvested fruits and vegetables are naturally attacked by a relatively 

small group of pathogens: approximately forty species. Moreover, each 

fruit or vegetable has its own typical pathogens out of this particular 

group. 

Fruits of tropical and subtropical origin, such as mango, papaya, 

avocado, citrus fruits, etc., are typically attacked by the fungus 

Colletotrichum gloeosporioides, which causes anthracnose. This pathogen 

penetrates into the fruit while still in the orchard, but the anthracnose 

symptoms in the rind break out only as the fruit ripens after harvest. In 

a similar manner, Gloeosporium musae attacks the unripe banana fruits 

in the orchard, to initiate a quiescent infection, which becomes active 

only during storage. 

Several fungi, such as Diplodia natalensis, Phomopsis citri or 

Dothiorella gregaria invade the cut stem of tropical and subtropical fruit, 

and induce the characteristic stem-end rot after harvest. D, natalensis, 

Alternaria citri and P. citri, the causal agents of postharvest stem-end rot 

of citrus fruit, all tend to lodge underneath the 'button' of the fruit, 

usually until after harvest, when the fungi progress from the button into 

the fruit and initiate stem-end rot. 

During and after harvest, the citrus fruit is typically attacked by 

Penicillium digitatum and P. italicum, the causal agents of green and 




blue mold, respectively, and Geotrichum candidum, the causal agent of 

sour rot: these are typical wound pathogens. P. digitatum is an example 

of a specific fungus that attacks only citrus fruits. On the other hand, 

P. italicum, which is also a significant citrus pathogen, can attack other 

fruits and vegetables, whereas Penicillium expansum, a significant apple 

and pear pathogen, does not naturally attack citrus fruits at all. 

The typical apple and pear pathogens include, among others, 

P. expansum, Botrytis cinerea, Gloeosporium spp., Alternaria alternata 

and Stemphylium botryosum, whereas the main pathogens of stone 

fruits, such as peaches, apricots, nectarines and plums, are Monilinia 

fructicola and Rhizopus stolonifer. Furthermore, while Penicillium spp. 

are responsible for major losses in harvested citrus or apple fruits, they 

are of little or no importance in most other crops. 

The harvested strawberry, too, has its own typical fungal pathogens. 

Out of the many airborne fungi found on the strawberry surface in the 

field or after harvest, two would normally develop during storage: the 

gray mold fungus (S. cinerea) and the soft watery rot fungus (R. 

stolonifer). In Great Britain, Mucor was found replacing Rhizopus as an 

additional pathogen to Botrytis in stored strawberries (Dennis and 

Mountford, 1975). 

Of the entire range of fungi that can be isolated from the surface of 

harvested tomatoes, A. alternata is the major storage decay agent; at 

times over 80% of the overall decay in the stored fruit is caused by this 

particular fungus (Barkai-Golan, 1973). The main causal agents of soft 

watery rot in harvested tomatoes are R. stolonifer, G. candidum and 

Erwinia spp. 

In much the same way, a celery head transferred to storage carries an 

abundance of air- and soil-borne fungi and bacteria. Here, too, the major 

decays that develop in the stored celery are generated by a small number 

of typical microorganisms: the fungi Sclerotinia sclerotiorum and 

B, cinerea, and the bacterium Erwinia cartovora. 

Each picked fruit and vegetable, therefore, has its own group of 

characteristic pathogens to which it is susceptible and for which it serves 

as a suitable host. 



Each Fruit or Vegetable and Its Characteristic Pathogens 27 

B. THE MAIN PATHOGENS OF HARVESTED FRUITS AND 

VEGETABLES 

The main pathogens of harvested fruits and vegetables, those that 

start their penetration in the field on the parent plant, and those that 

penetrate during or after the harvest, are listed in Table 1. Parallel to 

the pathogens are the disease symptoms and the primary hosts. The 

fungi are presented in alphabetical order followed by the bacteria. 

Summaries of the postharvest diseases of four specific groups of fruits, 

along with notes on their control, are presented in a separate chapter at 

the end of the book. 

TABLE 1 

The main pathogens of harvested fruits and vegetables * 

Pathogen Disease Primary Hosts 

Acremonium spp. 

Alternaria alternata (Fr.) 

Keissler 

Alternaria alternata (Fr.) 

Keissler 

Alternaria citri Ell. & Pierce 

crown rot banana 

fruit rot. 

dark spot. 

sooty mold 

stem-end 

rot; black 

spot 

stem-end 

rot 

stone and pome fruits. 

grapes, papaya, tomato. 

pepper, eggplant. 

cucumber, melon, 

watermelon, squash. 

cabbage, cauliflower. 

broccoli, corn, pea, bean. 

carrot, potato, sweet 

potato, onion 

avocado, mango, papaya 

persimmon 

citrus fruits 

* The main sources of the information in this Table are: Barkai-Golan, 

1981; Dennis, 1983; Bartz and Eckert, 1987; Snowdon, 1990, 1992. 




Pathogen 

Aspergillus niger v. Tieghem 

Botryodiplodia theobromae 

Pat. (see also Diplodia 

natalensis) 

Disease 

black rot 

crown rot 

finger rot 

stalk rot 

stem-end 

rot 

Botrytis allii Munn neck rot 

Perfect state: Botryotinia sp. 

Botrytis cinerea Pers. ex Fr. gray mold 

Perfect state: Botryotinia 

fuckeliana (de Bary) 

Whetzel 

Cladosporium herbarum 

(Pers.) Link 

Ceratocystis paradoxa 

(Dade) C. Moreau, 

Perfect state: Thielaviopsis 

paradoxa (de Seynes) 

Hohnel. 

Colletotrichum 

gloeosporioides (Penz.) Sacc. 

Perfect state: Glomerella 

cingulata (Stonem.) Spauld 

& V. Schrenk 

olive-green 

mold, sooty 

mold 

black rot, 

stalk rot, 

crown rot, 

soft rot 

Primary Hosts 

date, grape, tomato, 

melon, corn, onion, garlic 

banana 

citrus fruits, avocado, 

mango 

onion 

strawberry, raspberry, 

cherry, grape, pome and 

stone fruits, persimmon, 

citrus fruits, tomato, 

pepper, eggplant, 

cucumber, squash, melon, 

pumpkin, artichoke, 

cabbage, cauliflower, 

lettuce, broccoli, pea, 

bean, carrot, onion, 

potato, sweet potato 

date, grape, pome and 

stone fruits, papaya, fig, 

tomato, pepper, melon 

banana, pineapple 

anthracnose avocado, mango, papaya, 

guava, citrus fruits 




Pathogen Disease Primary Hosts 

Colletotrichum 

gloeosporioides (Penz.) Sacc. 

(syn. Gloeosporium 

fructigenum Berk.) 

Perfect state: Glomerella 

cingulata (see above) 

Colletotrichum musae 

(Berk. & Curt.) v. Arx (syn. 

Gloeosporium musarum 

Cke. & Massee) 

Diplodia natalensis P.E. 

Dothiorella gregaria Sacc. 

Perfect state: 

Botryosphaeria rihis Gross. 

& Duggar 

Fusarium pallidoroseum 

(Cooke) Sacc. 

Fusarium sambucinum 

Fuckel 

Perfect state: Giberella 

pulicaris (Fr.:Fr.) Sacc. 

Fusarium verticilloides 

(Sacc.) Nernberg (=F. 

moniliforme Sheldon) 

Perfect state: Gibberella 

fujikuroi (Saw.) Ito. 

bitter rot pome and stone fruits 

anthracnose 

crown rot 

stem-end 

rot, 

stalk rot, 

finger rot. 

crown rot 

stem-end 

rot 

crown rot 

banana 

citrus fruits, avocado. 

mango. 

banana 

avocado, mango, citrus 

fruits 

banana 

dry rot potato 

black heart banana, 

brown rot pineapple 




Pathogen 

Fusarium spp. 

Geotrichum candidum Link 

Gloeosporium album 

Osterw. 

Perfect state: Pezicula alba 

Guthrie 

Gloeosporium perennans 

Zeller & Childs 

Perfect state: Pezicula 

malicorticis (Jackson) Nannf. 

Helminthosporium solani 

Dur. & Mont. 

Monilinia fructicola (Wint.) 

Honey 

Monilinia fructigena (Aderh. 

& Ruhl.) Honey 

Monilinia laxa (Aderh. & 

Ruhl.) Honey 

Mucor hiemalis Wehmer 

Mucor piriformis Fischer 

Penicillium digitatum Sacc. 

Penicillium expansum 

(Link) Thom 

Disease 

dry or soft 

rot 

sour rot 

Primary Hosts 

tomato, pepper, eggplant, 

melon, squash, pumpkin, 

watermelon, cabbage, 

celery, artichoke, 

asparagus, corn, carrot, 

potato, sweet potato, 

onion, garlic 

citrus fruits, melon, 

tomato 

lenticel rot pome fruits 

lenticel rot pome fruits 

silver scurf 

brown rot 

brown rot 

brown rot 

watery soft 

rot 

watery soft 

rot 

green mold 

blue mold 


potato 

stone fruits mainly, but 

also pome fruits 

stone and pome fruits 

stone fruits mainly, but 

also pome fruits 

tomato, strawberry, 

raspberry, melon, corn 

tomato, strawberry, 

raspberry 

citrus fruits (exclusively) 

pome fruits mainly, but 

also stone fruits



Pathogen 

Penicillium italicum 

Wehmer 

Penicillium spp. 

Phoma caricae-papayae 

(Tarr) Punith. 

Perfect state: 

Mycosphaerella caricae H. & 

P. Sydow 

Phomopsis citri Fawc. 

Perfect state: Diaporthe citri 

Wolf 

Phytophthora citrophthora 

(Smith & Smith) Leon. 

Phytophthora infestans 

(Mont.) de Bary 

Pythium spp. 

Rhizopus stolonifer (Ehr. ex 

Fr.) Lind 

Sclerotinia sclerotiorum 

(Lib.) de Bary 

Disease 

blue mold 

blue mold 

stem-end 

rot, dry 

black rot 

stem-end 

rot 

brown rot 

late blight 

soft watersoaked 

lesion, 

cottony leak 

watery soft 

rot 

watery 

white rot, 

cottony rot 


Primary Hosts 

citrus fruits mainly 

tomato, cucumber, melon 

papaya 



potato, tomato 

tomato, pepper, eggplant 

stone and pome fruits. 

grape, avocado, papaya, 

strawberry, raspberry, 

cherry, tomato, pepper, 

eggplant, carrot, melon, 

pumpkin, squash, pea, 

bean, sweet potato 

citrus fruits, cabbage, 

cauliflower, lettuce. 

celery, broccoli, artichoke, 

pea, bean, carrot, 

eggplant, melon, 

cucumber, pumpkin, 

squash, onion, garlic



Pathogen 

Stemphylium botryosum 

Wallr. Perfect state: 

Pleospora herbarum (Pers.) 

Rabenh. 

Stemphylium radicinum 

(Meier, Drechsler & Eddy) 

Neerg. 

Trichoderma viride Pers. ex 

S.F. Gray 

Disease 

black lesion, 

dark spots 

black lesion 

black rot 

stem-end 

and fruit 

rot, 

green-yellow 

mold 

Trichothecium roseum Link pink mold 

Verticillium theobromae 

(Turc.) Mason & Hughes 

Erwinia carotovora ssp. 

carotovora (Jones) Dye 

Erwinia spp. 

Pseudomonas syringae 

(Kleb.) Kleb. 

Pseudomonas spp. 

Pseudomonas syringae pv. 

syringae van Hall 

crown rot, 

cigar-end 

rot 

bacterial 

soft rot 

spots or 

bacterial 

soft rot 

black pit 


Primary Hosts 

pome fruits, papaya. 

grape, tomato, lettuce 

carrot 


stone and pome fruits, 

banana, avocado, tomato, 

melon 

banana 

tomato, pepper, melon, 

squash, pumpkin, 

cucumber, cabbage, 

cauliflower, lettuce, 

celery, broccoli, spinach, 

asparagus, pea, bean, 

potato, sweet potato, 

onion 

tomato, cucumber, melon, 

squash, asparagus, 

cabbage, cauliflower, 

lettuce, celery, broccoli, 

spinach, pea, bean, onion 

citrus fruits


CHAPTER 4 

FACTORS AFFECTING DISEASE DEVELOPMENT 

A. PREHARVEST FACTORS, HARVESTING AND HANDLING 

The first preharvest factor which may affect postharvest quahty is the 

cultivar, since different cultivars may vary greatly in their susceptibihty 

to diseases. In fact, one of the aims of plant breeding and genetic 

engineering is to incorporate resistance genes in new varieties of crop 

plants. Differences in cultivar characteristics can markedly affect the 

keeping quality of the fresh produce. Thus, melons with a thick skin and 

raspberries with a firm texture are better able, than others, to withstand 

the rigors of harvesting and handling and should, therefore, have longer 

storage lives (Snowdon, 1990). Variability in postharvest decay among 

apple cultivars has been related to differences in the wounding resistance 

of their skins, a feature which may be of major importance for decay 

pathogens that depend on a wound to initiate infection (Spotts et al., 

1999). 

Another preharvest factor is the health of the planting material. 

Various pathogens may contaminate the planting material and cause 

disease in the field or in storage. Hence the importance of obtaining clean 

seeds and of the establishment of seed certification schemes (Jeffs, 1986). 

Environmental conditions during growth, such as unfavorable high or 

low temperature, wind, rain or hail, can all affect the crop and determine 

not only the yield but also the quality of the stored commodity (Sharpies, 

1984). High temperature was found to increase Botrytis cinerea infection 

of tomatoes via the flowers because it increased the rate of flower 

development and senescence (Eden et al., 1996). Environmental 

conditions may also affect the pathogens directly. Many pathogens 

persist in the soil or survive on plant debris in the field, from which 

winds and rain may be directly responsible for their dispersal to 

potential hosts. Other pathogens, such as Phytophthora spp. which infect 

potato tubers or citrus fruits, are actually dependent on rainwater for 

germination of their spores and initiation of infection (Lapwood, 1977; 

Feld et al., 1979). In fact, the percentage of brown rot caused by 

Phytophthora parasitica in orange orchards was directly related to the 




amount of rainfall in the principle infection period (Timmer and Fucik, 

1975). 

Cultural practices, such as pruning of fruit trees and destruction of 

crop debris, can markedly affect the survival of pathogenic 

microorganisms (Palti, 1981). Application of preharvest fungicides can 

directly reduce the level of infestation. However, preharvest chemical 

sprays, with the same chemical that is designated for postharvest 

application, can enhance the production of new fungal strains resistant 

to that fungicide (Eckert and Ogawa, 1988). Cultural practices can also 

be changed in order to reduce the inoculum level through sanitation, or 

to produce conditions less favorable for disease by modifying the canopy 

microclimate (Legard et al., 1997). Plant spacing within the row may also 

affect the incidence of rot. Legard et al. (2000) found that wider 

within-row plant spacing reduced Botrytis rot in strawberries compared 

with narrower spacing. This may be due to the increased number of 

target hosts available to intercept inoculum in the latter case, or to the 

fact that more fruit may escape timely harvesting and thus contribute to 

increased levels of inoculum. Increased plant density could also reduce 

the efficacy of fungicide applications by reducing plant coverage (Legard 

et al., 2000). Soil solarization, applied in hot countries by covering the 

soil with clear plastic film to heat the soil to a lethal temperature (Katan, 

J., 1987), or steam sterilization and fumigation applied in cooler 

countries, are practices which help to cleanse the soil by destroying 

harmful organisms. The sequence of crop rotation in the field can reduce 

the source of infection and thus influence the quality of the harvested 

commodity by affecting the health of the subsequent crop (Palti, 1981). 

Field nutrition, too, can have an impact on the development of storage 

decay. Thus, the rapid development of bacterial soft rot in tomato fruits 

depends, to a great extent, on the application of nitric fertilizer to the 

plant while in the field (Bartz et al., 1979), and the resistance of pears to 

postharvest decay increases after the trees have been given nitrogen and 

calcium (Sugar et al., 1992). Despite all these data, the linking 

relationship between stimulating nutrients within the host and its 

susceptibility to the pathogen is quite unclear. 

Harvesting by hand is the predominant method for fruits and 

vegetables destined for the fresh produce market (Kader et al., 1985). 

With proper training, pickers can select for the optimal maturity stage of 

a given commodity and can keep damage to a minimum. Mechanized 

harvesting, even when used correctly, can cause substantial damage to 

the commodity, which may serve as suitable areas of penetration for 



Factors Affecting disease Development 35 

'wound pathogens*. It is, therefore, confined mainly to less vulnerable 

commodities, such as carrots or potatoes, or to crops which are intended 

for immediate processing (Snowdon, 1990). The harvesting date may be 

of great significance for fruits intended for prolonged storage. However, 

despite the use of various criteria for determining the appropriate stage 

of maturity for harvesting (such as color, size, shape, flesh firmness and 

content of starch, sugar, juice or oil), the prediction of an optimal harvest 

date is often imprecise (Kader et al., 1985). Furthermore, unfavorable 

weather conditions, such as rain prior to harvesting, are liable to 

increase the risk of infection, even in fruits suitable for harvesting. The 

time of harvesting during the day may also affect the keeping quality of 

the produce. For most crops the cool hours of the night, or the early 

morning, can be advantageous. 

The risk of damage and subsequent infection accompanies the fresh 

produce along its way from harvesting and handling in the field, through 

transport from the field to the packinghouse, during the packinghouse 

operations and through the cooling processes - before and/or after 

packing - until its arrival in the storage room, and even onwards to its 

final destination. 

B. INOCULUM LEVEL 

For a pathological disease to occur, a meeting between a vital 

pathogen and its suitable host should take place. However, when such a 

meeting does occur a successful infection may be dependent on the level 

of the inoculum (generally comprising fungal spores or bacterial cells) 

available. Gauman (1946) and his followers have claimed that a 

minimum number of pathogen spores is necessary to establish certain 

diseases, even under favorable conditions. Such a theory denies, for many 

fungi, the possibility of a single spore infection and has not been accepted 

by other investigators (van der Plank, 1975). Infection studies with 

fungal spore suspensions on different hosts showed that increasing the 

spore concentrations frequently leads to higher rates of infection and 

lesion formation. Eden et al. (1996) found that infection of tomato flower 

parts by Botrytis cinerea increased as a function of inoculum 

concentration. This finding demonstrated the practical importance of 

reducing the inoculum level on the crop in order to minimize infection. 

For postharvest pathogens, which depend on a wound to enable them to 

penetrate into the host, it has generally been accepted that disease 




development is related to both the pathogen spore load on the fruit or 

vegetable surface and the availability of wounds for penetration. In fact, 

the control of postharvest infections initiated by *wound pathogens' is 

based primarily on the reduction of the inoculum level that may reach 

the host, along with continuing efforts to reduce pre- and postharvest 

injury. Regular sanitation in the field, in the packinghouse and in the 

storerooms, will all contribute to the reduction of the spore load on the 

harvested produce, while careful handling and prevention of mechanical 

damage will help to reduce the number of entry points for the pathogen. 

Discussing postharvest diseases of pome and stone fruits, Edney 

(1983) states that the incidence of rotting is influenced by the number of 

viable spores at potential infection sites when the fruit is at the stage of 

ripening suitable for infection to develop. However, the amount of 

inoculum present is closely related to weather conditions during the 

growing season, particularly when the spores are dispersed by rain as in 

the cases of Gloeosporium and Phytophthora species. 

An interaction between wounding and inoculum level has been 

described for the brown rot fungus, Monilinia fructicola on stone fruits 

(Hong et al., 1998). In the presence of a high level of fungal inoculum on 

the fruit surface, penetration of the unwounded fruit can take place 

through the stomata or directly through the peel. However, as the spore 

load on the fruit decreases below a certain level (5 or 50 spores per mm^ 

for nectarines or peaches, respectively) the significance of wounds as 

entry points increases. With plums, on the other hand, brown rot can 

develop only on wounded fruit, even at an inoculum level of 10^ 

spores/ml. 

Studying surface colonization and lesion formation by various 

inoculum densities of single conidia of Botrytis cinerea on grapes, Coertze 

and Holz (1999) found that individual conidia readily infected cold-stored 

berries, which are highly susceptible to infection, and that the number of 

lesions tended to increase at high inoculum concentrations. However, 

these conidia did not induce disease symptoms on fresh, resistant berries, 

irrespective of the inoculum density. These results suggested that 

infection was not always governed by conidial density on berry surfaces, 

but rather by the level of host resistance. 

The inoculum level of the pathogen may determine the success of 

biological control of postharvest diseases with antagonistic 

microorganisms (see the chapter on Biological Control). For various 

host-pathogen interactions, the efficacy of the antagonistic 

microorganism in reducing decay has frequently been affected by the 



Factors Affecting disease Development 3 7 

inoculum levels of both the pathogen and the antagonistic 

microorganism. At a high fungal spore concentration, low concentrations 

of the antagonist were not sufficient to reduce the incidence of infection. 

However, when wounds were inoculated with low spore concentrations, 

the percentage of infection could be effectively reduced even by low 

concentrations of the antagonist (Chalutz et al., 1991). Reducing the 

spore load of the pathogen on the fruit may, therefore, contribute to 

improved biological control. 

C. STORAGE CONDITIONS 

The actual pathogen penetration does not in itself ensure that the 

disease will develop. For the fungi or bacteria that have penetrated the 

host to become established and to continue their development, they have 

to encounter tissues under conditions suitable for their settling and 

developing. These conditions include appropriate temperature and 

relative humidity, the presence of available nutrients, suitable pH level, 

and other environmental conditions. 

1. TEMPERATURE 

The spore germinating and mycelial growing temperatures constitute 

basic or even limiting factors in the development of the disease. The 

optimal temperature for growth of most storage fungi is 20-25°C, though 

some species prefer higher or, more rarely, lower temperatures. The 

optimum can vary according to fungal species and at times even to its 

strain or isolate. However, the optimum for growth is not necessarily 

identical to the optimum for germination. The further from the optimal 

temperature for the fungus the longer the time required for initiation of 

germination and mycelial growth, and the longer the duration of the 

incubation period of the disease (the time until the appearance of decay 

symptoms). In parallel to the prolongation of its incubation period, the 

subsequent development rate of the pathogen is reduced. Fig. 8 displays 

the retardation of the gray mold development process in tomatoes 

infected with Botrytis cinerea spores, as a consequence of shifting the 

temperature away from the optimum (17-20°C). 

The minimal growing temperature can differ among the various 

storage fungi (Table 2). However, it should be noted that different 

isolates of the same fungus, or the same isolate under different growing 



38 

-• Half fruit rot 


4 6 8 

Days after inoculation 

Ô 

.^^'^ -.^^^ 

Fig. 8. Rate of development of Botrytis rot in inoculated tomato fruit at different 

temperatures. (Barkai-Golan, unpublished). 

conditions, can have different minimal temperatures, which explains why 

the differing minimal temperatures are cited in the literature for a given 

fungal species. For some fungi, the minimal developmental temperature 

is below 0°C (Sommer, 1985). That is why Botrytis cinerea, whose 

minimum is -2°C, continues to develop in cabbage, celery, lettuce and 

other vegetables stored at 0°C, and why various isolates of Penicillium 

expansum and Alternaria alternata, whose minimum is -3°C, continue 

their development in certain apples stored at 0°C or below. 

On the other hand, some pathogens, such as species of Colletotrichum 

or Aspergillus niger have a mycelium susceptible to low temperatures, 

and their growth minima are 9°C and 11°C, respectively. Nevertheless, 

the Colletotrichum fungus, which is characteristic of tropical and 

subtropical fruits, can easily develop in these fruits during storage since, 

because of their susceptibility to low temperatures, they are stored at 

relatively high temperatures. Rhizopus stolonifer spores are also 

susceptible to low temperatures and do not develop at temperatures 

under 5°C; though a certain percentage of the spores can germinate at 

2°C, their germ tubes cannot continue their growth at such a 

temperature (Dennis and Cohen, 1976). 




TABLE 2 

Minimal temperatures for postharvest 

decay fungi * 

Fungus Minimal Temperature °C 

Alternaria alternata -3 

Aspergillus niger 11 

Botrytis cinerea -2 

Cladosporium herbarum -4 

Colletotrichum gloeosporiodes 9 

Colletotrichum musae 9 

Diplodia natalensis -2 

Geotrichum candidum 2 

Monilinia fructicola 1 

Penicillium digitatum 3 

Penicillium expansum -3 

Penicillium italicum 0 

Phomopsis citri -2 

Rhizopus stolonifer 2; 5** 

* Reproduced from Sommer (1985) with permission of the Canadian 

Journal of Plant Pathology. 

** Reproduced from Dennis and Cohen (1976). 

As to Rhizopus, significant differences were found between dormant 

and germinating spores in their susceptibility to low temperatures. 

Dormant spores in the pre-germinating phase were not damaged by the 

chilling temperatures, but when they entered the primary germination 

stages, their susceptibility to chill increased, and at 0°C they perished 

within a few days (Matsumoto and Sommer, 1968); in some fungi it was 

found that a young mycelium within a tiny decay spot was more 

susceptible to low temperatures than a mature mycelium within a 

developed decay spot. 

One should bear in mind that the environmental temperature affects 

both the host and the pathogen simultaneously. Exposing the fruit or 

vegetable to temperatures that cause tissue damage does not inhibit decay 

development, but even enhances the infection rate of those pathogens that 

penetrate through a wound or a damaged tissue (Segall, 1967). 




The interrelations among the environmental temperature, the 

pathogen and the host can be depicted as a triangle displaying the effect 

of the temperature on each of the others, which have their own mutual 

interrelations: 

TEMPERATURE 

^ HOST 

PATHOGEN 

In fact, a significant proportion of the decay that occurs in the 

markets, particularly that of tropical fruits, results from overexposure to 

damaging temperatures. For some tropical fruits and vegetables, storage 

below 10°C is sufficient to cause chilling injuries, even when external 

physiological symptoms are not visible. 

The use of low temperatures to depress decay by maintaining host 

resistance will be described in a separate chapter (See: Means for 

Maintaining Host Resistance - Cold Storage). 

2. RELATIVE HUMIDITY AND MOISTURE 

The high relative humidity (RH) required for the protection of fruits 

and vegetables from dehydration and weight loss might stimulate 

pathogen development during storage. The danger of decay is enhanced 

by the condensation of mist over the fresh fruit or vegetable surface. 

Similarly, wrapping the fruit or vegetables in sealed plastic film with low 

permeability to water vapor often causes enhanced decay when these 

films are not punctured and ventilated. Rodov et al. (1995c) found that 

the RH within an airtight polymeric wrap can be reduced by adding a 

hygroscopic substance such as sodium chloride. This reduction results in 

a reduced decay of peppers wrapped in a polyethylene film and stored at 

8°C. Without the insertion of the hygroscopic substance, the relative 

humidity within the wrapping might approach saturation, with 

condensed water drops on the fruit and on the wrapping inner surface. 

Leafy vegetables and, especially, leafy herbs, characterized by rapid 

dehydration following harvest, store better under high RH and 

subsequently decay less because of the reduced accumulation of dead 

tissues, which may provide nutrients for decay organisms (Yoder and 

Whalen, 1975). Packing such crops in polyethylene-lined crates has the 

advantage of preventing dehydration and preserving turgidity. The main 

limitation to the use of polymeric film packaging is the water condensation 




within the package, which results from extreme changes in the storage 

and shipping temperatures. Perforating the hning is of primary 

importance, since the holes allow limited vapor emission, with acceptable 

weight loss (with no visible shriveling), prevent ethylene accumulation in 

the herbal atmosphere, and protect the leaves from decay (Aharoni, 1994). 

Many fruits and vegetables are more susceptible to the pathogen when 

their tissues are in a turgid state through being under high RH. In many 

cases, the increased decay rate should be attributed to moisture held 

within the wounds, lenticels or stomata under these conditions. Fungal 

spores use this moisture when germinating, prior to their penetration 

into the tissues. Crops such as apples and pears, with well developed 

cuticle and epidermal layers, tolerate relatively lower RH levels, which 

helps to prevent storage decay due to the inhibition of fungal spore 

germination (Spotts and Peters, 1982b). However, when Anjou pears were 

wounded prior to inoculation, spores of some of the important pathogens, 

such as Botrytis cinerea and Penicillium expansum, did germinate and 

cause decay at 97% RH in cold storage (-l.l^C). It is well known that 

accumulated water drops on the surface of citrus fruits stimulate the 

development of the green mold (Penicillium digitatum). Therefore, slightly 

drying the fruit might reduce its susceptibility to decay (Eckert, 1978). 

The water content of host tissues (water status) is a major 

contributing factor in the development of soft rot bacteria. In various 

vegetables, such as bell peppers and Chinese cabbage, high water status 

has frequently been associated with high potentials for bacterial soft rot 

(Bartz and Eckert, 1987). Accumulation of free water on potatoes stored 

in a relative humidity of over 95% and at a low temperature increases 

the rate of soft rot caused by bacterial penetration through the lenticels. 

Ventilated storage rooms and dried-up tubers can, therefore, reduce the 

bacterial decay (Perombelon and Lowe, 1975). On the other hand, 

Botrytis and Rhizopus easily attack carrots that have commenced 

withering and have lost over 8% of their water volume (Thorne, 1972). 

The increased susceptibility of the carrot under these conditions is 

attributed to the cell separation and enlargement of the intercellular 

spaces, which accompany the dehydration. 

Studies conducted by van den Berg and Lentz (1973, 1978) found that 

the decay rates in cabbages, Chinese cabbages, cauliflowers, carrots, celery 

and other vegetables, in cold storage with relative humidity of 98-100%, 

were no higher than those under lower relative humidity, and at times 

were even lower. Under hyper-humidity, the firmness and freshness of 

vegetables were preserved much longer, and the pectolytic action of the 




fungi pathogenic to carrots (especially that of Botrytis cinerea and 

Sclerotinia sclerotiorum) was weaker when the relative humidity was close 

to saturation than when it was at the 90-95% level (van den Berg and 

Lentz, 1968). Since these enzymes play an important part in the 

development of the pathogen within the host tissues, the pectolytic activity 

levels of the pathogens under the various humidity conditions might 

constitute a significant factor in the development of decay. 

3. THE STOREROOM ATMOSPHERE 

The gas composition of the storeroom atmosphere directly affects the 

development of fungal mycehum in the plant tissue, as it also affects spore 

germination on fruit and vegetable surfaces. Yet, hke the other 

environmental conditions, the storeroom atmosphere, too, simultaneously 

affects the host, the pathogen and their interrelations. Since the 

atmospheric gas composition can significantly affect the ripening and 

senescence processes of the host, and since the physiological state of the 

host can affect its susceptibility to disease, the atmospheric gas composition 

indirectly affects decay development in the tissue (Barkai-Golan, 1990). The 

direct and indirect effects of the atmospheric gases (the levels of O2, CO2 

and their combinations) on the development of pathogens, and the use of a 

suitable atmosphere composition for inhibiting disease in stored fruits and 

vegetables is discussed in the chapter on Means for Maintaining Host 

Resistance - Modified and Controlled Atmosphere. 

D. CONDITIONS PERTAINING TO THE HOST TISSUES 

1. ACIDITY LEVEL (pH) 

The low pH that characterizes many fruits (below pH5) is probably an 

important factor in their general resistance to bacterial decay agents, but 

it furthers the postharvest development of various fungi. In fact, fungi 

cause most of the decays in harvested fruits, whereas bacteria are 

important mainly in vegetables. Unlike the fruit, the various other plant 

organs such as roots, tubers, stems or leaves, where the pH level is 

generally higher and ranges between 4.5 and 7.0, might often be attacked 

by soft-rot bacteria (Lund, 1983; Bartz and Eckert, 1987). Exceptions to 

that rule are some "fruit-vegetables", such as tomatoes, bell peppers and 

cucumbers, in which bacterial decay is quite common. The pH of soft rot 

lesions on tomato fruits is higher (pH>5.0) than that of surrounding 

healthy tissue (pH= 4.3 - 4.5), suggesting that the bacteria are capable of 



Factors Affecting disease Development 

buffering their environment in a range suitable for their growth and the 

activity of their pectolytic enzymes (Bartz and Eckert, 1987). 

2. GROWTH STIMULI 

Certain compounds in the host tissues might, on certain occasions, affect 

the host susceptibiUty to infection by stimulating pathogen growth. 

Generally, spores o{ Penicillium digitatum do not germinate in water on the 

surface of citrus fruit until the peel is injured (Smoot and Melvin, 1961). 

However, when the peel is wounded, during harvesting or subsequent 

handling and processing, P. digitatum spores germinate and their germ 

tubes may penetrate the fruit, to initiate infection and, eventually, to 

develop the typical green mold symptoms. Ascorbic acid and a number of 

terpene compounds in citrus fruits, much hke their stimulating effect on the 

germination of P. digitatum spores, can also stimulate mycehal growth of 

this fungus, which is specific to citrus fruits (Pelser and Eckert, 1977; 

French et al., 1978). Thus, the presence of citrus juice with the appropriate 

acidity level can determine both the germination rate and the germ tube 

growth and, in turn, the rate of fungal development and the incubation 

period of the disease (Pelser and Eckert, 1977) (Fig. 9). 

100 

Fig. 9. Conidia germination and germ tube growth of Penicillium digitatim in 

1% (v/v) orange juice dialysate at several pH values (Reproduced from Pelser 

and Eckert, 1977 with permission of the American Phytopathological Society). 


I 

O) 

0) 

3 

CD 

43



Eckert and Ratnayake (1994) found that a mixture of limonene, 

acetaldehyde, ethanol, CO2 and other volatile compounds emanating 

from wounded oranges induced germination of P. digitatum conidia on 

water agar. Fungal conidia that accumulate in water condensate on 

citrus fruits can also be induced to germinate by volatile compounds 

emanating from adjacent wounds. On the other hand, washing wounded 

lemon fruit peel (epicarp) was found to greatly suppress P. digitatum 

infection, when the fruit was inoculated with fungal spores, so that only 

2% of the fruit showed green mold symptoms. However, when a 

comparable amount of isolated lemon peel oil was topically applied to the 

washed wounds, 92% of the inoculated wound sites did develop complete 

green mold symptoms (Arimoto et al., 1995). 

In the light of the finding that peel oil extracts applied to wounded 

epicarps can restore disease development potential to the pathogen, 

Arimoto et al. (1995) investigated the possibility that some components of 

the lemon peel oil might be essential for fungal development in the peel 

tissues. These studies indicated that lemon epicarp oil, with the limonene 

removed, promoted the production of green mold symptoms on only 28% 

of the wounds, suggesting that limonene was one facilitator of green mold 

formation on wounded fruits. However, another promoting factor was 

isolated from the oil and was identified as prangolarin by spectrometric 

analysis. Prangolarin by itself enhanced P. digitatum development on 

wounded epicarps of lemons, resulting in the development of green mold 

symptoms because of the production of masses of conidiophores and 

conidia (Arimoto et al., 1995). 

In potato tubers a close connection was found between the reducing 

sugar content and the susceptibility of the tuber to bacterial soft rot 

during storage at various temperatures (Otazu and Secor, 1981). A 

correlation was also found between the sugar contents of nectarine and 

plum fruits and their susceptibility to Botrytis cinerea infection (Fourie 

and Holz, 1998); this finding will be discussed below in regard to 

enhanced susceptibility to decay during the ripening stage. Edlich et al. 

(1989) found that certain sugars taken up by B. cinerea could stimulate 

fungal growth and enhance its infection capability, but suggested that 

the stimulation was due to the active oxygen formed rather than to a 

nutritional effect. Pollen exudates from weeds commonly found in stone 

fruit orchards have also been found to be stimulators of J3. cinerea growth 

(Fourie and Holz, 1998). When added to the fungus conidia during the 4 

weeks prior to the picking-ripe stage the exudates significantly increased 

the aggressiveness of the pathogen on plum and nectarine fruits. The 




investigators believe that weed pollens with high sugar contents are 

likely to lead to further stimulation of the pathogen on fruit in the 

orchard (Fourie and Holz, 1998). 

3. THE FRUIT RIPENING STAGE 

The susceptibility of harvested fruits and vegetables to decay agents 

depends mainly on their ripening stage at the time of picking; it 

increases as ripening progresses. Many types of fruits are more 

susceptible to injuries as they ripen and, therefore, become more 

susceptible to pathogen attack (Eckert, 1975). However, in general, the 

susceptibility of the fruit to pathogen invasion increases regardless of its 

susceptibility to injury. Various tissue characteristics, such as the acidity 

level, the turgor state of the tissues, or nutrient availability, change 

throughout the senescencing and ripening stages and might, separately 

or in combination, enhance the susceptibility to disease. Other factors 

affecting the impact of the ripening stage on disease susceptibility 

involve the enhanced virulence of the pathogen, on the one hand, and 

weakened host resistance and protection, on the other hand. These 

factors include: 

a. increased availability of compounds that induce formation of 

pectolytic enzymes by the pathogen; 

b. enhanced susceptibility of the host cell walls to the pectolytic 

activity of the pathogen; 

c. reduced concentrations of compounds toxic to the pathogen or of 

those that inhibit its enzymatic activity within the host tissues; 

d. changes in nutrient availability during ripening. 

One of the primary factors enhancing the susceptibility of the fruit to 

infection is the enhanced susceptibility of the plant cell walls to the 

activity of pectolytic enzymes produced and secreted by the pathogen. 

The pectic substances that comprise the cell wall of the young fruit are 

present in the form of insoluble protopectin. The insolubility of 

protopectin stems from both its high molecular weight and its close links 

with the cell wall cellulose, which comprise bridges of neutral 

polysaccharides and perhaps proteins as well (Eckert, 1978). At this 

stage, the cell walls withstand the pectolytic activity, which constitutes a 

most significant attack mechanism for many pathogens. As the fruit 

ripens, the links connecting the pectic substances to the cell wall break, 

so that the increased solubility of the pectic substances increases, and 

the tissue starts to soften (Eckert, 1978; PauU et al., 1999). At this stage, 




the fruit can provide pectic compounds that induce the formation of 

pectolytic enzymes and their activity, and the plant tissue becomes more 

susceptible to the pectolytic enzymes secreted by the pathogen. Recent 

studies with papapya cell walls suggested that, in addition to pectin 

hydroljdsis, the modification of another cell-wall compound - the 

hemicellulose - is also involved in fruit softening (PauU et al., 1999). 

Another factor that changes during the ripening process is the ability 

of the tissues to produce toxic compounds, which inhibit the pathogen 

growth; this diminishes as the tissues ripen. These compounds include: 

tomatine, which can be found in high concentrations in the green tomato 

skin (Verhoeff and Liem, 1975); tannins in young bananas (Green and 

Morales, 1967; van Buren, 1970); monoene and diene compounds in 

young avocado skin; resorcinols in the skin of mangoes in their early 

developmental stages (Prusky and Keen, 1993); and various toxic 

compounds found in the oil glands of young citrus fruit rind (Rodov et al., 

1995b). Polyphenols and tannins, highly concentrated in the younger 

fruits, have been described both as germination and growth inhibitors of 

microorganisms, and as suppressors of enzymatic activity. 

Regarding the nutrient availability changes during the advanced 

development or ripening of the fruit, a direct relationship was found 

between the sugar contents in nectarines and plums and the late-season 

susceptibility of these fruits to B. cinerea infection (Fourie and Holz, 

1998). During their early developmental stages, the young unwounded 

fruits are resistant to the pathogen. It is not until the early phase of 

rapid cell enlargement of the nectarine and the last phase of rapid cell 

enlargement of the plum that the fruit becomes susceptible to infection, 

and disease development occurs (Fourie and Holz, 1995). 

Studying the sugar content in exudates of immature plum and 

nectarine fruits, Fourie and Holz (1998) found that fructose, glucose and 

sorbitol were the only sugars present, whereas sucrose was first detected 

during fruit maturation, which occurs approximately two weeks before 

harvest. The sugars were exuded at low concentrations by immature 

fruits but their concentrations increased as the fruit ripened. In 

nectarines there was a pronounced increase in the sugar concentrations, 

especially that of sucrose, near the picking-ripe stage. Fungal growth was 

found to increase when the reducing sugars or sucrose were added to 

culture media in excess of 0.27 or 0.14 mM and when sorbitol was 

supplied in excess of 0.66 mM. With the progress in fruit maturity during 

the last two weeks before harvest, total sugars in plum exudates 

approached these values, whereas in nectarine exudates they far 




exceeded them. Fourie and Holz (1998), therefore, concluded that the 

stimulatory effect of fruit exudates on B. cinerea growth coincided with 

the period of rapid sugar release from the fruit and the exhibition of fruit 

susceptibility to decay. 

4. EFFECTS OF ETHYLENE 

A strong link between the ripening stage of a fruit and its sensitivity 

to decay may explain why conditions or chemical substances that 

stimulate ripening, generally also enhance decay. A classic example of 

conditions that stimulate ripening is the exposure of various citrus fruit 

cultivars to low concentrations (50 ppm) of ethylene for degreening. 

Ethylene treatments are applied commercially at the beginning of the 

citrus fruit picking season, to degreen the fruits that have reached 

maturity but have not yet developed the desired color. However, this 

economically important procedure, which enhances chlorophyll 

decomposition and exposes the yellow, orange or red color in the fruit 

peel, is accompanied by enhanced sensitivity of the fruit to decay (Brown, 

G.E. and Lee, 1993). It was found that along with the enhancement of 

ripening, ethylene also stimulates senescence and disruption of the 

stem-end ^button', thus activating the quiescent infection of Diplodia 

natalensis at this location (Brown, G.E. and Wilson, 1968; Brown, G.E. 

and Burns, 1998) and leading to increased incidence of stem-end rot 

(McCornack, 1972). 

An increase in rot incidence generally follows the use of ethylene at 

concentrations above those needed for adequate degreening. In vitro 

studies showed that the pathogen growth was stimulated by exposure to 

high ethylene concentrations (Brown, G.E. and Lee, 1993). However, the 

increased growth rate of the pathogen in response to high ethylene is not 

the only factor responsible for enhanced susceptibility of the fruit to 

disease. Following the finding of the correlation between ethylene 

treatment and the activities of the enzymes, polygalacturonase (PG) and 

cellulase (Cx) during citrus abscission (Riov, 1974; Ratner et al., 1969), it 

was recently found that the increase in disease incidence caused by the 

use of high concentrations of ethylene may be related to the activity of 

the abscission enzymes (Brown, G.E. and Burns, 1998). Enzymes formed 

during abscission degrade pectic substances in the middle lamella and 

lead to cell separation in the abscission zone. The activity of these 

enzymes in oranges was found to increase rapidly on exposure to high 

ethylene concentrations (0.055 ml l-i) and a larger number of cells were 

degraded within the abscission layer by these enzymes than by those 




exposed to lower, more typical degreening concentrations (0.002 ml l-i). 

Furthermore, the addition of commercial or partially purified 

preparations of the abscission enzymes to abscission areas of debuttoned 

oranges, before inoculation with D. natalensis, caused a significant 

increase in stem-end rot development. 

Another way to study the relationship between ethylene-dependent 

abscission enzymes and the susceptibility of the fruit to stem-end rot was 

the use of metabolic inhibitors that reduce enzyme activity during 

high-ethylene treatment (Brown, G.E. and Burns, 1998). When citrus 

fruits were dipped before degreening in 2,4 dichlorophenoxyacetic acid 

(2,4-D), which is known to reduce PG and Cx activity (Riov, 1974) or 

silver thiosulfate, which is an inhibitor of ethylene action (Veen, 1983), 

they showed enhanced resistance to stem-end rot when inoculated after 

degreening (Fig. 10). 

•o 

CD 

I 

E 

d) 

(/) 

CO 

JO 

Q. 

80 

60 

40 

S- 20 

b 

4- 

Control 

• 

2,4-D 

-^- 

STS 

—B— 

\ 1 ^ 

6 10 

14 

Days (300C) 

Fig. 10. Incidence of stem-end rot in Valencia oranges dipped in water (control), 

2,4-dichloro-phenoxyacetic acid (2,4-D), or silver thiosulfate (STS) before 

degreening with 0.055 ml l-i of ethylene for 42h. Debuttoned fruit were 

inoculated with Diplodia natalensis mycelium after ethylene treatment. Bars 

represent standard deviation. (Reproduced from Brown and Burns, 1998 with 

permission of Elsevier Science). 




Similarly, the addition of cycloheximide, which is a protein-synthesis 

inhibitor (Riov, 1974), to the abscission zone of debuttoned fruits during 

early stages of degreening with high ethylene concentrations, also 

suppressed disease development. In other words, the inhibition of the 

enzymatic activity led to reduced fruit susceptibility to stem-end rot 

development. 

The correlation between ethylene application to citrus fruit and the 

enhancement of fruit susceptibility to the green mold rot (Penicillium 

digitatum) has been attributed to the reduction in the antifungal activity 

of the peel: ethylene treatment stimulates the natural reduction in the 

amount of the antifungal citral and thus stimulates decay development 

(Ben-Yehoshua et al., 1995). Brown, G.E. and Barmore (1977) showed 

that anthracnose development in tangerines infected with Colletotrichum 

gloeosporioides occurred only when the fruits had been exposed to 

ethylene immediately after inoculation. In this case, disease induction by 

ethylene probably resulted from the stimulation of the infection hyphae 

production by the appressoria found on the fruit exposed to ethylene, and 

their penetration into the epidermis cells. 

Flaishman and Kolattukudy (1994) reported that ethylene treatment 

at concentrations much lower than those produced during ripening of 

climacteric fruits, was capable of inducing both conidia germination and 

appressoria formation in Colletotrichum musae and C. gloeosporioides, 

and that endogenic ethylene, produced in climacteric fruits during 

ripening, can serve as a signal for the termination of the appressoria 

latency on the fruit. The induction of appressoria formation does not take 

place in Colletotrichum species attacking non-climacteric fruits, such as 

citrus fruits. When spores of this fungus come into contact with citrus 

fruits, which normally do not produce a considerable amount of ethylene 

after harvest, appressoria will not be produced in large quantities, and 

the fungus will not cause a serious decay. However, when such a fruit is 

exposed to ethylene during degreening, an abundance of spores is 

produced and a massive infection of the fruit takes place, as was 

previously reported for ethylene-treated citrus fruits (Brown, G.E., 1975). 

In a later study, however, Prusky et al. (1996) differentiated between 

the effect of ethylene on the climacteric process and ripening of avocado 

fruits, on the one hand, and, on the other hand, the process of initiation of 

lesion growth from quiescent appressoria of C. gloeosporioides. They found 

that ethylene treatment at 45 |il l-i, applied to immature avocado fruits. 




enhanced appressorium formation and induced an early climacteric, but 

did not trigger earlier lesion development: progress in lesion development 

in ethylene-treated fruits was similar to that in untreated fruits (Fig. 11). 

It was concluded that exposure of avocado fruits to exogenous ethylene did 

induce multiple appressorium formation but did not activate disease 

development. The activation of appressoria during avocado ripening may 

rather be related to the decline in the antifungal diene in the peel, to 

subfungitoxic concentrations (Prusky et al., 1982). 

Ethylene application also stimulates Botrytis cinerea development in 

strawberries (El-Kazzaz et al., 1983), and enhances J3. cinerea and 

Alternaria alternata development in tomatoes (Barkai-Golan et al., 

1989a; Segall et al., 1974). For strawberries, which are non-climacteric 

fruits and are, therefore, regarded as independent of ethylene for 

ripening, it was found that storage life was extended by reducing the 

ethylene level (Wills and Kim, 1995). 

4 6 


Fig. 11. Effect of ethylene on decay development caused by Colletotrichum 

gloeosporioides (A) and changes in fruit firmness (•) in overmature inoculated 

avocado fruits (cv. Reed) as compared to decay (A) and firmness (o) in air 

treated fruit. Fruit firmness was determined by a penetrometer and expressed 

by N(=Newton, a parameter of fruit ripening). (Reproduced from Prusky et al., 

1996 with permission of the American Phytopathological Society). 




Relatively high levels of ethylene, that were found to accumulate in 

strawberry punnets overwrapped with polyethylene during marketing, 

were suggested to be the contributing cause to enhanced postharvest 

loss. The addition of an absorbent containing potassium permanganate 

was found to be an effective means of reducing ethylene in strawberry 

punnets during marketing, at ambient or reduced temperatures, with a 

corresponding increase in storage life. The potassium permanganatetreated 

fruit was less susceptible to B. cinerea, which is the main cause 

for deterioration at 20°C (Wills and Kim, 1995). With regard to the effect 

of ethylene on Botrytis in tomatoes, this gas was found directly to 

stimulate germ-tube elongation of fungal spores when introduced into 

tomato fruit tissue; but when the fruit is exposed to ethylene prior to its 

inoculation by the fungus, the gas may stimulate the pathogen 

development indirectly because of the stimulation of fruit ripening 

(Barkai-Golan et al., 1989a). Indirect stimulation of fungal growth occurs 

in normal tomato fruits at the mature-green stage, on which exogenous 

ethylene can normally act, but will not occur in non-ripening tomato 

mutants (nor) in which ripening cannot be induced by ethylene 

application (Barkai-Golan et al., 1989b). Exposure to ethylene also 

resulted in the stimulation of ripening in young mango fruits. Along with 

fruit ripening, the concentration of the antifungal resorcinol compounds 

was decreased and the sensitivity of the fruit to A. alternata was 

enhanced (Droby et al., 1986). 

In contrast to cases in which exposure to ethylene stimulates disease 

development, in other cases it does not affect disease development, and 

may even enhance host resistance to pathogen attack. Induction of 

resistance to decay by ethylene was reported for sweet potatoes infected 

by Ceratocystis fimbriata (Stahmann et al., 1966), and tangerines 

exposed to ethylene for 3 days prior to their inoculation with C. 

gloeosporioides (Brown, G.E. and Barmore, 1977). 

E. HOST-PATHOGEN INTERACTIONS 

A pathogen that has reached a suitable host and also finds conditions 

suitable for germination, for penetration into the host, and for 

establishment within its tissues, will develop and cause a disease only if 

it succeeds in withstanding the protective means of the host. In other 

words: the struggle between the pathogen attack capability and the 




defense capability of the host will dictate the success or failure of the 

infection. The attack mechanisms of the pathogen and the defense 

mechanisms of the host are summed up in Table 3. 

TABLES 

Attack mechanisms of the pathogen and defense 

mechanisms of the host 

Attack mechanisms of the pathogen: 

1. The ability to produce cuticle- and cell wall-degrading enzymes. 

2. The ability to produce toxins causing cell death. 

3. The ability to detoxify resistance compounds in the host. 

4. The ability to develop on the fruit and vegetable surfaces, and within 

their tissues. 

Defense mechanisms of the host: 

1. The ability of the intact cuticle to provide a barrier to fungal 

penetration; its ability to prevent diffusion of cellular solutions, so 

limiting water and nutrient availability on the plant surface. 

2. The resistance of the young fruit and vegetable cell wall to 

degradation by the pathogen enzymes. 

3. The presence of preformed (constitutive) compounds inhibiting 

pathogen growth and enzymatic activity, and the ability to further 

induce, or enhance their production. 

4. The ability to produce low molecular-weight secondary metabolites 

with antimicrobial activities, phytoalexins, in response to fungal 

penetration or other stress conditions. 

5. Wound healing and the formation of barriers against the progress of 

the pathogen and its enzymatic activity; reinforcement of plant cell 

walls by formation of glycoproteins, callose, lignin and other phenolic 

polymers; the formation of papillae and plugging of intercellular 

spaces. 




6. The hypersensitive response, which is thought to function by 

isolating and kiUing microorganisms in a small region of the plant 

tissue by localized cell death. 

7. The induction of host resistance by the formation of 

pathogenesis-related proteins, such as chitinase and P-1,3 glucanase, 

in pathological situations or in response to other stresses. 

8. The production of active oxygen in response to pathogens and 

elicitors; this oxygen has an antimicrobial effect and plays a role in 

various defense mechanisms. 

Attack mechanisms of the pathogen and defense mechanisms of the 

host will be discussed separately in the following chapters. 



CHAPTER 5 

ATTACK MECHANISMS OF THE PATHOGEN 

A. ENZYMATIC ACTIVITY 

Many plant pathogens produce extracellular products that may 

influence their growth, be determinant factors in their pathogenic 

capability, or contribute to their pathogenicity or virulence. Such 

products may include enzymes capable of hydrolyzing the cuticle layer of 

the epidermic plant cells (Kolattukudy, 1985), cell wall degrading 

enzymes, toxins, hormones, siderophores, DNA and signaling molecules 

(Salmond, 1994), or factors capable of degrading host chemical defense 

agents (Staples and Mayer, 1995). We will concentrate on the secretion of 

extracellular enzymes and toxins, the two major attack means with 

which the pathogen may be equipped, and the ability of the pathogen to 

detoxify host defense compounds. 

Cutinase 

The enzyme cutinase, which hydrolyzes the primary alcohol ester 

linkages of the polymer, cutin (Kolattukudy, 1985), has been found in 

many plant pathogenic fungi, including Botrytis cinerea (Shishiyama et 

al., 1970). However, the importance of this enzyme for penetration of the 

cuticle seems to be a matter of debate (Stahl and Schafer, 1992). Many 

lines of evidence suggest that cuticular penetration of Colletotrichum 

gloeosporioides into the host is associated with the induction of 

extracellular cutinase production, and that insertion of the cutinase gene 

into this pathogen facilitates infection of an intact host (Dickman et al., 

1989). Following the finding that cutinase secretion was also directed 

towards the region that penetrates the host, the infection peg of an 

appressorium produced by Colletotrichum spores (Podilia et al., 1995), it 

was suggested that the inhibition of this enzyme by active inhibitors 

could result in delay in the penetration of germinating appressoria into 

the host (Prusky, 1996). 

Secretion of extracellular cutinase has been reported for Aspergillus 

flavus (Guo et al., 1996) when grown on purified cutin as the sole carbon 

source. Pretreatment of corn kernels with A flavus culture filtrate or 

with bacterial cutinase promoted increased levels of aflatoxin production 



Attack Mechanisms of the Pathogen 55 

typical of this fungus in wounded kernels. Furthermore, cutinase activity 

was strongly inhibited by an inhibitor specific to fungal cutinase, 

supporting the suggestion that cutinase may have a role in the 

pathogenicity of A flavus. 

In a recent study, the involvement of phenolic acids, which are found 

in the peach fruit surface, in disease resistance to Monilinia fructicola, 

the fungal cause of brown rot, has been related to their effect on cutinase 

production by the fungus (Bostock et al., 1999). Chlorogenic and caffeic 

acids, the major phenolic acids in the epidermis and subtending cell 

layers of peach fruits, were found in higher concentrations in a peach 

genotype with a high level of resistance; they declined as the fruit 

matured, with a corresponding increase in disease susceptibility. These 

phenolic compounds, however, did not show any suppressive effects on 

fungal development. On the other hand, in cultures amended with either 

of these phenolic acids, the cutinase activity of the fungus was markedly 

reduced. It was, therefore, suggested that the relationship between the 

high concentration of the phenolic acid in the immature peach fruit and 

the resistance of this fruit to infection may be because they can interfere 

with the capability of the pathogen to degrade the host polymers. (See 

also the chapter on Host Protection and Defense Mechanisms - 

Preformed Inhibitory Compounds.) 

Pectolytic and Cellulolytic Enzymes 

The pectic substances, which constitute an important component of the 

primary cell wall of plants, and form the major component in the middle 

lamella (Bateman and Basham, 1976), are responsible for the tight link 

between the cells and the integrity of the plant tissue. These substances 

are composed mainly of a high-molecular weight polymer comprising 

linear chains of D-galacturonic acid (1,4-a-galacturonic acid) which 

underwent various degrees of methylation, combined with side chains of 

neutral sugars (Bateman and Basham, 1976). Non-methylated chains are 

termed pectic acid; chains with about 75% or more of the units 

methylated are termed pectin, while chains with smaller percentages of 

the galacturonic acid units methylated are referred to as pectinic acid. 

The development of disease within the harvested fruit or vegetable is 

dependent, to a great extent, on the ability of the pathogen to secrete 

pectolytic enzymes, which are capable of decomposing the non-soluble 

pectic compounds and so causing cell separation and tissue 

disintegration. This process leads to enhanced permeability of the 

plasma membranes of attacked cells and to cell death, and facilitates the 




diffusion of nutrients which can serve as a medium for pathogen 

development (Mount, 1978). 

Two groups of pectolytic enzymes take part in the disintegration of the 

pectic substances. The first group includes pectin methylesterase (PME), 

which breaks the ester bonds and separates the methyl groups from the 

carboxylic groups of the pectin or pectinic acid, to yield pectic acid and 

methanol. Enzymes of this group do not break the pectin chain. The 

second group includes pectolytic enzymes that break the 1,4-glycoside 

bonds between the galacturonic acid subunits. The glycoside bonds may 

be broken by the hydrolytic activity of polygalacturonases (PG), which 

act on pectic acid chains, or by that of pectin methylgalacturonases 

(PMG), which act on chains of pectin or pectinic acid. The glycosidic 

bonds may also be broken by the action of pectin or pectate lyase (PL), 

also called transeliminase (PTE). These enzymes cleave the 1-4-glycoside 

linkages by unsaturating the ring between the fourth and the fifth 

carbon atoms in the subunits of the chain (Bateman and Millar, 1966). 

Thus, both the hydrolytic enzymes and the lyases attack the 

galacturonide links of the polymeric chain and cause it to break, but they 

differ from one another in their mode of action: the polygalacturonases 

act hydrolytically, while the lyases function in a lytic way by 

transelimination. 

According to the point of attack in the pectic compound, the 

polygalacturonases and the lyases are classified as endo-enzymes, that 

break the galacturonide connections at random while producing 

oligogalacturonides, or as exo-enzymes, which attack only the terminal 

linkages. Disintegration and softening of the tissues (maceration), 

resulting in the formation of soft to watery rot caused by species of 

Rhizopus, Penicillium, Geotrichum and Sclerotinia, and by the bacterium 

Erwinia carotovora in many fruits and vegetables during storage, is 

generally related to the extracellular activity of endo-PG (Rombouts and 

Pilnik, 1972). However, the involvement of exo-PG in Penicillium 

digitatum pathogenesis in citrus fruits has been reported (Barash and 

Angel, 1970; Barmore and Brown, 1979). Enzymes from the 

endo-pectin-lyase group have been described as the main cause of the 

maceration of citrus fruits infected by Penicillium italicum or P. 

digitatum (Bush and Codner, 1970). However, in more recent studies the 

enzymes causing maceration of citrus fruit peel by these fungi were 

identified by Barmore and Brown (1979, 1980) as endo-PG and exo-PG, 

respectively. 




The virulence of a pathogen has been related to the total PG activity or 

to specific PG isozymes. The production of PG isozymes may be 

influenced by various factors, such as growth conditions of the pathogen, 

nutrients and fungal strains (Cleveland and Cotty, 1991; Tobias et al., 

1993). Recently an increase in total PG activity and in that of PG 

isozymes has also been correlated with reactivation of latent infections 

and the beginning of tissue maceration by the reactivated pathogen 

(Zhang et al., 1997). 

Endo-pectate lyase, which attacks the pectic acid at random, is 

produced by various pectolytic bacteria and is probably the major cause 

of cell wall decomposition of potatoes infected by these bacteria (Hall and 

Wood, 1973), but enzymes of the endo-PG group may also be involved in 

tuber maceration (Beraha et al., 1974). The involvement of pectate lyase 

(PL) in fungal decay has been described for anthracnose (Colletotrichum 

gloeosporioides) in stored avocados (Wattad et al., 1994); PL was found to 

cause softening in unripe avocado fruits, whereas the addition of 

antibodies to this enzyme inhibited tissue maceration. 

Saprophytic fungi and bacteria, which are not included in the 

postharvest pathogens' category, may also produce pectolytic enzymes on 

various substrates; some of these are capable of macerating discs of 

potato tuber or of other plant organs (Ishii, 1977). Therefore, the fact 

that a given microorganism can produce pectolytic enzymes in vitro 

cannot serve, by itself, as evidence of pathogenic capability. To determine 

the capability of a pathogen to decompose the pectic compounds of plant 

cell walls, its pectolytic activity should be examined in the host tissues 

themselves. 

Simmonds (1963) suggested that the failure of the pathogen to produce 

adequate levels of pectolytic enzymes in the host may be related to the 

establishment of infections that remain quiescent until the ripening 

process leads to suitable changes in the cell wall structure. An 

inadequate level of pectolytic activity in the plant tissue may be caused 

by several factors (Swinburne, 1983): (1) Pectolytic enzymes are not all 

constitutive and they may need inducers for their production (Bateman 

and Basham, 1976); the inducing substrates could be supplied to the 

pathogen only as the fruit ripens. (2) Even if such enzymes were 

produced, access to the labile sites within the wall could be blocked by 

cation cross-linking. (3) Enzymes may be inactivated by inhibitors 

present at higher levels in the immature fruit than in the mature one 




(see the chapter on Host Protection and Defense Mechanisms - 

Inhibitors of Cell Wall Degrading Enzymes). 

Many fungi are capable of utilizing cellulose for their growth, when it 

is present as the sole source of carbon in the culture medium, thanks to 

their ability to produce and secrete cellulases. Cellulose, which is a linear 

polymer of D-glucose, the configuration being that of a 1,4-P-glucoside, is 

the principal polysaccharide of plants, being an important component of 

the plant cell wall, mainly of the secondary wall. Cellulose is by far the 

largest reservoir of biologically utilizable carbon on the surface of the 

earth, and its decomposition, in which fungi probably play a major role, 

is of great ecological importance. Most fungal cellulases are induced 

enzymes, the inducing substrate being cellulose in the medium. However, 

various microorganisms, such as the fungi, R, stolonifer, Penicillium 

digitatum and P. italicum, produce and secrete cellulase independently of 

the presence of cellulose (Spalding, 1963; Barkai-Golan and Karadavid, 

1992). 

Although plant tissue decomposition and cell death during disease 

development are strongly connected with the activity of extracellular 

pectolytic enzymes, the pathogen cellulases, as well as hemicellulases 

(the enzymes responsible for breakdown of hemicelluloses - plant cell 

wall constituents, particularly polysaccharides and polyuronides), may 

also play a role in this complex process. The description of the 

involvement of cellulases in pathogenesis has generally addressed the 

late stages, the saprophytic stages of pathogen development (Bateman 

and Basham, 1976). However, studies of the Cx-cellulase (the enzyme 

splitting the polymeric chain of cellulose to form cellobiose residues) of P. 

digitatum and P. italicum in citrus fruits, revealed high levels of 

enzymatic activity during the incubation period of the disease, before the 

appearance of disease symptoms; moreover, a correlation was found 

between cellulase activity of the pathogen in the fruit and the severity of 

the disease symptoms (Barkai-Golan and Karadavid, 1992). These 

findings led to the suggestion that the cellulolytic enzymes of the two 

Penicillia may play an active role in the early stages of pathogenesis, 

including the penetration of the fungus into the tissue. In tomato 

genotypes a correlation was drawn between the level of cellulolytic 

activity in the fruit prior to infection and the extent of cellulolytic activity 

of Alternaria alternata in infected fruit tissues. Thus, the cellulolytic 

activity of the pathogen in the ripening normal fruit was higher than 

those in the non-ripening nor mutant and its hybrid at each stage of 




maturity. In fact, the enzymatic levels recorded for the infected mutant 

and hybrid fruits at their mature stage were similar to those recorded for 

the infected ripening normal fruit at its mature-green stage 

(Barkai-Golan and Kopeliovitch, 1989). 

B. TOXIN PRODUCTION 

Some pathogenic fungi secrete toxic compounds into the plant tissues, 

inflicting damage on them. These compounds are frequently called 

'phytotoxins'; however, this term may be misleading since it is sometimes 

used to designate toxins originating in higher plants. 

Toxins are pathogen-produced metabolites, most of which are 

low-molecular-weight compounds that cause histological and 

physiological changes in the host (Knoche and Duvick, 1987). They may 

be the primary cause of the disease induced by the pathogen or they may 

constitute only a part of the disease process (Scheffer, 1983; Walton and 

Panaccione, 1993). Toxins that have an injurious effect only on plant 

species or cultivars that serve as hosts of the toxin producing pathogen 

are termed host-specific or host-selective toxins. Conversely, toxins that 

affect a wider range of plant species, including both host and non-hosts of 

the pathogen, are termed non-host-specific or non-host-selective toxins 

(Rudolph, 1976; Mitchell, 1984). These toxins are not necessarily related 

to disease initiation. Pathogen-produced toxins can also be classified, 

according to Yoder (1980), as bearers of a 'pathogenicity factor', a factor 

essential for a pathogen to cause disease, and those bearing a 'virulence 

factor', that only enhances the extent of disease. Toxins that are 

considered as virulence factors turn out to be the non-host-specific toxins, 

whereas those considered as pathogenicity factors are generally 

host-specific toxins (Mitchell, 1984). 

Host-Specific Toxins 

Host-specific toxins were for a long time considered to be the only 

agents of specificity in plant/microbe interaction (Walton and 

Panaccione, 1993). They represent widely differing chemical categories, 

including peptides, glycosides, esters and other compounds (Macko, 

1983), which function as a chemical interface between pathogens and 

their host plants. However, in order to study the biosynthesis and the 

role of the toxins in pathogenesis, structural information is needed. In 




other words: the structure of a toxin yields clues about the biosynthetic 

process and the mechanism of action. 

Among the microorganisms producing host-specific toxins, we find 

pathogens that survive saprophytically when they lose their capacity to 

produce the toxin (Macko 1983; Scheffer 1983). These include various 

species of Helminthosporium and Alternaria, as well as other fungal 

species not related to postharvest diseases. Among the Helminthosporium 

species we find: H, sacchari, the causal fungus of eyespot disease of 

sugarcane, which is the producer of HS-toxins; H. victoriae, which 

attacks oats and produces HV-toxin; H, maydis, the causal fungus of 

specific blighting on leaves of sensitive corn, which is the producer of 

T-toxin; and H. carbonum, which attacks corn and produces HC-toxin. 

The corn fungus Phyllosticta maydis produces PM-toxin, a host-specific 

toxin similar to but not identical to T-toxin. The pathogenic species of 

Alternaria in this class were identified mainly on the basis of their 

specific host. They include: Alternaria alternata f. sp. kikuchiana 

(Alternaria kikuchiana), the causal fungus of black spot disease of the 

Japanese pear, which produces AK-toxin; Alternaria alternata f. sp. 

lycopersici, the causal fungus of stem canker disease of the tomato, which 

produces AL-toxin; A. alternata f. sp. mali (Alternaria mali), which 

attacks leaves, shoots and fruits of apples and produces AM-toxin; A. 

alternata f. sp. citri (Alternaria citri), which attacks rough lemon fruits 

and tangerines and produces AC-toxins; and A, alternata f. sp. fragariae 

(Alternaria fragariae), which attacks strawberries and produces 

AF-toxin. A toxin released from germinating spores of Alternaria 

tenuissima, causing leaf spot of pigeon pea in the field, has also been 

found to have host-specific properties (Nutsugah et al., 1994). Each of 

these fungi has its specific toxin that may be responsible for the selective 

pathogenicity of the fungus. 

According to Nishimura and Kohomoto (1983), the host range of the 

pathogenic strains of Alternaria is limited to cultivars that are 

susceptible to their host-specific toxins, while other cultivars are highly 

resistant. However, as reviewed by Knoche and Duvick (1987), such a 

definition might be too sharp, since AM-, AK-, and AF-toxins were able to 

exert a slight effect on hosts other than their specific hosts. Most of the 

Alternaria species or subspecies are similar morphologically and are 

classified today in the extended group of Alternaria alternata, being 

considered as pathotypes of this fungal species. Although these fungi are 

generally not included among the common postharvest pathogens, they 




belong to the same group that includes the weak pathogens which attack 

various harvested fruits and vegetables via wounds, following 

physiological damage, or through aging tissues, without being selective 

to a specific host. 

Nishimura and Kohomoto (1983) related the origin of the host-specific 

toxins of A, alternata to the wild strains of A. alternata. Since the 

transition from a non-specific Alternaria type to a host-specific type is 

associated with the production of host-specific toxins, it might be 

assumed that a wild population of the non-host-specific A, alternata 

initially contained toxin producers which have apparently arisen by 

mutation. Mutation to toxin production probably affected an extremely 

minute proportion of the field population of A, alternata before the 

introduction of susceptible plants. The introduction of new susceptible 

host genotypes might have led to multiplication of the toxin-producing 

mutants (Nishimura and Kohomoto, 1983). Monocultures of the 

susceptible plants act as a selection medium and increase the proportion 

of toxic mutants. If, on the other hand, there is no chance to meet with 

uniformly genetically susceptible genotypes, the property of toxin 

production will not have a chance to appear and will soon be lost. 

Non-Host-Specific Toxins 

In contrast to the host-specific toxins, many fungi produce non-specific 

toxins, which are toxic to hosts and non-hosts of the pathogen, and may 

attack many different plants (Rudolph, 1976). A wide range of 

non-host-selective toxins of fungal and bacterial origin have been 

described and identified. Some of them seem to be necessary for 

successful infection of the host, whereas others play only a secondary 

role. Also, many non-selective toxins are metabolites that are toxic in 

bioassays, although this property may be unrelated to disease 

development. For many of the non-specific toxins, the toxicity to plants is 

just another aspect of pathogenesis, acting in addition to penetration 

mechanisms, enzymatic activity, recognition capability and other factors. 

In spite of the damage caused by the non-specific toxins to the plant 

tissues, the relationship between their synthesis and disease inducement 

has not yet been proven. However, even if these toxins are not necessary 

for disease initiation, they may enhance the pathogen virulence by 

causing phytotoxic phenomena in the attacked tissues (Mitchell, 1984). 

Rudolph (1976) lists several criteria to be investigated before deciding 

upon the pathogenic role of a phytotoxic compound. Among these are: 




(1) The inducement of disease symptoms by the isolated toxin; 

(2) Demonstration of the presence of the toxin within the diseased 

host; 

(3) Demonstration of a correlation between the rate of toxin 

production in vitro and the pathogenicity of the isolate within the 

host; 

(4) A correlation between disease susceptibility and toxin sensitivity. 

Although this is the main criterion for host-specific toxins, it may 

also be applicable to some non-host-specific toxins at the cultivar 

level, e.g., tenuazonic acid from Alternaria alternata or the raw 

toxin solution from Fusarium moniliforme; 

(5) Indications of the role of toxins during pathogenesis can be 

obtained when their activity in vivo is inhibited within the host or 

by specific environmental conditions, and these manipulations 

result in suppression of disease development. 

Examples of non-host-specific fungal toxins that have a role in disease 

development, or have been suggested to be involved in disease, are 

produced by fungi that may be responsible for postharvest diseases. They 

include: fumaric acid, produced by Rhizopus species; oxalic acid, 

produced by Sclerotium rolfsii and Sclerotinia sclerotiorum; ophiobolins 

produced by Helminthosporium species; fusaric acid produced by 

Fusarium oxysporum; malformin produced by Aspergillus niger; 

coUetotrichins produced by Colletotrichum species; and tentoxin 

produced by A alternata f. tenuis (Macko, 1983; Scheffer, 1983). In fact, 

most Alternaria species may produce non-host-specific toxins, which are 

not prerequisite for infection. 

A. alternata, one of the most common postharvest pathogens of various 

fruits and vegetables (Barkai-Golan, 1981), has been reported to produce 

several toxic compounds in harvested fruits. Stinson et al. (1981) 

reported that the major toxin produced in tomato fruits was tenuazonic 

acid, with a maximal concentration in tomato tissue of 13.9mg/100g; 

alternariol, alternariol monomethyl ether, and altenuene were produced 

in much smaller amounts. The major toxins recorded in apples were 

alternariol and alternariol monomethyl ether, with maximum 

concentrations in tissue of 5.8 and 0.23mg/100g, respectively. Inoculation 

of oranges and lemons with the same fungal isolate showed that the 

predominant toxins in these hosts were tenuazonic acid, alternariol 

monomethyl ether and alternariol, with concentrations in tissue of only 

2.66, 1,31 and 1.15mg/100g, respectively. Working with another strain of 




A, alternata, Ozcelik et al. (1990) found that concentrations as high as 

120.6mg of alternariol and 63.7mg of alternariol methyl ether per lOOg 

tissue were recorded in inoculated tomatoes after 4 weeks at 15°C. The 

highest in-tissue concentration of altenuene (19mg/100g) was found after 

3 weeks at 25^C. However, tenuazonic acid was not detected in 

inoculated tomatoes, regardless of the storage temperature or type of 

packaging. Inoculations of Red Delicious apples indicated that alternariol 

was also the predominant toxin in this fruit, although its in-tissue 

concentrations were lower than those produced in tomatoes (e.g., 

49.8mg/100g at 25°C); relatively low concentrations were recorded for the 

other two toxins, while tenuazonic acid was not detected in infected 

apples (Ozcelik et al., 1990). Alternariol, alternariol methyl ether and 

tenuazonic acid were recorded in A. alternata-infected peppers (Bottalico 

et al., 1989). 

In addition to their phytotoxic effects, non-specific toxins may also 

function as mycotoxins - toxic to both humans and animals - as 

antibiotic substances, as plant growth regulators, etc. Examples of 

mycotoxins that also function as phytotoxins (Scheffer, 1983) are: 

aflatoxins, produced by Aspergillus flavus and related fungi; citrinin, 

patulin and penicillic acid, which are produced mainly by species of 

Penicillium and Aspergillus; moniliformin and fumonisins produced by 

strains of Fusarium moniliforme; and trichothecenes produced by various 

fungi, such as species of Fusarium, Cephalosporium, Myrothecium, 

Trichoderma and Stachybotrys. In addition to their mycotoxic effects on 

humans and animals and their toxic effects on plants and plant organs, 

the trichothecenes may also have insecticidal, antifungal, antibacterial, 

antiviral, and antileukemic effects (Macko, 1983). 

Of special interest to postharvest pathology is the production of 

patulin by Penicillium expansum in pome and stone fruits. The amount 

of patulin produced by P, expansum may vary greatly according to the 

strain involved. Sommer et al. (1974) found that patulin production by 

different strains of P. expansum in Golden Delicious apples ranged from 

2 to 100 |xg/per gram of tissue. On the other hand, various apple and pear 

cultivars may differ in their sensitivity to the same fungal strains. The 

storage temperature is another factor affecting patulin production in a 

given fruit cultivar. Paster et al. (1995) demonstrated that while more 

patulin was produced in Starking apples than in Spadona pears held at 

0-17°C, higher toxin levels were produced in pears than in apples held at 




25°C. However, in spite of the differences in the amounts of toxin 

produced, no significant differences in pathogenicity were recorded 

among fungal strains, as determined by lesion diameter on each of the 

fruits. In other words, no correlation was found between patulin 

production and disease development. Even strains that did not produce 

patulin at 0° and 25°C infected the fruits to the same degree as strains 

capable of growth and patulin production at these temperatures. 

Furthermore, patulin production was totally inhibited when inoculated 

apples were held in a 3%C02/2%02 atmosphere (25''C), although the level 

of infection under the controlled atmosphere conditions was fairly high, 

reaching 70% of that of the control. 

Non-host-specific toxins are generally acknowledged to be an element 

of virulence for many bacteria as well. Most of the known bacterial toxins 

are produced by various pathovars of Pseudomonas syringae (Scheffer, 

1983; Gross, 1991). These toxins constitute a family of structurally 

diverse compounds, usually peptide in nature, that, in some cases, 

display wide-spectrum antibiotic activity. Progress has been made in 

identifsdng gene clusters associated with toxin production, and there is 

some evidence that toxin genes may be activated in response to specific 

plant signals (Gross, 1991). 

C. DETOXIFICATION OF HOST DEFENSE COMPOUNDS 

BY PATHOGENS 

In addition to the activities of cell-wall-degrading enzymes and toxins 

of the pathogen, the ability to degrade plant chemical defense 

compounds, such as the tomato a-tomatine, has also been discussed as a 

potential pathogenicity determinant (Staples and Mayer, 1995). 

The saponin, a-tomatine is a steroidal glyco-alkaloid that occurs in 

many plant species and provides a preformed barrier to fungal invasion 

(Verhoeff and Liem, 1975; Osbourn, 1996). It is found in all parts of 

tomato plants but is especially plentiful in the peel of green tomato fruits 

(see the chapter on Host Protection and Defense Mechanisms - 

Preformed Inhibitory Compounds). The toxicity of a-tomatine to fungi is 

due to its ability to interact with membrane sterols, and thus cause 

membrane leakage (Fenwick et al., 1992; Quidde et al., 1998). 

Tomato pathogens have developed two primary strategies to overcome 

this chemical defense (Osbourn et al., 1994): either they are resistant to 




a-tomatine by virtue of their membrane composition or they are capable 

of detoxifsdng a-tomatine. Detoxification of a-tomatine has been reported 

for various fungi, such as Fusarium oxysporum f.sp. lycopersici (Ford et 

al., 1977), Alternaria solani (Schlosser, 1975), Alternaria alternata 

(Sandrock and VanEtten, 1997), Septoria lycopersici (Arneson and 

Durbin, 1967), and Botrytis cinerea (Verhoeff and Liem, 1975). 

In a recent study, Quidde et al. (1998) showed that B. cinerea 

metabolizes a-tomatine by removal of the terminal D-xylose, to yield 

Pi-tomatine, and not by deglycosylating a-tomatine completely, as was 

previously suggested (Verhoeff and Liem, 1975). It was concluded that 

the tomatinase of B, cinerea is a p-xylosidase. Using a tomatinasedeficient 

strain of S. cinerea, Quidde et al. (1998) found that Pi-tomatine 

is far less toxic to axenic culture than a-tomatine, confirming that the 

removal of the xylose moiety represent a detoxification process. Most B. 

cinerea strains tested, originating from various host plants, were found 

capable of detoxifying a-tomatine in this way. It was thus suggested that 

a-tomatine degradation has a role in the interaction between B, cinerea 

and tomato. 



CHAPTER 6 

HOST PROTECTION AND DEFENSE MECHANISMS 

A. THE CUTICLE AS A BARRIER AGAINST INVASION 

The cuticle provides a barrier to pathogen penetration. Wounding and 

treatments that disrupt or dissolve the cuticle result in more rapid 

infection by various pathogens (Elad and Evensen, 1995). The cuticle 

may, however, function not only as a physical barrier but also as a 

chemical barrier, since it may contain substances antagonistic to fungi 

(Martin, 1964). The cuticle thickness has been correlated with the 

resistance of tomato fruit to Botrytis cinerea (Rijkenberg et al., 1980), or 

of peach cultivars to Monilinia fructicola (Adaskaveg et al., 1989, 1991). 

Biggs and Northover (1989) showed that the cuticle and cell-wall 

thicknesses were correlated with longer incubation periods of the brown 

rot in cherry fruits, accompanied by a lower incidence of infection. Elad 

and Evenson (1995) listed several ways by which a thicker cuticle might 

enhance host resistance: (a) by being more resistant to cracking and thus 

to penetration of wound pathogens (Coley-Smith et al., 1980); (b) by 

providing mechanical resistance to penetration (Howard et al., 1991); 

(c) by presenting a greater quantity of substrate to be degraded by fungal 

enzymes, and preventing diffusion of cellular solutions, thus limiting the 

access of water and nutrients to the microenvironment where they are 

required for spore germination and the infection process (Martin, 1964). 

Studies with various genotypes of peaches clearly showed that 

genotypes resistant to brown rot caused by Monilinia fructicola had a 

thicker cuticle and denser epidermis than susceptible genotypes. Their 

resistance was correlated with both a delay in fungal penetration and a 

longer incubation period for infection initiation, and resulted in the 

extension of the quiescent period of the pathogen (Adaskaveg et al., 1989, 

1991). 

B. INHIBITORS OF CELL WALL-DEGRADING ENZYMES 

Pathogen development within the host is correlated in many cases 

with the activity of cell wall-degrading enzymes, which are responsible 



Host Protection and Defense Mechanisms 67 

for cell death and the liberation of nutrients, which then become 

available to the pathogen. Liberation of nutrients results in a stimulation 

of pathogen growth and accelerated disease development. This may 

explain the great importance attributed to compounds that are capable of 

suppressing or preventing enzymatic activity. 

Trials with various pathogens have emphasized the point that sugars 

in the culture medium, which serve as available nutrients for the 

pathogen and stimulate its growth, may inhibit pectolytic and cellulolytic 

enzyme production and activity (Spalding et al., 1973; Biehn and 

Dimond, 1973; Bahkali, 1995). It was found that the presence of glucose 

in the culture medium, either as a sole source of carbon or in addition to 

malic or citric acid, inhibited pectin lyase activity of Penicillium 

expansum, while resulting in enhanced fungal growth (Spalding and 

Abdul-Baki, 1973) 

Polyphenols and tannins have been described by Byrde et al. (1973) as 

inhibitors of polygalacturonase (PG) activity of Sclerotinia fructicola 

(Monilinia fructicola) in apples and other pathogen/host combinations, 

although they have no effect on the pathogen itself. Decay development 

in many fruits and vegetables may be the result of the delicate balance 

between active production of enzymes and their inhibition by 

polyphenols, mainly in their oxidized condition, as a result of 

polyphenol-oxidase activity (Dennis, 1987). 

Another class of inhibitors of cell wall-degrading enzymes comprises 

the PG-inhibitory proteins present in both infected and uninfected plant 

tissue (Albersheim and Anderson, 1971; Abu-Goukh et al., 1983). A 

PG-inhibiting protein from a given plant source may act on pectic 

enzymes produced by different fungal species, both pathogenic and 

non-pathogenic, or on various PG isozymes from the same fungus. 

Research carried out with pepper fruits has shown that cell wall proteins 

of the host inhibited pectolytic enzyme production by Glomerella 

cinigulata, whereas the pectolytic activity of Botrytis cinerea was much 

less affected by these proteins (Brown, A.E. and Adikaram, 1983). The 

fact that B, cinerea can rot an immature pepper fruit whereas G. 

cingulata can attack only the ripened fruit, suggested that protein 

inhibitors might play a role in the quiescent infections of pepper fruits by 

Glomerella, 

Purified pear inhibitory proteins were found to inhibit various fungal 

PGs, including that of B, cinerea, but did not affect endogenous PG 

activity in pear fruits (Abu-Goukh and Labavitch, 1983). In a later study, 

Stotz et al. (1993) reported on the molecular characterization of the 




PG-inhibiting protein from pears, a step that may lead to the expression 

of inhibitory proteins in transgenic plants and contribute to the 

inhibition of decay development. Following the finding that ripening 

tomato fruits from transgenic plants expressing the pear inhibitory 

protein gene were more resistant to B. cinerea infection than the control 

fruits (Powell et al., 1994), PG-inhibiting proteins have been considered 

as pathogen infection resistance factors. 

A PG-inhibiting protein purified from mature apple fruits (Yao et al., 

1995) showed differential inhibitory activity against five PG isozymes 

purified from B, cinerea grown in liquid culture. However, inhibition was 

not detected against PG extracted from apple fruits inoculated with the 

same fungus; in contrast to the several isozymes isolated from fungal 

culture, only one PG was isolated from apple fruits inoculated with B. 

cinerea. The absence of multiple PG isozymes in inoculated fruits may 

suggest that the PG-inhibiting protein is involved in limiting the 

production of most PG isozymes during fungal infection. In other words, 

although the apple fruits did not show inhibitory activity against the PG 

isozyme isolated from inoculated fruits when assayed in vitro, this does 

not exclude the possibility that an apple inhibitor may contribute to the 

general resistance mechanism of the fruit when fighting fungal infection. 

Its application has even been considered as a possible alternative method 

for controlling postharvest diseases (Yao et al., 1995). A new protein 

inhibitor that may be involved in the inhibition of enzymes necessary for 

microbial development was isolated from cabbages (Lorito et al., 1994); it 

significantly inhibited the growth of B. cinerea by blocking chitin 

synthesis, so causing cytoplasmic leakage. Several studies supported the 

theory that natural protein compounds within the plant tissue may act 

as inhibitors of pathogen enzymes, and that these inhibitors may be 

responsible for the low levels of PG and PL found in infected tissue 

(Fielding, 1981; Barmore and Nguyen, 1985; Bugbee, 1993). 

Recent studies show a close correlation between the changes in the 

level of epicatechin in the peel of avocado fruits and the inhibition of 

pectolytic enzyme activity of Colletotrichum gloeosporioides. The 

epicatechin was found in unripe fruits at concentrations much higher 

than those required for in vitro inhibition of purified PG and pectate 

lyase (PL) produced by the fungus (Wattad et al., 1994). This led to the 

suggestion that epicatechin may contribute to the quiescence of the 

fungus by inhibiting the activity of pathogenicity factors. The importance 

of pectate lyase as a pathogenicity factor during the activation of 




quiescent infections has been demonstrated in two experiments: (a) 

antibodies produced against PL of C. gloeosporioides incubated with 

fungal spores, inhibited anthracnose development in ripe fruits; and (b) a 

nonsecreting PL mutant was found to be non-pathogenic to susceptible 

avocado, indicating that even when concentrations of antifungal 

compounds were reduced, PL production was required for infection 

(Wattad et al., 1995). 

C. PREFORMED INHIBITORY COMPOUNDS 

Phenolic compounds have long been implicated in disease resistance in 

many horticultural crops (Ndubizu, 1976). Some occur constitutively and 

are considered to function as preformed or passive inhibitors, while 

others are formed in response to the ingress of pathogens, and their 

appearance is considered as part of an active defense response (Kurosaki 

et al., 1986a; Nicholson and Hammerschmidt, 1992). 

Activity of Preformed Compounds 

Preformed resistance involves the presence in healthy tissues of 

biologically active, low-molecular-weight compounds whose activity 

affords protection against infection (Ingham, 1973). These are regarded 

as constitutive antimicrobial barriers (Schonbeck and Schlosser, 1976; 

Prusky, 1997). Phenolic compounds contribute to resistance through 

their antimicrobial properties, which elicit direct effects on the pathogen, 

or by affecting pathogenicity factors of the pathogen. However, they may 

also enhance resistance by contributing to the healing of wounds via 

lignification of cell walls around wound zones. Evidence strongly 

suggests that esterification of phenols to cell-wall materials is a common 

aspect of the expression of resistance (Friend, 1981). 

The antifungal properties of phenolic compounds and their derivatives, 

and the fact that these compounds are frequently found in the young 

fruit at concentrations higher than in the ripe fruit, led to the hypothesis 

that these compounds play an important role in the maintenance of 

latency in the unripe fruit. 

In vitro assays have shown that the phenolic compounds, chlorogenic 

acid and ferulic acid directly inhibited Fusarium oxysporum and 

Sclerotinia sclerotiorium, respectively. Benzoic acid derivatives have 

been shown to be the best inhibitors of some of the major postharvest 

pathogens, such as Alternaria spp., Botrytis cinerea, Penicillium 




digitatum, S. sclerotiorium and F, oxysporum (Lattanzio et al., 1995). 

The principal phenols in the peach fruit epidermis and subtending cell 

layers are chlorogenic and caffeic acids. The concentrations of these 

phenols decline as fruits mature, with a corresponding increase in fruit 

susceptibility to the brown rot fungus Monilinia fructicola. The levels of 

chlorogenic and caffeic acids are higher in peaches with a high level of 

resistance than in fruits of similar maturity, of more susceptible 

genotypes (Bostock et al., 1999). Studying the direct effects of chlorogenic 

and caffeic acids on M fructicola growth indicated that fungal spore 

germination or mycelial growth were not inhibited by concentrations 

similar to or exceeding those that occur in the tissue of immature, 

resistant fruit. Examination of the effects of phenolic compounds on other 

factors that may be involved in pathogenicity of the fungus showed that 

the presence of either of these phenolic acids in culture during fungal 

growth resulted in a sharp decrease in cutinase activity. It was further 

proposed that the high concentration of chlorogenic acid present in 

immature fruit and in fruit from highly resistant genotypes may 

contribute to the brown rot-resistance of the tissue by interference with 

the production of factors involved in the degradation of cutin, rather than 

by direct toxicity to the pathogen (Bostock et al., 1999). 

The saponin, a-tomatine, present in high concentrations in the peel of 

green tomatoes, is inhibitory to B, cinerea and other fungal pathogens, 

and its involvement in the development of quiescent infection has been 

suggested (Verhoeff and Liem, 1975). In this case, although no further 

development of fungal lesions occurred after ripening, tomatine was not 

detected in mature fruits (Table 4). 

TABLE 4 

Tomatine content of tomato fruits* 

Fruit tissue 

Green fruits (2-3 cm diameter) 

Skin of green fruits (2-3 cm in diameter) 

Red, ripe fruits 

Tomatine content 

% of dry 

weight 

0.5 

0.95 

none 

mg/100 g 

fresh weight 

43 

82 

none 

* Reproduced from Verhoeff and Liem, 1975 with permission of 

Blackwell Wissenschafts-Verlag GmbH. 




However, because of the marked antifungal activity of tomatine and 

other saponins, these preformed compounds are behoved to be involved 

in host resistance towards saponin-sensitive fungi. Tomatine presumably 

owes its toxic properties to its ability to bind to 3p-hydroxy sterols in 

fungal membranes (Steel and Drysdale, 1988). Most tomato pathogens, 

on the other hand, can specifically degrade tomatine and detoxify its 

effect through the activity of tomatinase (Osbourn et al., 1994). 

In a recent study, Sandrock and VanEtten (1998) examined 23 strains 

of fungi for their sensitivity to a-tomatine and to its two breakdown 

products, P2-tomatine and tomatidine. The two saprophytes and the five 

non-pathogens of tomato included in this study were found to be 

sensitive to a-tomatine, while all the other fungi tested, except for two 

tomato pathogens, were determined to be tolerant to a-tomatine. All the 

tomato pathogens tested except Phytophthora infestans and Pythium 

aphanidermatum were able to degrade a-tomatine. These included 

Alternaria alternata, the common postharvest pathogen of tomato fruit. 

A strong correlation was found between tolerance to a-tomatine, ability 

to degrade this compound, and pathogenicity to tomato. However, 

P2-tomatine and tomatidine, which were less toxic to most pathogens 

because of their inability to complex with membrane-bound SP-hydroxy 

sterols (Steel and Drysdale, 1988), were inhibitory to some of the 

non-pathogens of tomato, suggesting that tomato pathogens may have 

multiple tolerance mechanisms to a-tomatine (Sandrock and VanEtten, 

1998). 

Tannins in young banana fruits (Green and Morales, 1967) and 

benzylisothiocyanate in unripe papaya fruits (Patil et al., 1973) are 

additional examples of in-fruit toxic compounds. Since the concentration 

of these compounds decreases with fruit ripening, it has been considered 

that they have a role in resistance to decay in young fruits. 

Antifungal substances isolated from unripe avocado fruit peel include 

monoene and diene compounds, of which diene is the more important 

(Prusky and Keen, 1993). Resistance of unripe avocado fruits to 

postharvest pathogens has been suggested to be closely related to the 

presence in the peel of the antifungal diene compound (l-acetoxy-2 

hydroxy-4-oxo-heneicosa 12,15) (Prusky and Keen, 1993). This compound 

inhibits spore germination and mycelial growth of Colletotrichum 

gloeosporioideSy at concentrations lower than those present in the peel 

(see Fig. 3). The concentration of diene diminishes considerably as the 

fruit ripens and, in parallel, the dormant fungus in the peel starts its 

development (Prusky et al., 1982) (see Fig. 4). The decrease in the 




antifungal compound in the ripe fruit, which tends to decay, has been 

attributed to the enzymatic activity of hpoxygenase in the fruit. Several 

findings supported the suggestion that this enzyme is the cause for diene 

decomposition and increased sensitivity of the ripe fruit to decay (Prusky 

and Keen, 1993): 

(a) The enzyme activity increased considerably during ripening; 

(b) Partially purified lipoxygenase oxidized diene under in vitro 

conditions; 

(c) Treatment with tocopherol acetate, an antioxidant compound that 

non-specifically suppresses lipoxygenase, or with an enzymespecific 

inhibitor, delayed the reduction in diene concentration 

and inhibited the appearance of disease symptoms elicited by 

C gloeosporioides. 

The activity of lipoxygenase in ripe avocado fruits is affected by the 

activity of the epicatechin, a natural antioxidant present in the fruit peel. 

The concentration of this compound decreases during ripening thus 

allowing the activity of lipoxygenase to increase. A considerable 

reduction in the epicatechin concentration in sensitive avocado cultivars 

occurs along with the reduction in fruit firmness, and disease symptoms 

are expressed only when the concentration is reduced to the lowest levels 

(Prusky and Keen, 1993). Thus, epicatechin appears to play a key role in 

fruit susceptibility during ripening, by indirectly controlling the level of 

the antifungal compound. 

A mixture of resorcinols, found in the peel of unripe mango fruits, 

showed antifungal activity against A. alternata, the causal agent of the 

black spots in harvested fruit. The presence of this mixture was related 

to the latent stage of the fungus in young mango fruits (Droby et al., 

1986): during ripening, the resorcinol concentrations in the peel are 

reduced, the fruit loses its resistance to the fungus and the quiescent 

Alternaria infections become active. Similarly to the preformed 

compounds in young avocados, the preformed resorcinols in unripe 

mangoes can also be further induced in the fruit. The fact that the 

resorcinol concentrations in Kelt mango, which is disease-sensitive, do 

not decrease to non-toxic levels during ripening also supports the 

assumption that the resorcinols mixture has a role in disease 

suppression. Large quantities of resorcinols generally occur in the fruit 

peel, whereas the fruit flesh contains only very small quantities. 




Several natural antifungal compounds have been isolated from lemon 

peel and have been identified. Among these are citral, limetin, 

5-geranoxy-7-methoxycoumarin and isopimpenellin. Of these, only the 

citral, which is a monoterpene aldehyde, has been found in the flavedo in 

concentrations sufficient to inhibit decay development. By adding citral 

to a green lemon, previously inoculated with Penicillium digitatum, it 

was possible to prevent decay development under shelf-life conditions, 

but high concentrations of citral damaged the peel (Ben-Yehoshua et al., 

1992). The presence of antifungal compounds, mainly citral, in young 

lemon fruits, may explain why citrus fruits, in their developmental 

stages on the tree, are resistant to decay, in spite of their constant 

contact with Penicillium spores and the injuries that inevitably occur in 

the grove and provide points for penetration. 

The antifungal activity of citral has been exhibited against various 

fungi (Onawunmi, 1989). Introducing P. digitatum spores into the oil 

glands of the peel of young lemon fruits revealed that citral is the main 

factor within the glands responsible for the inhibition of the pathogen 

development (Rodov et al., 1995b). However, during long-term storage of 

lemon fruits - during which fruit senescence progresses - the citral 

concentration within the glands declines, resulting in decreased 

antifungal activity of the peel and increased incidence of decay. The 

changes in citral concentration in the lemon peel may thus determine the 

fruit sensitivity to postharvest decay. In parallel with citral decline, the 

flavedo of yellow lemons exhibited an increased level of neryl-acetate, 

which is a monoterpene ester which exhibits no inhibitory activity 

against P. digitatum, and which, in low concentrations (of less than 500 

ppm) may even stimulate development of the pathogen. The increase in 

the level of monoterpenes in the peel may explain, at least in part, the 

stimulatory effect on P. digitatum and P. italicum development, of the 

etheric oil derived from stored citrus fruit, a phenomenon recorded 

earlier by French et al. (1978). The resistance of citrus peel to 

inoculations applied between the oil glands, found in early studies by 

Schiffmann-Nadel and Littauer (1956), was attributed to another factor, 

which does not change with fruit ripening and may be related to its 

chemical composition or its anatomic structure (Rodov et al., 1995b). 

Inducible Preformed Compounds 

Over the last two decades several studies have indicated that 

preformed antifungal compounds, which are normally present in healthy 

plant tissues, can be further induced in the host in response to pathogen 




attack or presence, as well as to other stresses. Induction of existing 

preformed compounds can take place in the tissue in which they are 

already present, or in a different tissue (Prusky and Keen, 1995). 

An example of a preformed compound being induced in the same 

tissue where fungitoxic concentrations are naturally accumulated is the 

antifungal diene in the peel of unripe avocado fruits. Prusky et al. (1990) 

found that inoculation of unharvested or freshly harvested avocado fruits 

with C. gloeosporioides, but not with the stem-end fungus Diplodia 

natalensis, resulted in a temporarily enhanced level of this compound. 

The response to this challenge doubled the amount of the preformed 

diene after 1 day, and the effect persisted for 3 days, suggesting 

persistence of the elicitor (Prusky et al., 1990). A significant increase in 

the concentration of the antifungal diene, with longer persistence of the 

elicitors but without symptom expression, has been induced by a 

nonpathogenic mutant of Colletotrichum magna (Prusky et al., 1994). 

The inducement of preformed antifungal compounds by two different 

Colletotrichum species suggests that nonspecific eliciting factors are 

probably present in the hyphae of both species (Prusky, 1996). 

Wounding is an abiotic factor that may enhance diene concentration. 

However, whereas wounding of freshly harvested fruit resulted in a 

temporarily enhanced diene accumulation in the fruit, inducement did 

not occur in fruits 3-4 days after harvest (Prusky et al., 1990). Gamma 

irradiation is another abiotic factor capable of inducing diene 

accumulation. Following irradiation at 5 or 20 krad, the diene 

concentration was doubled, but a day later its level was lower than in the 

untreated controls. This phenomenon was probably due to the fact that 

irradiation also enhanced fruit ripening and, in turn, the decline in the 

diene concentration (Prusky and Keen, 1995). 

An inducement of the antifungal diene also followed a high-C02 

application. Exposing freshly harvested avocado fruits to 30% CO2 

resulted in increased concentration of the diene upon removal from the 

controlled atmosphere storage. The concentration of diene then decreased 

but this decrease was followed by a second increase which correlated 

with suppressed decay development (Prusky et al., 1991). The increased 

concentration of the antifungal diene in the fruit could result from direct 

synthesis or from inhibition of its breakdown during fruit ripening. 

Several findings have pointed to the importance of lipoxygenase activity 

in the breakdown of the diene, and to its being affected by the activity of 

epicatechin in the fruit peel. In the flesh of avocado fruits both diene and 

monoene antifungal compounds are among the compounds that 




accumulate within special oil cells named idioblasts (Kobiler et al., 1994). 

It is not clear whether induction of the antifungal diene involves de novo 

synthesis or a release of the compound from the idioblasts. However, the 

fact that about 85% of all antifungal compounds within the fruit 

mesocarp are located in idioblasts suggests that their synthesis might 

occur in these specific cells (Kobiler et al., 1994). 

Inducement of preformed antifungal compounds has also been 

described in mango fruits. A mixture of resorcinolic compounds normally 

occurs in fungitoxic concentrations (154-232 \ig ml^ fresh weight) in the 

peel of unripe mangoes, whereas only very low concentrations are 

present in the flesh of the fruits (Droby et al., 1986). Peeling of unripe 

fruits and exposing them to atmospheric conditions for 48 h enhanced the 

concentrations of resorcinols in the outer layers of the flesh to fungitoxic 

levels (Droby et al., 1987). This enhancement was accompanied by an 

increase in fruit resistance to fungal attack. Fruit peeling resulted in 

browning of the flesh accompanied by enhanced activity of phenylalanine 

ammonia lyase (PAL). There are indications that PAL activity is 

connected with production of phytoalexins and other compounds involved 

in the defense mechanisms of the plant (Kuc, 1972). However, it was 

further found that cycloheximide application inhibited both the 

enhancement of PAL activity and the browning of the tissue, but did not 

interfere with the production of resorcinols and the resistance of the fruit 

to Alternaria alternata (Droby et al., 1987). These results confirm the 

assumption that the resorcinols that accumulate in the fruit flesh 

following peeling are responsible for the decay resistance of the peeled 

fruit. 

Exposure of the fruit to a controlled atmosphere containing up to 75% 

CO2 was found to enhance the level of resorcinols in the peel itself, where 

they are normally present; this enhancement was accompanied by decay 

retardation, as indicated by a delay in the appearance of the symptoms of 

A. alternata infection (Prusky and Keen, 1995). These results showed 

that it is possible to induce the preformed resorcinols both in the tissue 

in which they are already present, and in a different tissue. 

In carrot roots, high concentrations of the antifungal polyacetylene 

compound, falcarindiol, were recorded. This compound is found in 

extracellular oil droplets within the root periderm and the pericyclic 

areas (Garrod and Lewis, 1979). The high concentrations of the 

antifungal compound were suggested to result from the continuous 

contact of the carrots with organisms in the rhizosphere or with various 

pathogens. One of the important antifungal compounds in carrot roots is 




the polyacetylenic compound, falcarinol. Heat-killed conidia of Botrytis 

cinerea applied to freshly cut slices of carrot root further induced the 

accumulation of falcarinol and, in parallel, increased the resistance of the 

tissue to living cells of the pathogen (Harding and Heale, 1980). 

The fact that specific factors may enhance the levels of some 

preformed compounds, and lead to the prevention of fungal attack, has a 

physiological importance in fruit resistance. Furthermore, understanding 

the mechanisms behind these phenomena may enable us to modulate 

them to control the levels of the natural preformed inhibitory compounds, 

as a means of disease control. 

D. PHYTOALEXINS - INDUCED INHIBITORY COMPOUNDS 

Phytoalexins are low-molecular-weight toxic compounds produced in 

the host tissue in response to initial infection by microorganisms, or to an 

attempt at infection. In other words, in order to overcome an attack by 

the pathogen, the host is induced by the pathogen to produce antifungal 

compounds that would prevent pathogen development. However, the 

accumulation of phytoalexins does not depend on infection only. Such 

compounds may be elicited by fungal, bacterial or viral metabolites, by 

mechanical damage, by plant constituents released after injury, by a 

wide diversity of chemical compounds, or by low temperature, irradiation 

and other stress conditions. Phytoalexins are thus considered to be 

general stress-response compounds, produced after biotic or abiotic 

stress. The most available evidence on the role of phytoalexins shows 

that disruption of cell membranes is a central factor in their toxicity 

(Smith, 1996), and that the mechanism is consistent with the lipophilic 

properties of most phytoalexins (Arnoldi and Merlini, 1990). 

Earlier studies by MuUer and Borger (1940) already provided strong 

evidence that resistance of potato to Phytophthora infestans is based on 

the production of fungitoxic compounds by the host. A terpenoid 

compound, rishitin, produced in potato tubers following infection by P. 

infestans, was first isolated by Tomiyama et al. (1968) from resistant 

potatoes that had been inoculated with the fungus. It accumulates 

rapidly in the tuber and reaches levels much higher than those required 

to prevent fungal development. The relationship between the 

accumulation of rishitin in the tuber and its resistance to late blight may 




point to its role in resistance development (Kuc, 1976). It was clarified 

that the rapidity and magnitude of phytoalexin accumulation were 

important in disease resistance, and that potato tubers could be 

protected against disease caused by a compatible race of P. infestans by 

inoculation with an incompatible race. Rishitin has also been found to be 

induced in potato tuber discs 24 h after inoculation with Fusarium 

sambucinum, which causes dry rot in stored potatoes (Ray and 

Hammerschmidt, 1998). Other sesquiterpenoids that have been found in 

potatoes may also play a role in tuber disease resistance; they include 

rishitinol (Katsui et al., 1972), lubimin (Katsui et al., 1974; Stoessl et al., 

1974), oxylubimin (Katsui et al., 1974), solavetivone (Coxon et al., 1974), 

and others. The terpenoid phytuberin was found to be constitutively 

present in tuber tissues at low levels, but it was further induced after 

inoculation with F. sambucinum. The phytoalexins, phytuberol and 

lubimin appeared in potato discs by 48 h after inoculation, while 

solavetivone was produced in very low quantities. At least eight additional 

terpenoid compounds were induced in potato tubers in response to 

inoculation with pathogenic strains of F. sambucinum, and they appeared 

48-70 h after inoculation. Successful infection was necessary for terpenoid 

accumulation. No inducement of terpenoid accumulation occurred 

following inoculation of tuber discs with non-pathogenic strains of 

F. sambucinum (Ray and Hammerschmidt, 1998). 

Rishitin and solavetivone were recently isolated from potato tuber 

slices inoculated with Erwinia carotovora. These sesquiterpenoids 

suppressed mycelial growth of the potato pathogen P. infestans on a 

defined medium (Engstrom et al., 1999). A similar effect, however, was 

recorded for the naturally occurring plant sesquiterpenoids abscisic acid, 

cedrol and farnesol, although these compounds are found in healthy 

plant tissue and are not associated with post-infection responses. At 

concentrations of 50 or 100 ^g ml^ rishitin and cedrol showed the 

strongest inhibitory effects on P. infestans, while solavetivone, abscisic 

acid and farnesol were somewhat less inhibitory. P. infestans has 

frequently been used for testing the activity of potato phytoalexins, the 

criteria for inhibition being the rates of spore germination, germ-tube 

elongation, mycelial growth and the inhibition of fungal glucanases 

(Ishizaka et al., 1969; Hohl et al., 1980). The effects of these 

sesquiterpenoids - phytoalexins as well as non-phytoalexins - were found 

to be much lower than the effect of the fungicide metalaxyl. In general, 

phytoalexins are not considered to be as potent as antibiotic compounds 

(Kuc 1995; Smith 1996). 




Several phytoalexinic compounds, such as umbelliferone, scopoletin 

and esculetin, are produced in sweet potato roots infected by the fungus 

Ceratocystis fimbriata, and it was noted that these compounds 

accumulate more rapidly in roots resistant to this fungus than in 

sensitive roots (Minamikawa et al., 1963). 

The resistance of celery petioles to pathogens has been attributed over 

the years to psoralens, linear furanocoumarins which are considered to 

be phytoalexins (Afek et al., 1995a, b; Beier and Oertli, 1983). These 

compounds, which have deleterious effects on the skin of humans, were 

previously thought to be mycotoxins produced by Sclerotinia sclerotiorum 

in celery (Ashwood-Smith et al., 1985). However, mechanical damage 

during harvesting and storage, and other elicitors, such as low 

temperatures and UV, have also been shown to induce furanocoumarin 

production (Beier and Oertli, 1983; Chaudhary et al., 1985). The psoralen 

content of celery increases during storage (Chaudhary et al., 1985), and 

the levels of various psoralens found in older celery stalks are higher 

than those in younger stalks (Aharoni et al., 1996). Furthermore, 

infection of celery with S, sclerotiorum or Botrytis cinerea, the main 

fungal pathogens of stored celery, was found to stimulate psoralen 

production during storage (Austad and Kalvi, 1983). Recent studies with 

celery (Afek et al., 1995b) indicate that (+) marmesin, the precursor of 

linear furanocoumarins in this crop, and not psoralens, is the major 

compound involved in celery resistance to pathogens; it has at least 100 

times greater antifungal activity than psoralens. Also, increased 

susceptibility of stored celery to pathogens is accompanied by a decrease 

in (+) marmesin concentration and a corresponding increase in total 

psoralen concentration. However, treatment of celery prior to storage 

with gibberellic acid (GA3), a naturally occurring phytohormone in 

juvenile plant tissue, resulted in decay suppression during 1 month of 

storage at 2°C (see the chapter on Means for Maintaining Host 

Resistance - Growth Regulators), although GA3 does not have any effect 

on fungal growth in vitro (Barkai-Golan and Aharoni, 1980). It was 

suggested that the phytohormone retards celery decay during storage by 

slowing down the conversion of (+) marmesin to psoralens, thereby 

maintaining the high level of (+) marmesin and low levels of psoralens 

and, thus increasing celery resistance to storage pathogens (Afek et al., 

1995b). 

Another phytoalexin found in celery tissue is columbianetin, which 

probably also plays a more important role than psoralens in celery 




resistance to decay (Afek et al., 1995c). This supposition is derived from 

the following facts: (a) columbianetin exhibits stronger activity than 

psoralens against Alternaria alternata, B, cinerea and S, sclerotiorum, 

the main postharvest pathogens of celery; (b) the concentration of 

psoralens in celery is much lower than that required to inhibit the 

growth of celery pathogens, while the concentration of columbianetin in 

the tissue is close to that required for their suppression; (c) increased 

sensitivity of celery to pathogens during storage occurred in parallel with 

the decrease in the concentration of columbianetin, and with the increase 

in that of psoralen (Fig. 12). 

The phytoalexin capsidiol is a sesquiterpenoid compound produced by 

pepper fruits in response to infection with a range of fungi (Stoessl et 

al., 1972). Pepper fruits inoculated with B. cinerea and Phytophthora 

capsici contain only small quantities of capsidiol, whereas fruits 

inoculated with saprophytic species or with weak pathogens may 

produce higher concentrations of the phytoalexin, which inhibit spore 

germination and mycelial growth. Inoculating peppers with Fusarium 

c 

o 

CO 

c 

0 

o 

c 

o 

o 

80 

60 + 

40 

20 + 

0 

Coiumbianetin 

Furanocoumarins 

1 2 

Time (weeks) 

Fig. 12. Concentration of columbianetin and total furanocoumarins in celery 

during 4 weeks of storage at 2°C. Vertical bars indicate standard error. 

(Reproduced from Afek et al., 1995 with permission of Elsevier Science). 




species results in increased capsidiol concentration 6 to 12 h after 

inoculation. In such cases, the capsidiol accumulation in the tissue is 

rapid, whereas in other cases it is rapidly oxidized to capsenone, which 

is characterized by a much weaker toxic effect than that of the 

capsidiol. When unripe pepper fruits were inoculated with Glomerella 

cingulata, the cause of anthracnose, a phytoalexin was readily 

identified in tissue extracts (Adikaram et al., 1982). This compound, 

possibly related to capsidiol but much less water soluble, has been 

named capsicannol (Swinburne, 1983). 

The resistance of unripe banana to anthracnose incited by 

Colletotrichum musae has been attributed to the accumulation of five 

fungitoxic phytoalexin compounds that were not present in healthy 

tissue. As the fruit ripened these compounds diminished and, at a 

progressive stage of disease development, no phytoalexins were detected 

(Brown, A.E. and Swinburne, 1980). Elicitors composed of a glucan-like 

fraction of the cell walls of hyphae and conidia of C. musae elicited both 

necrosis and the accumulation of the two major phytoalexins found with 

the live inoculum. 

Benzoic acid is a phytoalexin produced in apples as a result of 

infection by Nectria galligena and other pathogens. Fruit resistance to 

this pathogen at the beginning of a long storage period, was attributed to 

the formation of this phytoalexin (Swinburne, 1973). Nectria penetrates 

apples via wounds or lenticels prior to picking, but its development in the 

fruit is very limited. Benzoic acid is the compound isolated from the 

limited infected area; its production is correlated with the activity of 

protease produced by the pathogen (Swinburne, 1975). 

The accumulation of this antifungal compound in the necrotic area 

inhibits the progress of Nectria within the fruit. Benzoic acid has proved 

to be toxic only as the undissociated molecule and it is expressed only at 

low pH values, such as can be found in unripe apples where the initial 

development of the fungus was indeed halted. With ripening and the 

decline in fruit tissue acidity, in conjunction with increasing sugar levels 

(Sitterly and Shay, 1960), the benzoic acid is decomposed by the 

pathogen, ultimately to CO2, and the fungus can resume active growth 

(Swinburne, 1983). The elicitor of benzoic acid synthesis was found to be 

a protease produced by the pathogen (Swinburne, 1975). This protease is 

a non-specific elicitor and a number of proteases from several sources 

may elicit the same response. On the other hand, Penicillium expansum, 




B, cinerea, Sclerotinia fructigena, and Aspergillus niger, which do not 

produce protease in the infected tissue and do not induce the accumulation 

of benzoic acid, can rot immature fruit (Swinburne, 1975). 

Inoculating lemon fruits with Penicillium digitatum, the pathogen 

specific to citrus fruits, results in the accumulation of the phytoalexin 

scoparone (6,7-dimethyloxycoumarin). The induced compound has a 

greater toxic effect than that of the preformed antifungal compounds 

naturally found in the fruit tissue, such as citral and limetin, as indicated 

by the inhibition of P. digitatum spore germination (Ben-Yehoshua et al., 

1992). A considerable increase in the scoparone level was found in lemons 

which had been preheated at 36°C for 3 days, and then inoculated with P. 

digitatum spores. The increased concentration of scoparone in the fruit 

was in good correlation with the enhanced antifungal activity of the fruit 

extract. This finding led to the supposition that scoparone plays an 

important role in the increased infection resistance of heated fruit. It is 

worth mentioning that various citrus fruits (lemon, orange, grapefruit, 

etc.) differ from one another in their ability to produce scoparone in 

response to fungal infection (Ben-Yehoshua et al., 1992). 

Scoparone production can also be induced in the peel of various citrus 

fruits by ultraviolet (UV) illumination (Rodov et al., 1992). In 

UV-irradiated (254 nm) kumquat fruits the accumulation of scoparone 

reached its peak (530 |Lig gi dry weight of the flavedo tissue) 11 days after 

illumination. However, its level then declined rapidly, returning to the 

minimal trace levels characteristic of the untreated fruit, 1 month after 

treatment (Fig. 13). Increasing the radiation dose and raising the storage 

temperature (from 2 to 17°C) enhanced scoparone production. 

A correlation has been drawn between the level of phytoalexin 

accumulated in the flavedo of irradiated fruits and the increased 

antifungal activity of the flavedo. It has also been found that irradiating 

kumquat fruits prior to their inoculation with P. digitatum resulted in 

decreased incidence of green mold decay, whereas irradiating fruits 

which had previously been inoculated with P. digitatum failed to prevent 

decay development (Fig. 14). The finding that decay reduction was 

achieved when irradiation was applied to the fruit prior to its 

inoculation, and therefore without any direct exposure of the pathogen to 

the radiation, led to the suggestion that disease inhibition stems from 

increased resistance of the fruit to infection and not from the direct 

fungicidal effect of UV on the pathogen (Rodov et al., 1992). 




100 

80 

60 

I 40 

20 

Treatments: 

• Control 

o UV 18h after inoculation 

Q UV 48h before inoculation 

10 15 


Fig. 13. Effect of ultraviolet (UV) treatment on decay development in kumquat 

fruit inoculated with dry spores of Penicillium digitatum. Control: 

non-illuminated fruit. UV dose: 1.5x10^ m-2. Bars indicate standard deviations. 

(Reproduced from Rodov et al., 1992 with permission of the American Society 

for Horticultural Science). 

600 

UV treatment 

(1.5x103jm-2)^ 

10 15 20 

Days after Treatment 

Fig. 14. Time course of scoparone accumulation and depletion in UV-treated 

kumquat fruit. Bars indicate standard deviations. (Reproduced from Rodov et 

al., 1992 with permission of the American Society for Horticultural Science). 


25 

20



Several studies with citrus fruits have also described gamma 

irradiation as a stress factor leading to the induction of antifungal 

phytoalexinic compounds in the treated fruit tissues. (See the chapter on 

Physical Means - Ionizing Radiation). 

Biosynthesis of toxic compounds as a result of wounding or other 

stress conditions, is a ubiquitous phenomenon in various plant tissues. 

An example of such a synthesis is the production of the toxic compound 

6-methoxymellein in carrot roots in response to wounding or to ethylene 

application (Chalutz et al., 1969; Coxon et al., 1973); the application of B. 

cinerea conidia and other fungal spores to the wounded area was found to 

stimulate the formation of this compound (Coxon et al., 1973). A similar 

result is also achieved by the application of fungal-produced pectinase, in 

spite of the fact that this enzyme does not affect cell vitality (Kurosaki et 

al., 1986b). This toxic compound probably has an important role in the 

resistance of fresh carrots to infection. Carrots that have been stored for 

a long period at a low temperature lose the ability to produce this 

compound and, in parallel, their susceptibility to pathogens increases. 

Enhanced resistance of carrots can also be induced by application of dead 

spores; carrot discs treated with J3. cinerea spores which had previously 

been killed by heating developed a marked resistance to living spores of 

the fungus, which was much greater than that of the control discs. The 

increased resistance of the tissue to active fungal spores, as 

evaluated by the inhibition of germ-tube elongation on the treated carrot, 

is shown in Fig. 15. The most effective inhibitors found in the tissues 

after the induction of resistance, as well as in the control tissues, were 

methoxymellein, p-hydroxybenzoic acid, and polyacetylene falcarinol 

(Harding and Heale, 1980). 

In spite of the demonstration that many plant organs produce 

low-molecular-weight antibiotic compounds in response to infection, 

there remain substantial questions as to the role of phytoalexins in 

disease interactions. The lack of clear answers to questions such as 

whether phytoalexin accumulates in suitable concentrations at the 

right place and the right time, has led to the conclusion that 

phytoalexin synthesis is but one of a vast array of phenomena that, 

taken together, contribute to the expression of resistance (Nicholson 

and Hammerschmidt, 1992). 



84 

0 

3 

120 

100 

£ 80 

D) 

C 

0) 

0) 

CD 

60 

40+1 

20 

0 


0 1 2 3 4 5 6 7 8 9 10 

Number of months at 4-7°C prior to treatment 

Fig. 15. Assessment of resistance induction in carrot root slices 16 h after 

treatment with heat-killed conidia of Botrytis cinerea. The criterion: germ-tube 

elongation on the surface of treated (•) and untreated (o) carrot slices. Vertical 

bars indicate standard error. (Reproduced from Harding and Heale, 1980 with 

permission of Academic Press). 

E. WOUND HEALING AND HOST BARRIERS 

Pathogen penetration through a wound, cut, scratch or other injury to 

the fruit or other plant organs, is a major way for initiating postharvest 

disease. However, a fresh fruit, which escaped infection because of a lack 

of appropriate spores or the absence of conditions suitable for 

germination, may, in many cases, become more resistant to later 

infection. In some instances, the reduced sensitivity of a wound to 

infection may be related to its natural drjdng. In other cases, some 

biochemical changes which take place at the wound area may lead to its 

protection. 

Mechanically injured fruit is characterized by enhanced respiration 

and ethylene evolution ("wound ethylene"). The injured cells lose their 

vitality, whereas living cells around the wound are exposed to stress 




conditions and their metabolic activity is enhanced. These cells may 

repair the damage sustained by the injured tissue. In response to injury, 

the host tissue is capable of forming protective "barriers" composed of 

tightly packed cells, but such barrier formation takes place only when 

the injured tissue is still capable of cell division, or the cells surrounding 

the wound can produce and deposit lignin and suberin in their walls 

(Eckert, 1978). These compounds protect the host from pathogen 

penetration or from the action of cell-wall degrading enzymes secreted by 

the pathogen. As a result of wounding, the production of antimicrobial 

polyphenolic compounds can also contribute to wound protection. 

Phytoalexins are other toxic compounds that can be formed at the wound 

area following inducement by initial infection. 

Ray and Hammerschmidt (1998) found that inoculation of potato 

tubers with Fusarium sambucinum, the fungal pathogen of potato dry 

rot, resulted in an increase in phenolic acids suggesting that phenolic 

acid biosynthesis was induced. Following such inducement, free phenolic 

acids are removed as they are converted into lignin or are cross-linked to 

cell walls. Lignin production is mediated by peroxidase, which is strongly 

induced in the infected tuber in a number of isoforms. While a lignin 

barrier may inhibit the advance of the fungus, microscopy has indicated 

that lignin barriers are repeatedly breached or circumvented as dry rot 

progresses. This may suggest that lignin is not in itself an effective 

defense against F. sambucinum or, alternatively, that it cannot be 

induced quickly enough to defend the tissue (Ray and Hammerschmidt, 

1998). Furthermore, the phenolic acid and total peroxidase contents are 

not higher in tubers of the resistant line than in more susceptible 

genotypes, suggesting that neither is critical to tuber infection 

resistance. 

Suberization of cells at the wound area, a process that heals the 

wound with a layer of phenolic and aliphatic compounds (Mohan and 

Kolattukudy, 1990), is an important anti-infection defense mechanism of 

the wounded tuber. When superficially wounded tubers are stored at 

high relative humidity and moderate temperature, suberin is formed in 

the walls and intercellular spaces of the living cells surrounding the 

injury. Under optimal environmental conditions this takes place within 

24 h after wounding (Fox et al., 1971). The suberized periderm which 

forms after several days is composed of tightly packed cells, the walls of 

which are impregnated with suberin and contain only a small amount of 

pectin (Fox et al., 1971). On the other hand, storing tubers at too low 




temperatures, which do not promote suberin formation by the cells 

surrounding the wound, facilitates the development of gangrene caused 

by Phoma infection in stored potatoes. This is because temperatures 

which prevent the formation of suberinic barriers by the host are still 

suitable for the development of the fungus (Boyd, 1972). 

Suberin also serves as a defensive layer on the lenticels of potato 

tubers. Lenticels of potatoes are normally covered with a suberin layer 

that prevents the entry of soft rot bacteria into the cortex of the tuber. 

Removal of this layer renders the lenticels susceptible to bacterial 

invasion (Fox et al., 1971; Perombelon and Lowe, 1975). 

Kolattukudy (1987) described suberin as a complex biopolyester 

comprising a phenolic (aromatic or lignin-like) domain attached to the 

cell wall, and an aliphatic (lipid, hydrophobic) domain, which is probably 

attached to the phenolic domain. Soluble waxes are imbedded within the 

suberin matrix and provide resistance to water vapor loss but have not 

been indicated to play a role in disease resistance (Lulai and Corsini, 

1998). However, rapid suberization of wounded tubers is critical in 

avoiding both bacterial soft rot (Erwinia carotovora subsp. carotovora) 

and fungal dry rot (F.. sambucinum). Lulai and Corsini (1998) 

emphasized the differential development of potato tuber resistance to 

bacteria and then to fungal penetration during suberization. They 

related this phenomenon to the differential deposition of the two major 

suberin components (the phenolic and the aliphatic domains) during 

wound healing (18°C at 98% RH). It was found that the initiation of 

suberin phenolic deposition at the wound site preceded that of suberin 

aliphatic deposition. Total resistance to Erwinia infection occurred after 

completion of the phenolic deposition on the outer wall of the first layer 

of cells (2-3 days). However, the phenolic suberin deposition offered no 

resistance to Fusarium infection. Resistance to fungal infection began to 

develop only after deposition of the suberin aliphatic domain was 

initiated. Total resistance to fungal infection was attained after 

completion of deposition of the suberin aliphatic domain within the first 

layer of suberized cells (5-7 days). This suberin domain was not required 

for tuber resistance to bacterial soft rot infection. 

Curing sweet potatoes for several days at high relative humidity 

(85-90%) and high temperatures (26-32°C) facilitates the formation of 

suberized periderm, which protects harvest injuries against attack by 

Rhizopus stolonifer (Clark, 1992). In fact, curing is a standard prestorage 

treatment to prevent decay in various varieties of sweet potatoes. 

Similarly, holding carrots at high relative humidity and 22-26°C for 2 




days prior to removal to cold storage (5°C) results in the suberization of 

cells at the wound area, and the formation of a barrier against Botrytis 

cinerea and other pathogens. Such a treatment considerably reduces the 

incidence of decay at the end of prolonged storage (Heale and Sharman, 

1977). 

Studies with cultured carrot cells indicated that phenolic compounds 

with low molecular weight, which are a link in lignin biosynthesis, and 

free radicals produced during its polymerization, may take part in 

resistance inducement by damaging fungal cell membranes, fungal 

enzymes or toxins (Kurosaki et al., 1986a; Vance et al., 1980). 

Accumulation of phenolic compounds and callose deposition in cell walls 

of young tomato fruits, following inoculation with B. cinerea, were found 

to arrest fungal development, thus retarding or preventing decay 

(Glazener, 1982). The mechanism by which phenolic compounds 

accumulate in the host is not yet clear, but research carried out with 

wheat leaves suggested that the chitin in the fungal cell walls acts as a 

stimulator to lignification in the leaves (Pearce and Ride, 1982). 

Curing, involving the use of warmth and high humidity to harden 

wounds and promote the development of resistance to infection, was long 

ago recommended for control of Penicillium decay in sweet oranges 

(Tindale and Fish, 1931). Since then, the efficiency of curing in protecting 

against infection by various wound pathogens has been described for 

several citrus fruits. It was thus found that holding wounded citrus fruits 

for several days at 30-36°C and high relative humidity (90-96%) 

markedly suppressed decay development during storage (Baudoin and 

Eckert, 1985; Brown, G.E., 1973; Hopkins and Loucks, 1948). The 

resistance of the fruits developed under these conditions has been 

attributed to the deposit of lignin or lignin-like material by the living 

cells of the flavedo (the external, colored layer of the peel) adjacent to the 

injury, leading to the formation of a barrier which prevents fungal 

penetration and restricts its growth (Brown, G.E. and Barmore, 1983). In 

addition, lignification of cell walls at the site of infection protects them 

against the activity of cell-wall-degrading pectolytic enzymes produced 

by the fungus, and prevents degradation of the middle lamella of cell 

walls (Ismail and Brown, 1979; Vance et al., 1980). 

The combination of moderate temperatures with high relative 

humidity was found to be suitable for rapid production of lignin at the 

wounded area, while such a temperature was too high for Penicillium 

digitatum development (Brown, G.E., 1973), but lowering the 

temperature to 2TC allowed rapid fungal development, which preceded 




lignin formation. Thus, holding the fruit at this temperature facihtated 

decay development. On the other hand, lowering the relative humidity 

below 75% dried up the cells around the wound, and such injured cells 

were incapable of lignin production (Brown, G.E., 1973). In an earlier 

study, the infection resistance of unripe papaya fruits was similarly 

attributed to the ability of the tissue to produce lignin, which prevented 

fungal progress within the tissue (Stanghellini and Aragaki, 1966). 

Decay reduction has also been achieved through the curing and 

lignification of superficial wounds in the peel of citrus fruits, by using 

plastic film to seal-package individual fruits for 1-3 days at high relative 

humidity and moderate or high temperatures (32-36°C) prior to storage 

(Brown, G.E. and Barmore, 1983; Eckert et al., 1984; Golomb et al., 

1984). Healing of wounds under these conditions was attributed to the 

water-saturated atmosphere formed within the plastic wrap. The 

water-saturated atmosphere, when applied together with high 

temperatures, in addition to retarding tissue aging and preserving the 

integrity of all membranes, may also stimulate the biosynthesis of lignin 

by the living cells surrounding the wound. Under these conditions, 

lignification results in a considerable reduction in decay incidence, 

without affecting the taste of the fruit (Ben-Yehoshua et al., 1987). 

Wrapping various citrus fruits (lemon, orange, grapefruit and pomelo) 

in sealed plastic film protects them from damage caused by the high 

temperature; the optimal temperature and duration of curing can be 

determined for each fruit/fungus system (Ben-Yehoshua et al., 1988). 

(Figs. 16, 17). 

In more recent studies Stange and Eckert (1994) indicated that 

improved decay control in lemons could be achieved by dipping the fruits 

in a surfactant solution, prior to curing at 32°C in a water-saturated 

atmosphere. The enhanced decay control was probably due to the 

increased mortality of P. digitatum conidia and germ tubes. In other 

studies with citrus fruits, however, Stange et al. (1993) came to the 

conclusion that the evidence for the involvement of lignin in wound 

protection in citrus fruits was based on histochemical tests which were 

not entirely specific for lignin; they suggested that the resistance of the 

wound to infection had to be attributed to the deposition of induced 

antifungal gum materials, rather than lignin, in the resistant wound. In 

support of this hypothesis, Stange et al. (1993) presented the finding that 

extracts from hardened, resistant wounds contained several induced, 

low-molecular-weight aldehydes, some of which had marked antifungal 

activity. The resistance of the wound to infection may be associated 




10 20 30 

Days after curing 

Fig. 16. Effect of 72 h curing at various temperatures on percent decay of 

inoculated pummelo held at 11° and 17°C. (Reproduced from Ben-Yehoshua et 

al, 1988). 

8 12 16 20 24 

Days after curing 

Fig. 17. Effect of duration of the curing period at 36°C on percent decay of 

inoculated pummelo at 17°C. (Reproduced from Ben-Yehoshua et al. 1988). 




directly with the inducement of these compounds at the wound site. It 

was further found that dipping the fruits in the non-toxic compound, 

potassium phosphonate, increased the resistance of the wound to 

P. digitatum infection (Wild, 1993). This was only noticeable when at 

least 24 h was allowed between dipping the freshly wounded fruits and 

inoculating them with Penicillium spores. Since this compound has no 

direct effect on Penicillium growth in vitro, it was concluded that the 

potassium phosphonate was eliciting enhanced production of these 

antifungal compounds in the host, and this conclusion was further 

supported by the finding that cycloheximide, which is a protein synthesis 

inhibitor, negated the effect of the phosphonate (Wild, 1993). 

Studies with young apricot fruits indicated that the mechanical 

barrier formed in response to fungal penetration may also prevent 

reactivation of a quiescent infection. The cell-wall suberization of living 

cells surrounding the infection point of Monilinia fructicola in the young 

fruit and the accompanying accumulation of phenolic compounds, have 

been hypothesized to be the reason for the non-activation of the quiescent 

fungus growth (Wade and Cruickshank, 1992). When the fruit ripens, 

however, viable hyphae of M fructicola may escape from the arrested 

lesions and cause fruit decay. Curing effects and decreased incidence of 

Botrytis cinerea can be obtained in kiwifruit by prolonging the time 

between harvesting and cool storage (Pennycook and Manning, 1992). In 

determining the critical conditions required during curing of kiwifruit in 

order to minimize Botrytis rot during storage, Bautista-Bafios et al. 

(1997) concluded that temperatures between 10 and 20°C, together with 

relative humidity (RH) higher than 92% for a three-day curing period, 

should provide adequate disease control without lowering fruit quality 

during cold storage (0°C); increased temperature or decreased RH 

generally resulted in increased fruit weight loss. 

F. ACTIVE OXYGEN 

Active oxygen, produced by plant cells during interactions with 

potential pathogens and in response to elicitors, has recently been 

suggested to be involved in pathogenesis. "Active oxygen species", 

including superoxide, hydrogen peroxide and hydroxyl radical, can affect 

many cellular processes involved in plant-pathogen interactions (Baker 

and Orlandi, 1995). The direct antimicrobial effect of active oxygen 




species has not yet been clarified, but they are considered to play a role 

in various defense mechanisms, including lignin production, lipid 

peroxidation, phytoalexin production and hypersensitive responses. 

However, active oxygen can be difficult to monitor in plant cells because 

many of the "active oxygen species" are short lived and are subject to 

cellular antioxidant mechanisms such as superoxide dismutases, 

peroxidases, catalase and other factors (Baker and Orlandi, 1995). 

A first report on the production of active oxygen in potato tubers 

undergoing a hypersensitive response was given by Doke (1983), who 

demonstrated that O2 production occurred in potato tissues upon 

inoculation with an incompatible race of Phytophthora infestans (i.e., a 

race causing a hypersensitive response), but not after inoculations with a 

compatible race (i.e., a disease-causing race). Following this early report 

the connection between active oxygen species and phytoalexin synthesis 

has been found in several host-pathogen interactions (Devlin and 

Gustine, 1992). 

In a recent study Beno-Moualem and Prusky (2000) found that the 

level of reactive oxygen species in freshly harvested unripe avocado fruit, 

which is resistant to infection, was higher than that in the susceptible 

ripe fruit. Moreover, inoculation of resistant fruits with Colletotrichum 

gloeosporioides further increased their reactive oxygen production, 

whereas inoculation of susceptible fruits had no such effect. It was also 

indicated that reactive oxygen production could be induced by avocado 

cell cultures treated with cell-wall elicitor of C. gloeosporioides. 

When isolated avocado pericarp tissue was treated with H2O2 (1 mM) 

the reactive oxygen production was enhanced and, in parallel, a rapid 

increase in the levels of epicatechin was detected. Epicatechin appears to 

play a key role in fruit susceptibility during ripening, by indirectly 

controlling the levels of the antifungal compounds naturally present in 

the peel of unripe fruits (Prusky et al., 1982); phenylalanine ammonia 

lyase (PAL), one of the enzymes involved in epicatechin synthesis, also 

increased, in parallel with the enhanced production of reactive oxygen 

species. On the other hand, pre-incubation of avocado tissue in the 

presence of protein kinase inhibitors inhibited the release of H2O2 from 

avocado cell cultures and PAL activity was no longer induced. 

Beno-Moualem and Prusky (2000) suggested that the inducement of 

reactive oxygen species by fungal infection of unripe fruits may modulate 

fruit resistance and lead to the inhibition of fungal development and the 

continuation of the quiescent stage of the pathogen. 




G. PATHOGENESIS-RELATED PROTEINS 

Several glucanohydrolases found in plants, such as chitinase and 

P-l,3-glucanase, have received considerable attention as they are 

considered to play a major role in constitutive and inducible resistance 

against pathogens (El Ghaouth, 1994). These enzymes are lowmolecular-weight 

proteins, frequently referred to as pathogenesis-related 

(PR) proteins. They hydrolyze the major components of fungal cell walls 

which results in the inhibition of fungal growth (Schlumbaum et al., 1986). 

The chitinases, which are ubiquitous enzymes of bacteria, fungi, 

plants and animals, hydrolyze the P-l,4-linkage between the 

N-acetylglucosamine residues of chitin, a polysaccharide of the cell wall 

of many fungi (Neuhaus, 1999). The glucanases, which are abundant, 

highly regulated enzymes, widely distributed in seed-plant species, are 

able to catalyze endo-type hydrolytic cleavage of glucosidic linkages in 

P-l,3-glucans (Leubner-Metzger and Meins, 1999). 

The chitinases and P-l,3-glucanases are stimulated by infection and in 

response to elicitors (Bowles, 1990). Chitosan, a P-l,4-glucosamine 

polymer found as a natural constituent in cell walls of many fungi, is 

capable of both directly interfering with fungal growth and eliciting 

defense mechanisms in the plant tissue (El Ghaouth, 1994). Postharvest 

treatment with this elicitor has been found to activate antifungal 

hydrolases in several fruits: treatment of strawberries, bell peppers and 

tomato fruits with chitosan induced the production of hydrolases, which 

remained elevated for up to 14 days after treatment and reduced lesion 

development by Botrytis cinerea (El Ghaouth and Arul, 1992; El Ghaouth 

et al., 1997). When applied as a stem scar treatment to bell peppers, 

chitosan stimulated the activities of chitinase, chitosanase and 

p-l,3-glucanase. Being capable of degrading fungal cell walls, these 

antifungal hydrolases are considered to play a major role in disease 

resistance (Schlumbaum et al., 1986; El Ghaouth, 1994). 

In chitosan-treated bell peppers the production of lytic enzymes was 

followed by a substantial reduction of chitin content of the cell walls of 

invading fungal hyphae, expressed as a reduction of chitin labeling in the 

walls (El Ghaouth and Arul, 1992). The hypothesis of fungal infection 

suppression was also supported by the finding that invading fungal hyphae 

were mainly restricted to the epidermal cells ruptured during wounding (El 

Ghaouth et al., 1994). It was thus suggested that the deliberate stimulation 

and activation of PR proteins in the fruit tissue might lead to disease 

suppression by enhancing host resistance to infection. 




Peroxidases are another group of PR proteins whose activity has been 

correlated with plant resistance against pathogens. Plant peroxidases, 

which are glycoproteins that catalyze the oxidation by peroxide of many 

organic and inorganic substrates, have been implicated in a wide range of 

physiological processes, such as ethylene biosynthesis, auxin metabolism, 

respiration, lignin formation, suberization, growth and senescence. 

Findings of correlations between deposition of cell wall strengthening 

materials, such as lignin, suberin and extensin, and peroxidase activities 

are consistent with a role for this enzyme in defense through 

wall-strengthening processes (Chitoor et al., 1999). The importance of 

peroxidase lies in the fact that the host cell wall constitutes one of the first 

lines of defense against pathogens, and peroxidase is a key enzyme in the 

wall-building processes. Such processes include the accumulation of lignin 

and phenolic compounds, which has been correlated with enhanced 

resistance in various host-pathogen interactions, and suberization, which 

leads to enhanced resistance by healing wounds with a layer of phenolic 

and aliphatic compounds (Mohan and Kolattukudy, 1990). However, the 

resistance against pathogens may also be related to the highly reactive 

oxygen species such as H2O2 or oxygenase, which are likely to be toxic to 

pathogens and which are formed by peroxidase activity during the 

deposition of cell wall compounds (Goodman and Novacky, 1994). 

In a recent study Ray and Hammerschmidt (1998) found that the 

activity of peroxidase in potato tubers increased greatly following 

infection with Fusarium sambucinum, the cause of potato dry rot. Such 

an infection induces several isoforms of peroxidase whose role has not 

been identified. Lignin levels also increased but the lignified zones could 

be breached, allowing the infection to develop further into the tuber. The 

increased peroxidase levels in infected tubers were expected to increase 

their potential to synthesize lignin and so enhance their resistance to 

infection. However, infection of tubers of transgenic potatoes following 

introduction of a foreign peroxidase gene from cucumbers, which was 

associated with an induced resistance response, had no effect on disease 

caused by several potato pathogens (Ray et al., 1998). These results 

indicated that the incorporation of a putative defense-associated 

peroxidase and the formation of transgenic potato plants does not 

necessarily result in the enhancement of resistance. It has been 

suggested, however, that understanding the function of peroxidase and 

determining the availability of the substrates needed for the enzyme 

activity are important for using transgenic studies to evaluate the role of 

peroxidase in defense responses (Ray et al., 1998, Chitoor et al., 1999). 



CHAPTER 7 

PHYSIOLOGICAL AND BIOCHEMICAL CHANGES 

FOLLOWING INFECTION 

Postharvest infection of fruits and other plant organs may induce a 

number of alterations in their physiological and biochemical processes or 

in the host tissue constituents, as a result of host-pathogen interactions. 

Process changes may include acceleration of ethylene evolution, 

stimulation of the respiratory enzymes, enhanced pectolytic activity, 

altered protein synthesis or polyamine-synthesis enzyme activity, etc.; and 

tissue changes may include increased cell-wall soluble pectin, and changed 

organic acid and sugar contents. The infection, or sometimes an attempted 

infection, may also induce the accumulation of low-molecular-weight 

defense chemicals - the phytoalexins - or enhance the production of 

preformed antimicrobial compounds in the host. The latter changes are 

dealt with in the chapter on Host Protection and Defense Mechanisms. 

A. CHANGES IN FRUIT RESPIRATION AND ETHYLENE 

EVOLUTION 

Studies carried out over the last few decades have found enhanced 

respiratory activity and increased ethylene production following injury or 

exposure to other stresses, such as low temperature, gamma irradiation, 

etc. Invasion by pathogens, including fungi, bacteria and viruses, seems 

to be no different in this respect from other plant stresses. 

Both respiration and ethylene evolution are important indicators of the 

physiological state of the fruit, and their enhancement following infection 

highlights its effects on the postharvest ripening and senescence 

processes. Stimulation of respiration and ethylene evolution processes in 

citrus fruits infected by Penicillium digitatum, was first recorded at the 

beginning of the 1940s (Biale, 1940; Biale and Shepherd, 1941; Miller et 

al., 1940), and subsequent studies of various citrus fruits inoculated with 

divers postharvest fungi confirmed that this was a general post-infection 

phenomenon (Schiffmann-Nadel, 1974). Similarly to the fungi, pathogenic 

bacteria such as Pseudomonas syringae can also enhance respiratory 



Physiological and Biochemical Changes 95 

activity and ethylene evolution in citrus fruits (Cohen et al., 1978). When 

we compare the effects of different fungi on a certain fruit, e.g., lemon 

(Schiffmann-Nadel, 1974), we see that the greatest enhancements, both of 

ethylene evolution and respiration, are caused by P. digitatum, followed by 

P. italicum, Diplodia natalensis and Geotrichum candidum. Much less 

enhancement is caused by the fungi Fusarium moniliforme and Alternaria 

citri (Figs. 18, 19). In general, a correlation was found between respiration 

and ethylene levels, on the one hand, and the rate of fungal development 

on the fruit, on the other hand. Thus, fungi with short incubation periods 

and rapid growth cause greater increases in the levels of ethylene and CO2 

evolution than those characterized by slow development. 

Acceleration of the respiratory activity, ethylene evolution, or both, 

has also been found in infected climacteric fruits, e.g., peaches infected 

by Monilinia fructicola (Hall, R., 1967); tomatoes infected by Rhizopus 

stolonifer, Botrytis cinerea and G. candidum (Barkai-Golan and 

Kopeliovitch, 1983; Barkai-Golan et al., 1989b); and bananas, mangoes 

O 

o 

E 

.0 

v.* 

(0 

0 

a: 

250 

200 -+- 

150 -+- 

100 -4- 

4 8 12 16 20 30 


Fig. 18. Respiration rate of green lemon fruit inoculated with postharvest fungi. 

(Reproduced from Schiffmann-Nadel, 1974). 



96 

c 

.g 

%-• 

_g 

o 

> 

0 

c 

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-C 

LU 

400 - 

200 - 

t 

25 " 

20 = 

15 ' 

10 = 

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0 = 

i 

i 

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f 

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n Alternaria 

o Control 

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-W-M-r "-"V^ 

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0 4 8 12 16 20 30 60 


Fig. 19. Ethylene evolution from green lemon fruit inoculated with postharvest 

fungi. (Reproduced from Schiffmann-Nadel, 1974). 

and apples infected by Penicillium expansum and species of Alternaria, 

Trichothecium, Colletotrichum, Dipiodia and Pestalotia (Schiffmann-Nadel 

et al., 1985). 

Similarly to non-climacteric fruits, correlations between the rates of 

fungal growth on the fruit and the enhancement of physiological processes 

was again recorded. Inoculating avocado fruits with Fusarium solani 

accelerated fruit respiration and ethylene evolution without enhancing the 

peak levels of these two physiological processes (Zauberman and 

Schiffmann-Nadel, 1974); i.e., in spite of the acceleration of these 

processes, no change in their general patterns had occurred (Figs. 20A, 

20B). Inoculating the fruits with other Fusarium spp., characterized by 

slow growth rates, only slightly accelerated these two processes. 

Inoculating tomato fruits at their pre-cUmacteric stage (mature-green 

fruit) with B, cinerea, typically stimulated ethylene evolution, while 

inoculations at the post-chmacteric stage (mature fruit) elicited new 

ethylene peaks (Barkai-Golan et al., 1989b) (Fig. 21). 




O) 

d 

o 

c 

o 

Q. 

to 

0 

iiUU 

160- 

120- 

80- 

_ B 


/ \ 

Ulnfected 

Fruit 

/H^ 

Fruit 

softening 

\ 1 — 5 "^^^^ 1 h-H 

1 h—' 

0 1 2 3 4 5 6 7 8 9 


Fig. 20. Ethylene evolution (A) and respiration rates (B) of avocado fruit 

infected by Fusarium solani in comparison to uninfected fruit. (Reproduced 

from Zauberman and Schiffmann-Nadel, 1974 with permission of the American 





0 

c 

LU 

6 8 10 0 2 4 


Fig. 21. Ethylene evolution from normal Taculta 38' tomato fruit, at its 

nature-green (A) and mature (B) stages of maturity, following inoculation with 

Botrytis cinerea (A), or following treatment prior to inoculation (•) as compared 

to uninfected fruit (•). (Reproduced from Barkai-Golan et al., 1989 with 

permission of Blackwell Wissenschafts-Verlag GmbH). 

Ethylene evolution was stimulated, not only in the normal ripening 

tomato fruit, but also in the non-ripening nor mutant fruit, which did not 

show any rise in the ethylene level when uninfected (Fig. 22). Thus, 

fungal infection of the non-ripening mutant tomatoes induced thylene 

synthesis by tissues which normally lack an active ripening system and 

do not even respond to exogenic ethylene treatment (Tigchelaar et al., 

1978). Earlier and higher ethylene peaks were detected in fruits infected 

with J5. cinerea than in those infected with G. candidum, corresponding 

to the faster growth of the former (Barkai-Golan et al., 1989b). 




nor tomatoes 


Fig. 22. Ethylene evolution from nor mutant tomato fruit, at its mature-green 

(A) and mature (B) stages of maturity, following inoculation with Botrytis 

cinerea (A), or following treatment prior to inoculation (•) as compared to 

uninfected fruit (•). (Reproduced from Barkai-Golan et al., 1989 with permission 

of Blackwell Wissenschafts-Verlag GmbH). 

Ethylene synthesis is also stimulated by mechanical wounding, such 

as occurs while introducing the inoculum into the host. However, 

comparison between rates of ethylene production elicited by wounds of 

certain dimensions and those elicited by fungal lesions of similar size, 

indicates that ethylene production in fungus-infected fruits is 

considerably greater than that in wounded ones. These results may 

suggest that, in addition to the mechanical stress produced during 

pathogenesis, which is responsible for the 'wound-ethylene', another 

factor is involved in the synthesis of 'infection-ethylene'. This 'biological 

factor' seems to differ for different fungi (Barkai-Golan et al., 1989b). 




B. ETHYLENE SOURCE IN INFECTED TISSUE 

The marked increase in ethylene evolution during infection stimulated 

the questions: What is the source of ethylene in the infected fruit? Is 

ethylene produced by the attacking pathogen or by the attacked host? 

It is known that various fungi do produce ethylene in vitro as a 

metabolic product during their development (Hag and Curtis, 1968). 

However, with the exception of a few species, such as Aspergillus 

clavatus, A, flavus, Penicillium digitatum and P. corylophilum, which 

produce considerable levels of ethylene, the production of ethylene by 

most species is low, reaching 0.5-1.0 ppm/g dry weight. Determination of 

ethylene production in vitro may be complicated due to the fact that 

different strains or even different isolates, of the same species, may differ 

in their capacity to produce ethylene. Moreover, when no ethylene is 

detected for a given fungus in culture, it is still possible that the 

pathogen can synthesize it in the fruit, because of the special substrate 

supplied by the living tissue. However, the healthy tissue is also capable 

of producing and emitting ethylene but, in contrast to climacteric fruits, 

in which ethylene evolution may reach high peaks during ripening, most 

plant tissues produce very low levels of this gas when healthy. The 

increased ethylene produced by the diseased fruit may, therefore, be from 

the damaged host tissue, from the activity of the pathogen within the 

host, or both. 

Examination of the source of ethylene in apples infected by the brown 

rot fungus, Sclerotinia fructigena (Hislop et al., 1973), and in tomatoes 

infected by Rhizopus stolonifer or Botrytis cinerea (Barkai-Golan and 

Kopeliovitch, 1983; Barkai-Golan et al., 1989b) showed that the rotted 

tissues themselves, which contained an abundance of actively growing 

mycelium, contributed only slightly or not at all to the production of 

ethylene by the infected tissue. On the other hand, the healthy fruit 

tissue at the periphery of the rot, where only a few hyphae could be seen 

penetrating the healthy tissue, were producing considerably higher levels 

of ethylene (Table 5). The induction of ethylene production by the host 

tissues in response to infection has also been exhibited by the 

non-ripening, non-ethylene-producing nor tomato mutant at various 

stages of maturity (Table 5). Moreover, none of the fungi mentioned 

produce ethylene in vitro. These results suggested that the ethylene 

recorded in the various host/pathogen systems is produced by the host in 

response to fungal infection. 




TABLE 5 

Ethylene production by fruit discs (3 mm^) otRhizopusinfected 

'Rutgars' tomatoes and non-ripening nor tomatoes* 

Tested 

tissue 

a 

b 

c 

d 

e 

'Rutgars' 

mature-green 

1.2 

8.1 

30.2 

2.0 

0.0 

Ethylene Production (|il kg-i h-i) 

mature-green 

0.4 

5.8 

19.9 

0.0 

0.2 

Nor 

Mature 

0.3 

0.5 

11.2 

0.3 

0.0 

a - healthy pulp; b - healthy pulp 1-5 cm from rotted area; c - healthy 

pulp at the periphery of rot; d - rotten pulp at the periphery of the 

healthy pulp; e - rotten pulp from the center of the lesion. 

* Reproduced from Barkai-Golan and Kopeliovitch (1983) with 

permission of the Academic Press. 

Barkai-Golan et al. (1989b) studied the source of ethylene in tomatoes 

inoculated with B. cinerea or Geotrichum candidum by using aminoxyacetic 

acid (AOA), a potent inhibitor of plant-originating ethylene (Yang, 1985). 

AOA application at the site of inoculation with B. cinerea was found to 

inhibit ethylene production by 55-60% in the normal tomato fruits and by 

about 80% in the non-ripening nor mutant fruits (see Figs. 21 and 22). 

Since AOA acts as a specific inhibitor of ethylene biosynthesis by higher 

plants, these results suggest that ethylene production in both normal 

and mutant fruits is of plant origin, and is induced by the pathogen. The 

inability of AOA to arrest the production of infection-ethylene totally 

may be due to the short lag time between its application and the fungal 

inoculation, which prevents sufficient uptake by the tissue. 

The source of ethylene in the infected tissue was also studied in citrus 

fruit inoculated with P. digitatum (Achilea et al., 1985a, b) which is one of 

the fungi capable of producing high levels of ethylene in vitro (Hag and 

Curtis, 1968). Inoculation of citrus fruit with the fungus, similarly to other 

stresses, enhances the production of both ethylene and the compound ACC 

(1-aminocylopropane-l- carboxylic acid), which is the precursor of ethylene 

production in higher plants (Yang and Hoffman, 1984). Achilea et al. 




(1985a, b) studied P. digitatum ethylene production in several ways: 

(a) By comparing an isolate of P. digitatum producing very high levels 

of ethylene with an isolate producing very low levels of the gas. 

(b) By adding ACC, which is typically connected with the production 

of ethylene of plant origin, or glutamic acid, which is probably the 

source of fungal ethylene (Chou and Yang, 1973). 

(c) By adding AVG (aminoethoxy vinyl glycine), which inhibits plant 

ethylene production (Lieberman et al., 1974) and only slightly affects 

its production by P. digitatum (Chalutz and Lieberman, 1978). 

(d) By the addition of Cu^ ions, which are toxic to the fungus but 

stimulate ethylene production in higher plants (Abeles and Abeles, 

1972). 

The application of this complex of methods to the study of the origin of 

the ethylene produced in infected hosts clearly proved that the high level of 

ethylene produced in the infected area itself is derived wholly or in part 

from the fungus, which is capable of producing it both in culture and in the 

fruit. The production of this ethylene was not affected by the presence of 

ACC, but was considerably enhanced by the addition of glutamic acid. The 

addition of AVG inhibited its production only slightly, whereas the addition 

of CuS04 resulted in a marked inhibition of ethylene synthesis. On the 

other hand, the ethylene produced by the healthy part of the fruit, at the 

periphery of the rot, must be of plant origin, since it was stimulated by ACC 

and suppressed by AVG application. 

C. PECTOLYTIC ACTIVITY AND ITS SOURCE IN INFECTED 

TISSUE 

An increase in polygalacturonase (PG) activity in infected tissue during 

disease development has generally been accepted as being of fungal origin 

(Wood, 1967). However, studies of the origin of pectolytic activity in various 

host/pathogen systems demonstrated that the fungus not only 'contributes' 

its enzymes to the host, but may also induce enzyme production by the host 

itself, as a response to infection (Barkai-Golan et al., 1986). It was thus 

found that PG production by avocado fruits following infection with 

Colletotrichum gloeosporioides was induced by the fungus, and that 

softening of the mesocarp during anthracnose development was primarily 

due to the induction of the fruit PG (Barash and Khazzam, 1970). 




The origin of PG in Rhizopus stolonifer-miected tomatoes has been 

studied in various tomato fruits (Barkai-Golan et al., 1986): (a) a non 

ripening nor tomato mutant, which contains no PG during any stages 

of development; (b) a green normal fruit, which does not contain PG 

while still unripe; and (c) a ripe red fruit, which contained PG of plant 

origin. Analysis by gel electrophoresis of Rhizopus PG, after in vitro 

growth, revealed the production of at least seven molecular forms of this 

enzyme. In each of the fruits tested, Rhizopus infection resulted in the 

accumulation of these typical PG isozymes, which are all of fungal 

origin; however, in the infected ripe tomato fruit, both fruit and fungal 

enzymes were found. An analysis by gel electrophoresis enabled 

differentiation between the isozyme bands of the tomato and those of 

the fungus which migrated in the gel faster than those of the fruit PG. 

It could not, however, provide a clear differentiation between the 

slower-migrating fungal isozymes and the fruit isozymes (Barkai-Golan 

et al., 1986). The differentiation between these isozymes was completed 

by immunological analysis with antiserum against specific forms of 

tomato enzymes; this analysis demonstrated that the infected fruit 

contained a higher level of tomato enzyme than the uninfected fruit of 

similar maturity. It was concluded that infection by R, stolonifer 

enhanced PG production in the normally ripening fruit or, in other 

words, that the infection advanced the ripening process in genetically 

competent tomato tissue. The infection did not, however, induce PG in 

the non-ripening nor fruits, which lack an active ripening system and 

are unable to produce PG naturally (Barkai-Golan et al., 1986). 

Clarification of the origin of PG in the infected tissue also suggests that 

the pectolytic enzymes of the mature tomato fruit can take an active 

part in fungal pathogenicity or contribute to disease development. 

D. STIMULATION OF FRUIT SOFTENING AND CHANGES 

IN THE PECTIC COMPOUND CONTENTS 

Examination of avocado fruits infected by Fusarium solani has 

revealed that their softening starts earlier than in uninfected fruits, and 

that it occurs during the incubation period of the pathogen, before the 

appearance of decay symptoms (Zauberman and Schiffmann-Nadel, 

1974). Furthermore, softening does not begin at the point of infection 

and progress outwards from there, but occurs simultaneously in the 

whole fruit. This finding raised the suggestion that the advance in fruit 




softening is not related to the local effect of the fungus but to its ability to 

induce a softening mechanism similar to that of the healthy fruit. The 

inducement of this mechanism may be directly related to the enhanced 

ethylene emission typical of infected fruit, and to the ability of the 

'infection ethylene', similarly to exogenic ethylene, to enhance fruit 

softening and lead to earlier senescence. 

Earlier studies of apples infected with various pathogens had already 

shown a considerable reduction in the insoluble pectin in their tissues, 

particularly in those cases where the pathogen was responsible for fruit 

soft rot (Cole and Wood, 1961). Studies with F, soZaAii-inoculated avocados 

indicated that fungal infection affected the rates of changes in the pectic 

substances: the increase in the soluble pectin and the reduction in the 

insoluble protopectin are more rapid in the infected than the uninfected 

fruit, even though the final values of each of the fractions are similar in 

the two cases. Since fruit softening starts with the increase in the soluble 

pectin and the decrease in the protopectin, it is clear that it occurs earlier 

in the infected fruit (Zauberman and Schiffmann-Nadel, 1974). 

Cell disintegration and tissue maceration were followed by an increase 

in soluble pectin in grapefruit peel treated with endo-PG produced by 

Penicillium italicum (Barmore and Brown, 1980). The increase in soluble 

pectin was recorded after 3-5 h incubation of the enzyme with host 

tissue, as presented in Table 6: 

TABLE 6 

Maceration of grapefruit peel mesocarp by endo-polygalacturonase 

from fruits infected with Penicillium italicum^ 

Treatment time Maceration Pectin solubilization^ 

(h) index2 (mg/g of peel) 

1 1 0.47 

3 3 0.88 

5 5 1.02 

1 Reproduced from Barmore and Brown (1980) with permission of the 

American Phytopathological Society. 

2 0 = no maceration; 5 = complete maceration 

3 Soluble pectin content resulting from the action of endopolygalacturonase 




E. CHANGES IN BIOCHEMICAL CONSTITUENTS OF 

INFECTED TISSUES 

In various host/pathogen systems, fruit infection results in the 

decrease or total disappearance of the sugar content of the fruit. Such 

systems include: lemon fruits infected by Phytophthora citrophthora 

(Cohen and Schiffmann-Nadel, 1972); pineapples infected by Ceratocystis 

paradoxa (Adisa, 1985a); other tropical fruits, such as guava, papaya and 

banana, infected by various pathogens (Ghosh et al., 1964; Odebode and 

Sansui, 1996); watermelons infected by Alternaria cucumerina strains 

(Chopra et al., 1974); and cucumbers infected by Pythium 

aphanidermatum (McCombs and Winstead, 1964). This common 

phenomenon has generally been related to the stimulation of respiration 

in the infected tissue. An interesting phenomenon was recorded in 

banana fruits: whereas the presence of glucose, sucrose, fructose, maltose 

and raffinose were recorded in healthy fruits, only sucrose appeared 

during storage of bananas infected with Botryodiplodia theobromae 

(Odebode and Sansui, 1996). On the other hand, inoculation of 

pineapples with the fungus Curvularia verruculosa resulted in an 

increased level of sugars, probably because of the activity of 

cell-wall-degrading enzymes of the fungus (Adisa, 1985a). 

Inoculation of citrus fruits with Penicillium digitatum considerably 

affected the D-galacturonic acid content of the peel (Achilea et al., 

1985a). During the infection process the fungus produces 

exo-polygalacturonase, which hydrolyzes the cell wall pectin into 

monomers of galacturonic acid (Barmore and Brown, 1979), which is 

responsible for cell disintegration and tissue maceration. 

Fungal infection may result in the reduction of the organic acid level. 

Reductions in ascorbic acid were recorded in lemons infected by 

Phytophthora citrophthora, in bananas infected by various storage 

pathogens, and in chili fruits infected by Colletotrichum gloeosporioides 

(Cohen and Schiffmann-Nadel, 1972; Khodke and Gahukar, 1995; 

Odebode and Sansui, 1996). The absence of citric acid from pineapples 

was recorded in fruit infected by Ceratocystis and Curvularia (Adisa, 

1985a). It has been suggested that the pathogenic fungi might use 

organic acids for their respiration, thus leading to their decreased level 

or disappearance from the tissue. 

Several cases have been reported of changes in the protein level in the 

plant tissue during disease development: a reduction in the protein 




content along with an increase in the free amino acids. However, in other 

cases infection had no effect on the protein content of the tissue, or even 

resulted in their increase (Adisa, 1985a; Khodke and Gahukar, 1995). 

Several amino acids, such as histidine, arginine, aspartic acid, glycine, 

alanine and leucine, increased in infected fruits, whereas the levels of 

lysine, threonine, glutamic acid, proline, cystein, valine, methionine and 

others all decreased following infection (Khodke and Gahukar, 1995). 

Polyamines, which are responsible for the control of nucleic acid 

synthesis and function (Slogum et al., 1984), play an important role in 

plant growth and development (Galston, 1983). The relationship between 

disease development and polyamine content in the fruit has been studied 

in Rhizopus stolonifer-infected tomato fruits (Bakanashvili et al., 1987). 

Marked reductions in the activities of the enzymes ornithine 

decarboxylase and arginine decarboxylase were recorded in the healthy 

tomato fruit during fruit development and towards ripening, but, as a 

result of the changes in the activities of these enzymes, the content of the 

polyamine putrescine, as well as those of the polyamines permidine and 

spermine which derive from it, were markedly reduced, and reached 

their lowest levels during ripening (Bakanashvili et al., 1987). 

Inoculation of preclimacteric tomato fruits with R, stolonifer was found 

to induce, in addition to fruit ripening, a reduction in the activity of the 

enzymes ornithine and arginine decarboxylase and, consequently, to 

reduce the polyamine level in the fruit. Since treatment with exogenic 

ethylene elicited phenomena similar to those associated with fungal 

infection or fruit ripening, it has been suggested that the ethylene 

induced by pathogen infection may be responsible for the altered 

polyamine content of the Rhizopus-iioSected tomato fruits. 

Numerous biochemical changes occurred in potato tubers inoculated 

with Fusarium sambucinum, the causal agent of dry rot in stored tubers 

(Ray and Hammerschmidt, 1998). Peroxidase activity, which is very low 

in the uninfected tuber, increased up to 500-fold after inoculation with 

the fungus. High levels were recorded in tissue just ahead of the 

advancing hyphae. Smaller increases were recorded in tubers following 

wounding alone. Lignin content increased in tubers following inoculation, 

the highest concentrations being detected 40 h after inoculation, when 

the infected surface was often partly or entirely decayed. As disease 

progressed, the lignified zone increased in depth until it comprised five or 

six cell layers. Here again, responses to F. sambucinum infection were 




stronger than responses to wounding alone, and the hgnified zone in 

wounded, non-infected tissue was much narrower than that in tissue 

adjacent to the infection (Ray and Hammerschmidt, 1998). The 

inducement of peroxidase activity and hgnin formation is included 

among the defense responses of the host to infection (see the chapter on 

Host Protection and Defense Mechanisms). 

Polyphenol oxidase activity, which is involved in quinone formation, 

browning and other reactions in potato tubers, increased following 

inoculation with F. sambucinum and wounding, but to a lesser extent 

than peroxidase activity. The greatest polyphenol oxidase activity was 

found in tissues adjacent to the diseased zone; it seemed to be associated 

with tuber resistance and was highest in the more resistant potato 

varieties than in the susceptible ones (Ray and Hammerschmidt, 1988). 

Furthermore, resistant tubers also browned more quickly than the 

susceptible ones following infection or wounding. 

Phenolic compounds were found to increase following potato tuber 

inoculation or wounding alone in both susceptible and moderately 

resistant tubers, suggesting that phenolic biosynthesis was induced. 

Examination of extracts of infected tuber discs revealed the presence of a 

number of phenolic compounds. Constant amounts of ferulic acid were 

detected, whereas chlorogenic acid, which is a phytoalexinic compound 

induced in apple fruit infected by Nectria galligena (Swinburne, 1983), 

accumulated to maximal levels at 48 h and then decreased. These results 

indicate that free phenolic acids may be induced by infection or 

wounding, and are then removed as they are converted into lignin or 

cross-linked to cell walls (Ray and Hammerschmidt, 1998). For 

discussion of the inducement of phytoalexins in tuber tissue in response 

to infection, see the chapter on Host Protection and Defense Mechanisms. 



CHAPTER 8 

MEANS FOR MAINTAINING HOST RESISTANCE 

It was previously pointed out that fruits become more susceptible to 

invasion by postharvest pathogens as they ripen (see the chapter on 

Factors Affecting Disease Development - The Fruit Ripening Stage). 

Treatments and conditions that lead to delayed ripening and senescence 

can, therefore, indirectly suppress postharvest disease development. These 

include low-temperature storage, I0W-O2 and high-C02 atmospheres, 

ethylene removal from the atmosphere, growth regulators, calcium 

application; these as well as other treatments or strategies may contribute 

to maintaining the natural resistance typical of the young fruit or 

vegetable. 

A. COLD STORAGE 

Storage at low temperature is the main method for reducing 

deterioration of harvested fruits and vegetables. The importance of cold 

storage in decay suppression is so great that all other control methods 

are frequently considered as supplements to refrigeration (Eckert and 

Sommer, 1967). 

Low temperatures affect both the host and the pathogen 

simultaneously. They prevent moisture loss from the host tissues and 

consequent shriveling; they retard metabolic activity and delay 

physiological changes that lead to ripening and senescence. Since fruits 

and vegetables become generally more susceptible to pathogens as they 

mature and approach senescence, the retardation in the physiological 

activity of the host is accompanied by a delay in decay development after 

harvest. As with the host, the metabolic activity of the pathogen is also 

directly influenced by the environmental temperature, and both its growth 

ability and enzymatic activity can be greatly retarded by low temperatures. 

Low temperatures can thus delay postharvest disease development in two 

ways: (a) indirectly, by inhibition of ripening and senescence of the host 

and extension of the period during which it maintains its resistance to 

disease; and (b) directly, by inhibition of pathogen development by 

subjecting it to a temperature unfavorable for its growth. 



Means for Maintaining Host Resistance 109 

We have seen that the minimum temperature for growth of some 

fungal species is around 0°C, while other species are capable of growth, 

although at a very slow rate, even at temperatures as low as -2°C or less 

(see Table 2). For these pathogens, storing the commodity at 0°C would 

not arrest their growth nor prevent disease development, but would only 

delay the appearance of the disease or, in other words, would prolong its 

incubation period. We have also seen that the closer the storage 

temperature approaches to the minimum for growth of the pathogen, the 

longer is the incubation period of the disease and the slower the progress 

of decay (see Fig. 8). It should be emphasized, however, that even if the 

pathogen is not eradicated at 0°C or at temperatures close to it, it is very 

significant that its rate of growth under these conditions is very slow; 

this enables us to understand our general desire to lower the 

temperature of the storage room atmosphere as much as possible. 

However, the possibility of lowering the storage temperature is limited 

by the sensitivity of the fruit or vegetable to chilling injury. This 

sensitivity varies among fruit and vegetable species, and even among 

cultivars of the same species, or depends on the state of maturity of a 

given cultivar. 

Storing cold-sensitive fruits and vegetables at temperatures below 

their resistance threshold results in the development of chilling injuries 

- physiological injuries caused by low, but above-freezing, temperatures. 

The severity of the damage depends on the cultivar sensitivity, on the 

temperature and on the duration of exposure to it (Ryall and Lipton, 

1979). Chilling injuries are generally associated with the destruction of 

groups of cells, which leads to the formation of sunken areas and to 

external or internal browning. The development of chilling injuries in 

cold-sensitive crops has been hypothesized to derive from changes in 

lipids, which leading, in turn, to changes in the permeability of cell 

membranes and the accumulation of toxic intermediate compounds, 

which may cause cell destruction or death (Lyons, 1973). 

Following chilling injury, the sensitivity of the fruit or vegetable to 

decay increases considerably, and decay incidence might reach higher 

levels than those in fruits held at higher temperatures even though the 

latter are unable to reduce decay. The increased decay in the chill-injured 

host may result from the easy penetration of the pathogen through 

damaged tissues (see the chapter on Postharvest Disease Initiation - 

Pathogen Penetration into the Host). Such increased decay in 

cold-sensitive fruits was exhibited in Navel oranges which had to undergo 

cold-sterilization against insect infestation, under Japanese quarantine 




regulations which require all imported South African citrus fruit to be cold 

sterilized (12 days at 0.5°C) in order to destroy all forms of insect 

infestation. The resulting incidence of core rot caused by Alternaria citri 

and Fusarium spp. may become unacceptably high in cold-sensitive 

oranges stored at 4.5°C after the cold treatment (Pelser and Grange, 

1995). 

The guiding principle for choosing storage temperature is the use of 

the lowest temperature that does not harm the host. The storage 

temperature for fruits and vegetables should, therefore, be just above 

their injury threshold (Hardenburg et al., 1986). Recommended 

temperatures and relative humidities for storing fruits and vegetables 

are summed up in Tables 7 and 8. 

Commodity 

TABLET 

Recommended storage conditions for fruits i' 2 

Apple 

Bramley's Seedling 

Cox's Orange Pippin 

Crispin (Mutsu) 

Discovery 

Golden Delicious 

Granny Smith 

Jonathan 

Mcintosh 

Red Delicious 

Rome Beauty 

Spartan 

Temperature 

(°C) 

3 to 4 

3 to 3.5 

1.5-2 

3.5 

1.5 to 2 

-ItoO 

3 to 3.5 

1.5 to 3.5 

3.5 to 4 

lto2 

0 to 0.5 

Relative humidity 

(%) 

90-95 

90-95 

90-95 

90-95 

90-95 

90-95 

90-95 

90-95 

90-95 

90-95 

90-95 

1 Reproduced with permission from A Colour Atlas of Postharvest Diseases 

and Disorders of Fruits and Vegetables, Vol. I, by Anna S. Snowdon, 

published by Manson Publishing, London, 1990. It also includes data 

from the Agricultural Research Organization, Bet Dagan, Israel. 

2 The leading lines in determining these data are the temperatures 

suitable for achieving the longest storage duration followed by 

appropriate 'shelf life'. 




Commodity 

Apricot 

Avocado 

unripe 

Booth 1 and 8, Taylor 

Ettinger 

Fuchs, PoUick, Waldin 

Fuerte, Hass 

Lula 

ripe 

Fuerte, Hass 

Banana 

green 

Cavendish 

Gros Michel 

Lacatan 

Foyo Robusta, Valery 

Colored 

Blueberry 

Carambola 

Cherry 

Sour 

Sweet 

Citrus Fruits 

Clementine 

Grapefruit 

California and Arizona 

Florida and Texas 

Israeli 

South African 

Lemon 

Lime 

Mandarin 

Minneola 

Temperature 

(°C) 

-ItoO 

4.5 

5.5 

10 to 13 

5.5 to 8 

4.5 

2 to 5 

13 

13 

13 to 15 

13 

13 to 16 

0 

5 to 10 

Oto 1 

-ItoO 

4 to 5 

14 to 15 

10 to 15 

10 to 11 

11 

10 tol4 

9 to 10 

4 to 8 

4 to 9 



(%) 

90-95 

85-90 

85-90 

85-90 

85-90 

85-90 

85-90 

85-90 

85-90 

85-90 

85-90 

85-90 

90-95 

90 

90-95 

90-95 

90 

90 

90 

90 

90 

90 

90 

90 

90



Commodity 

Orange 

Castellana, Spanish 

Jaffa, Shamouti 

Navel 

Australian 

Israeli 

Moroccan, Spanish 

Valencia 

California 

Cyprus 


Israeli 

Morrocan 

South African 

Spanish 

Pomelo 

Satsuma 

Tangelo 

Tangerine 

California and Arizona 


Cranberry 

Date 

Feijoa 

Fig 

Gooseberry 

Grape 

Guava 

Kiwifruit 

Litchi 

Loquat 

Temperature 

(°C) 

lto2 

8 

4.5 to 5.5 

6 to 8 

2 to 3 

2 to 7 

2 to 3 

Otol 

2 to 5 

2 to 3 

4.5 

2 

10 to 15 

4 to 5 

4 to 5 

4.5 to 7 

0 to 4.5 

2 to 4 

0 

4 

0 

-0.5 to 0 

-1 to - 0.5 

5 to 10 

-0.5 to 0 

5 to 10 

0 



m 

90 

90 

90 

90 

90 

90 

90 

90 

90 

90 

90 

90 

90 

90 

90 

90 

90 

90-95 

85-90 

90 

90 

90-95 

90-95 

90 

90-95 

90-95 

90-95



Commodity 

Mango 

Alphonso, Indian 

USA 

Bangalore, Safeda 

Haden, Keitt 

Zill 

Nectarine 

Papaya 

green 

turning 

Passion Fruit 

Peach 

Pear 

Persimmon 

Pineapple 

mature green 

ripe 

Plantain 

green 

colored 

Plum 

Pomegranate 

Rambutan 

Raspberry 

Strawberry 

Tamarillo 

Temperature 

(°C) 

7 to 9 

13 

5.5 to 7 

12 to 14 

10 

-ItoO 

10 

7 

7 to 10 

-ItoO 

-ItoO 

-ItoO 

10 to 13 

7 

10 

11 to 15.5 

-0.5 to 0 

5 to 6 

10 to 12 

0 

0 

3 to 4 



(%) 

90 

90 

90 

90 

90 

90-95 

90 

90 

85-90 

90-95 

90-95 

90-95 

90 

90 

85-90 

85-90 

90-95 

90 

95 

90-95 

90-95 

90



Commodity 

Artichoke 

Globe 

Jerusalem 

Asparagus (Asperge) 

Bean 

Common 

Faba 

Beet 

Broccoli 

Brussels sprout 

Cabbage 

White cabbage 

Chinese cabbage 

Carrot 

Cassava 

Cauliflower 

Celeriac 

Celery 

Chicory 

Chilli 

Cucumber 

TABLE 8 

Recommended conditions for vegetables i' ^ 

Temperature 

(°C) 

Oto 1 

-0.5 to 0 

Oto 2 

4 to 8 

Otol 

Otol 

Otol 

Otol 

Otol 

Otol 

Otol 

Oto 2 

Otol 

Oto 1 

Oto 1 

Oto 1 

7 to 10 

10 to 12 


(%) 

95 - 100 

95 - 98 

95 - 100 

95 - 100 

95 

95 - 100 

95 - 100 

95 - 100 

95 - 100 

95 - 100 

95 - 100 

85-90 

95-98 

95-98 

95 - 100 

95 - 100 

90-95 

90-95 

1 Reproduced with permission from A Colour Atlas of Postharvest 

Diseases and Disorders of Fruits and Vegetables, Vol. 2, by Anna S. 

Snowdon, published by Manson Publishing, London, 1992. It also 

includes data from N. Aharoni (1992). 

2 The leading lines in determining these data are the temperatures 

suitable for achieving the longest storage duration followed by 

appropriate 'shelf life'. 




Commodity Temperature Relative humidity 

(!C) (%) 

Eggplant 8 to 12 90 - 95 

Garlic 

fresh 0 to 1 60-70 

dry 0 to 1 60-70 

Ginger 12 to 14 70 

Herbs (fresh): 

Basil 12 95 - 98 

Chervil 0 to 1 95-98 

Chives 0 to 1 95-98 

Coriander 0 to 1 95-98 

Dill 0 to 1 95-98 

Endive 0 to 1 95-98 

Fennel 0 to 1 95-98 

Marjoram 1 to 3 95-98 

Mehssa 1 to 3 95-98 

Mint 1 to 3 95-98 

Oregano 1 to 3 95-98 

Parsley 0 to 1 95-98 

Parsnip 0 to 1 95-98 

Rosemary 1 to 3 95-98 

Sage 1 to 3 95-98 

Sorrel 0 to 1 95-98 

Tarragon 0 to 1 95-98 

Thyme 1 to 3 95-98 

Watercress 0 to 1 95-98 

Z'atar 1 to 3 95-98 

Horseradish 0 to 1 95 - 100 

Kohlrabi 0 to 1 95 - 100 

Leek 0 to 1 95 -100 

Lettuce 0 to 1 95 -100 



116 

Commodity 

Melon 

Galia 

Ogen 

Cantaloupe 

Honeydew 

USA 

Onion 

green 

dry 

Pea 

Pepper 

Potato 

culinary 

seed 

Pumpkin 

Radish 

Rhubarb 

Shallot 

Spinach 

Squash 

summer 

winter 

Sweet corn 

Sweet Potato 

Tomato 

mature-green 

turning 

ripe 

Turnip 

Watermelon 

Zucchini 


Temperature 

(°C) 

5 to 7 

6 to 7 

4 to 5 

10 to 15 

5 

Oto 1 

-ItoO 

Oto 1 

7 to 10 

10 to 14 

4 to 5 

10 to 13 

Oto 1 

Oto 1 

Otol 

Otol 

8 to 10 

10 to 13 

Otol 

12 to 14 

12 to 15 

10 to 12 

8 to 10 

Oto 1 

10 to 14 

7 to 10 



(%) . 

90-95 

90-95 

90-95 

90-95 

90-95 

95 - 100 

70 

95 

95 

90-95 

90-95 

60-70 

95 - 100 

95 - 100 

95 - 100 

95 - 100 

90-95 

60-70 

95-98 

85-90 

90 

90 

90 

90-95 

85-90 

90-95



The effect of refrigeration as a means for disease suppression is 

manifested mainly in systems in which the host is tolerant of 

near-freezing temperatures, while the pathogen requires higher 

temperatures for growth. Rhizopus stolonifer strains or isolates that do 

not develop at temperatures lower than 5°C (Dennis and Cohen, 1976) do 

not raise a serious problem for strawberries or carrots, which can be 

stored or shipped at temperatures below 5°C. For these vegetables, as 

well as for most fruits of temperate origin, such as certain cultivars of 

apples and grapes, the optimal temperature, which will preserve the 

required quality of the commodity for the maximum duration, is near 

0°C. Therefore storing strawberries at around 1°C retards decay both 

directly by retarding fungal growth (mainly of Botrytis cinered) and 

indirectly by delaying senescence, as exhibited in the maintenance of 

fruit and calyx freshness. 

On the other hand, fruits of tropical origin, such as bananas, 

pineapples, lemons, grapefruits and tomatoes, are already liable to suffer 

chilling injury after several hours at 10-15°C, and fruits of subtropical 

origin, such as avocado cultivars, melon cultivars and various citrus 

fruits, are sensitive to temperatures below 10°C. Therefore, the fungus 

Geotrichum candidum, although it is sensitive to low temperatures, will 

continue to develop and cause the sour rot in stored lemons, since this 

fruit has to be stored at 13-14°C because of its sensitivity to prolonged 

chilling. Similarly, cold storage is also unable to suppress Penicillium 

spp. in various cold-sensitive citrus fruits; the storage temperature 

suitable for these commodities allow fungal development. Tomato fruits 

stored at temperatures lower than 8-12°C are extremely sensitive to 

attack by microorganisms, such as Alternaria alternata and R, stolonifer, 

even before the appearance of external chilling injury symptoms. The 

optimal temperature for tomatoes is closely related to the stage of 

ripening: mature-green fruits are more sensitive to chilling and should 

not be stored at temperatures below 12°C; those at a more advanced 

stage of ripening may be stored at 8-10°C without their quality 

suffering. Similarly, sweet peppers are injured below 7°C and become 

susceptible to Alternaria and Botrytis decay (McCoUoch and Wright, 

1966) and sweet potatoes stored below 10°C are predisposed to 

Alternaria, Botrytis, Mucor and Penicillium decay (Lauritzen, 1931). 

Direct control of the pathogen in these cold-sensitive commodities is not 

possible, because of the relatively high temperatures required for their 

storage; decay in these cases is suppressed mainly indirectly by delaying 

the ripening and senescence processes. 




When low-temperature storage is used for decay suppression, it is 

necessary to take into consideration not only the storage temperature but 

also the temperature from which the commodity was removed to cold 

storage, and the shelf-life temperature to which it will be transferred. 

Maximum storage life prolongation demands the rapid removal of the 

Tield heat' from the commodity. Holding the commodity at a high 

temperature prior to refrigeration, or delasdng its removal to cold 

storage, will permit germination of many of the spores which infest 

wounds and will contribute to pathogen establishment in the tissues, 

thus shortening the way to decay initiation. In addition, we should 

remember that low temperatures during storage are generally not lethal 

to the pathogenic fungi and bacteria, but merely retard their 

development. When the commodity is transferred to higher temperature 

conditions to complete its ripening or for marketing, the pathogen 

resumes growth and develops rapidly into decay lesions. Thus, growth of 

B. cinerea, the gray mold fungus in strawberries, which is markedly 

suppressed during cold storage at 0-2°C, will develop rapidly upon 

removal of the fruit to shelf-life conditions. On the other hand, the 

exposure of germinating sporangiospores of R. stolonifer to temperatures 

close to freezing, causes their death and prevents decay development 

during shelf life. It is no wonder, therefore, that the incidence of 

Rhizopus rot in peaches stored at O^^C for one week prior to transfer to 

ambient temperatures was lower than that in fruits which were held 

continuously for a week at room temperature to ripen before marketing 

(Pierson, 1966). 

Chilling Injury Retardation 

In order to store fresh commodities, which are sensitive to chilling, at 

temperatures lower than their critical chilling range, their tolerance to 

chilling conditions should be enhanced or the appearance of chilling 

injury symptoms should be delayed. Various postharvest techniques for 

preventing or alleviating chilling injury have been reported during the 

recent decades (Wang, 1993). 

One approach is to condition fruits at temperatures above the critical 

chilling level, to increase their tolerance to chilling during subsequent 

low-temperature storage. This method has been found effective in 

alleviating chilling injury in several citrus fruits. Preconditioning limes 

at 7-20°C for 1 week prior to storage at 1.5°C, or exposure of lemons to 5 

or 15°C for 1 week prior to storage at 0-2.2°C, reduced chilling injury 

during cold storage (Houck et al., 1990; Spalding and Reeder, 1983). 




Low-temperature conditioning was similarly effective for grapefruits: 

preconditioning grapefruits at 10-16°C for 7 days or at IT'^C for 6 days 

resulted in reduced chilling injury and decay development during cold 

storage of this chilling-sensitive fruit (Chalutz et al., 1985; Hatton and 

Cubbedge, 1982). Exposure of papaya fruits to 12.5°C for 4 days 

markedly reduced chilling sensitivity at storage temperatures below 

7.5°C (Chen and PauU, 1986). 

For certain commodities, a double-step, or 'stepwise', temperature 

conditioning has been found more effective in reducing chilling injury 

symptoms than a single temperature conditioning. The advantage of this 

technique was exhibited in mangoes by Thomas and Oke (1983), who 

found that preclimacteric mangoes exposed to 20°C for 1 day and then to 

15°C for 2 days were more tolerant of storage at 10°C than those treated 

with a single temperature conditioning. Marangoni et al. (1990) showed 

that tomatoes can also be acclimated to cold storage by gradual cooling: 

this included exposure to 12°C for 4 days, to 8°C for 4 days and to 5°C 

for 7 days. 

Another way to prevent or retard chilling injury is by the application 

of intermittent warming. Studies with various horticultural crops have 

shown that periodic interruption of cold storage by exposure of fruits to 

short periods of warming can reduce chilling injury symptoms and 

maintain fruit quality for longer durations (Wang, 1993). The optimum 

storage conditions with intermittent warming may vary greatly 

according to the cultivar, fruit maturity stage and growing conditions. 

For each fruit, however, interruption of chilling exposure with warm 

treatments must be performed before chilling injury has developed 

beyond a reversible stage. Wang (1993) emphasized the importance of 

correct timing and duration for intermittent warming treatments to be 

effective; if the critical time at chilling temperature has been exceeded 

and chilling injury progressed beyond recovery, the high temperature 

would only accelerate the degrading processes and the appearance of 

injury symptoms. On the other hand, warming treatments applied too 

early or too frequently result in excessively soft tissues, which are 

vulnerable to invasion by postharvest pathogens. 

Intermittent warming was found to be effective in reducing chilling 

injury in various cold-sensitive citrus fruit cultivars. Early studies by 

Brooks and McCoUoch (1936) had already shown that removal of 

grapefruits from 2°C storage for 1 day after 1 week and again after 2 

weeks markedly reduced the development of chilling injury symptoms, 

including pitting, scald and watery breakdown. Hatton et al. (1981) 




found that intermittent warming treatments in grapefruit made it 

possible to reduce the recommended dosage of the fungicide benomyl by 

50%, from 600 to 300 ppm, while still maintaining good decay control. 

The advantage of intermittent warming in enhancing fruit resistance to 

chilling injury has been proven for Olinda oranges: 2 weeks of 

intermittent warming to 15°C every 3 weeks, during storage at 3°C, 

delayed the onset of chilling injury by 15 weeks and greatly suppressed 

the incidence of damage during the subsequent 10 weeks of storage 

(Schirra and Cohen, 1999). Experiments with lemons have shown that 

intermediate heating to 13°C for 7 days after every 21 days enables them 

to be stored at 2°C, a temperature much lower than the optimal storage 

temperature for the fruit. Under these conditions, the percentage of peel 

pitting, characteristic of lemon fruits stored at 2°C, was markedly 

reduced and decay was suppressed (Artes et al., 1993; Cohen et al., 

1983). The technique of storing lemons at 2*^C with intermittent warming 

at 13°C has been adopted as a commercial method for long-term storage 

of these fruits (Cohen, 1988). 

Mature-green tomato fruits are susceptible to chilling injury at storage 

temperatures below 12®C. Chilling injury symptoms include surface 

pitting, uneven ripening or failure to ripen, and altered flavor. These 

symptoms are accompanied by increased disease development (Hobson, 

1981; Cheng and Shewfelt, 1988). Intermittent warming treatments have 

proved to be effective in reducing chilling injury in tomatoes, to an extent 

which depends on the cultivar and the time-temperature regime. Artes 

and Escriche (1994) found that storing the fruit at 9°C (but not at 6°C), 

with cycles of intermittent warming at 20°C for 1 day every 6 days, was 

effective in reducing chilling injury symptoms in two tomato cultivars, 

while fruits stored at a continuous 9°C were susceptible to Alternaria 

decay, which typically developed on the pitted area. Intermittent 

warming was more beneficial than intermittent cooling; pitting 

developed at 2°C in the intermittently cooled fruits (Artes et al., 1998). 

Chilling injury is not limited to fruits of tropical or subtropical origin. 

Cultivars of peaches, which were recommended for storage under 

refrigerated conditions, may develop chilling injury symptoms, including 

breakdown or woolly breakdown, discoloration, low juice content, and 

failure to ripen or to ripen normally. Intermittent warming at 20°C or at 

higher temperatures, for 1-3 days every 2 weeks, during storage at 0 or 

1°C, gave good control of chilling injury in peaches (Ben-Arie et al., 1970; 

Buescher and Furmanski, 1978); while intermittent warming at 18^C for 




2 days every 3 or 4 weeks reduced chilling injury symptoms in stored 

nectarines (Anderson and Penney, 1975). 

The time taken for an irreversible chilling injury to occur is usually 

very short for perishable produce with short storage life, such as 

cucumber or zucchini squash. Therefore, intermittent warming of these 

commodities must be applied earlier and more frequently (Cabrera and 

Saltveit, 1990; Wang, 1993). 

B. MODIFIED AND CONTROLLED ATMOSPHERES 

A controlled atmosphere (CA) or modified atmosphere (MA) around the 

produce is created by alterations in the concentrations of the respiratory 

gases in the storage atmosphere; these alterations include elevation of 

carbon dioxide (CO2) level, the reduction of oxygen (O2) tension, or both . 

Whereas the term 'CA storage' generally implies precise control of O2 and 

CO2 concentrations in the atmosphere, the term MA storage' is broader 

and may indicate any synthetic atmosphere, arising intentionally or 

unintentionally, in which the composition of its constituent gases cannot 

be closely controlled. 

Keeping fruits and vegetables in modified atmospheres is an old 

technique. Wang (1990), in his review of physiological and biochemical 

effects of controlled atmosphere on fruits and vegetables, mentions an 

ancient Chinese poem that describes the fondness for lychees of an 

empress in the 8th century. She demanded that freshly picked lychees be 

transported, on horseback, from southern China to her palace, a distance 

of about 1,000 km, and the fruit carriers discovered that lychees would 

keep well if the fruits were sealed inside the hollow centers of bamboo 

stems, along with some fresh leaves. Wang (1990) adds that while the 

poem did not explain the reason for this, we know today that the 

atmosphere produced within the bamboo stems by the continued 

respiration of the fruits and the fresh leaves, was probably the reason for 

the freshness of those lychees. During the last few decades, 

understanding of the physiological and biochemical processes in the fruit 

associated with alterations in the CO2 and O2 contents of the atmosphere 

has considerably increased. Alterations in the levels of the atmospheric 

gases were primarily aimed at suppressing the respiration and other 

metabolic reactions of harvested fruits and vegetables, and so retarding 

the ripening and senescence processes. During storage, many volatile 

compounds are evolved from the stored fruits and vegetables, and these 




accumulate in the storage atmosphere. Of these compounds, ethylene is 

apparently the most important. Since the accumulation of ethylene 

above certain levels may hasten the ripening and enhance senescence of 

many fruits and vegetables, its removal from the atmosphere may help 

to suppress the physiological processes related to ripening and 

senescence. 

However, for many fruits and vegetables, the factor limiting the 

extension of their useful life is the development of postharvest diseases. 

Many studies have indicated that modifications of the storage 

atmosphere, apart from their effects on the physiological processes of the 

host, can also retard postharvest disease development during storage (El 

Goorani and Sommer, 1981; Barkai-Golan, 1990). The effect of low O2 

levels or high CO2 levels on postharvest disease development can be 

direct - by suppressing various stages of the pathogen growth, and its 

enzymatic activity - or indirect - by maintaining the resistance of the host 

to infection by keeping it in a superior physiological condition. 

1. CONTROLLED ATMOSPHERE 

Direct Effects on the Pathogen 

Oxygen is required for normal respiration and growth of the pathogen. 

Suppression of growth by low O2 is most likely due to effects of electron 

transport on the cytochrome system, although other oxidative enzyme 

systems present in cells may also be suppressed by low oxygen (Sommer, 

1985). In most fungi, no growth occurs in the absence of molecular O2, 

but the reduction in the level of O2 required to inhibit the various stages 

of fungal growth varies considerably among species. In general, however, 

lowering the level of O2 from 21 to 5% has little or no effect on fungal 

growth. In order to obtain appreciable reduction of spore germination, 

mycelial growth and sporulation in many fungal species, O2 

concentrations of less than 1% are required (Wells and Uota, 1970), 

although spore sensitivity to low O2 does not necessarily match hyphal 

sensitivity. Wells and Uota (1970) showed that spore germination of 

Rhizopus stolonifer and Cladosporium herbarum in 1% O2 was about 

50% of that in air, and they found that the rate decreased gradually as 

the O2 concentration decreased from 1 to 0.25%. Spore germination of 

Alternaria alternata, Botrytis cinerea and Fusarium roseum, on the other 

hand, decreased significantly only when the O2 concentration was 0.25% 

or less (Fig. 23A). All cultures resumed normal growth when returned to 

air. Mycelial growth (as indicated by mycelial dry weight) of these fungi 

was inhibited by more than 50% when the O2 concentration was 4%. 




— 

•50 

0) CD 

o .^ 

a c 

0) Q) 

3: CJ) 

O o^ 

•P O 

05 O 

C T- 

1 « 

120 - 

lUU - 

80 - 

+-* c 

c — 

0 C 60 - 

P



For R, Stolonifer, however, a 50% inhibition was caused only at 2% 

O2 and some growth was still recorded at 0% (FoUstad, 1966, Wells and 

Uota, 1970). Differences between the sensitivity of spores and sporangia 

to low O2 were recorded for R, stolonifer: whereas fungal sporangia 

remained viable after 72 h of anoxia, only a few of the sensitive 

sporangiospores survived under these conditions (Bussel et al., 1969). 

The response of fungal sporulation to low O2 may also differ with the 

species: B. cinerea produces an abundance of aerial mycelium when 

grown in 1% O2, but no spores develop in this level of O2; sporulation of 

A. alternata and C. herbarum, on the other hand, was found even in 

0.25% O2, although mycelial growth was markedly reduced (FoUstad, 

1966). Lowering of O2 concentrations also suppressed sclerotium 

formation in species that naturally produce them during their life cycle. 

It was thus found that sclerotium production by Sclerotinia minor was 

more sensitive to low O2 than was radial growth and at 1% O2 no 

sclerotia were produced, although some growth was recorded (Imolehin 

and Grogan, 1980). 

Low oxygen tension of 1-3%, a range tolerated by many agricultural 

commodities in storage, also reduces growth of various decay-causing 

bacteria, such as Erwinia carotovora, E, atroseptica and Pseudomonas 

fluorescens, although some growth was recorded even at 0% O2 (Wells, 

1974). 

Carbon dioxide is essential for the growth of many aerobic 

microorganisms, since it is fixed in lactic, fumaric, citric and other acids 

of the Krebs cycle. However, although these microorganisms can fix CO2 

for their use, they cannot use it as a sole source of carbon for 

metabolism. High concentrations of CO2 may directly suppress fungal 

growth by retarding various metabolic functions, so causing lowered 

respiration (Sommer, 1985). The early studies of Brown, W. (1922b) and 

Brooks et al. (1932) had already demonstrated the inhibitory effect of 

high-C02 atmospheres on mycelial growth and spore germination of B, 

cinerea, R. stolonifer, Mucor spp. and other fungi. The retarding effect of 

CO2 on fungal growth is greater in the early phases of growth and at low 

storage temperatures (Brown, W. 1922b). 

Similarly to O2 concentration, that of CO2 required to inhibit spore 

germination and mycelial growth varies with the species (Wells and 

Uota, 1970) (Fig. 23B): spore germination of R. stolonifer, C. herbarum, 

and B. cinerea was inhibited by over 90% at 16% CO2, but levels as high 




as 32% had no effect on A, alternata spore germination. Similarly, growth 

of A, alternata, B, cinerea and C. herbarum were inhibited by 50% in an 

atmosphere of 20% CO2, whereas growth of F, roseum was inhibited by 

50% only when the CO2 level was elevated to 45%. Growth of colonies of 

the two bacteria, E, carotovora and E. atroseptica was also inhibited by 

high CO2 concentrations. On the other hand, in the absence of CO2 no 

growth of these fungi occurred at any O2 concentration (Wells, 1974). 

Combining low O2 with high CO2 concentrations does not necessarily 

result in additive effects. For certain fungi, such as Sclerotinia 

sclerotiorum, growth retardation induced by low O2 concentrations is not 

enhanced by elevating the CO2 content of the atmosphere. On the other 

hand, certain fungi, such as A. alternata and Penicillium expansum have 

been found to grow more slowly in CAs composed of low O2 (2.3%) and high 

CO2 (5%) than in air, although neither low O2 nor high CO2 alone caused 

any significant growth retardation (El-Goorani and Sommer, 1981). 

In contrast to the fact that CA usually retards the growth of the 

pathogen, it has been found that atmospheres low in O2 stimulated the 

fungus, Geotrichum candidum. Furthermore, a combination of low O2 

concentrations with 3% CO2 increased the stimulatory effect of the low 

O2 alone (Wells and Spalding, 1975). CO2, at concentrations higher than 

0.03%, has been found to stimulate the growth of several pathogenic 

fungi even when the O2 level was sufficiently low to limit fungal 

development, and the CO2 concentrations were not in themselves 

inhibitory (Wells and Uota, 1970). The growth stimulation by CO2 under 

these conditions has been attributed to fixation of CO2 into acids of the 

Krebs cycle, by fungal cells, and utilization of the derived energy for 

fungal growth. 

Several investigations have addressed the effects of CA on the 

enzymatic activity of the pathogen. Holding R, stolonifer cultures in a 

I0W-O2 atmosphere retarded their growth and also repressed fungal 

enzymatic activities. Pectin methyl-esterase (PME), polygalacturonase 

(PG) and cellulase (Cx) activities were directly related to fungal growth 

on potato-pectin culture filtrates, with the highest activities found in the 

normally aerated cultures (Wells, 1968). Similarly, a controlled 

atmosphere of 5% CO2 and 3% O2, which may be suitable for storing 

various apple cultivars, markedly reduced mycelial growth as well as the 

enzymatic activities of PG and polymethylgalacturonase (PMG) produced 

by Gloeosporium album and G. perennans (Edney, 1964). 




Indirect Effects on Pathogen Development 

It is well known that susceptibility of fruits to postharvest decay 

organisms generally increases with ripening. A controlled atmosphere 

may retard ripening and senescence processes of fruits and vegetables, 

through the suppression of respiration and other metabolic functions. 

Low O2 levels and high CO2 levels may also affect ripening by retarding 

or inhibiting the production and activity of ethylene, whose accumulation 

at certain levels in the storage atmosphere may result in the initiation of 

ripening or the enhancement of tissue senescence in fruits (Yang and 

Hoffman, 1984). 

A study carried out by Aharoni et al. (1986) showed that ethylene 

played a primary role in the initiation of broccoli inflorescence 

senescence, and that the senescence-retarding effects of a C02-enriched 

atmosphere were related to its ability to block ethylene action. 

Furthermore, the marked decrease in rot development observed in 

broccoli florets inoculated with Botrytis cinerea spores and stored in a 

C02-enriched atmosphere, was completely nullified by the addition of 10 

ppm ethylene to the atmosphere (Table 9). 

Removal of ethylene from long-term CA storage also delayed ripening 

in certain cultivars of apples, as expressed by improved firmness 

retention. This was especially apparent in fruit picked in its 

preclimacteric state (Knee, 1990). Along with the prolongation of the 

physiological life of apples, these conditions also led to decay suppression 

during storage (Bompeix, 1978; Sams and Conway, 1985). 

TABLE 9 

Effect of CO2 and ethylene on decay development in broccoli florets 

inoculated with Botrytis cinerea spores^ 

^ Index of decay^ 

(after 120 h of incubation) 

Air 4.0 

CO2(10%) 2.6 

C2H4 (10 ml 1-1) 4.2 

CO2 + C2H4 4^2 

1 From Aharoni, N., et al., (1986) 

2 1, no decay; 5, maximum decay development 




The ability of modified or controlled atmospheres to extend the 

physiological life of fruits have been closely related to their action on the 

host cell walls. As fruits ripen the pectic substances of the middle 

lamella, which are initially laid down by the cell in a relatively insoluble 

form, protopectin, become more soluble, and the tissue begins to soften. 

The increased solubility of the pectic substances and the softening render 

the tissue more vulnerable to maceration by pectolytic enzymes released 

by the pathogen. Storing the fruit in CA may retard changes in the pectic 

constituents, with longer retention of a firm texture, which is less 

vulnerable to pathogen attack (Smock, 1979). 

Under CA storage, when ripening and senescence are retarded, the 

fruit may retain its ability to produce antifungal compounds, normally 

characteristic only of young tissues. Such effects lead to disease 

inhibition and delay the transformation from quiescent to active 

infections, by prolonging the pathogen latency (Barkai-Golan, 1990). An 

exception to the association between ripeness and disease development 

is the bacterial soft rot of tomatoes caused by Erwinia carotovora, 

which develops even more rapidly in mature-green fruits than in 

mature ones (Parsons and Spalding, 1972). This phenomenon may 

explain why CA conditions, which delay ripening and reduce decay by 

Rhizopus and Alternaria in mature-green tomatoes do not result in 

bacterial soft rot control. 

In general, atmospheres containing about 2.5% O2, a level which is 

commonly maintained in CA storage, are most likely to suppress decay 

indirectly, by acting upon host resistance, rather than by acting directly 

on the pathogen, since most pathogenic fungi grow under these 

conditions. In order to suppress the main pathogens directly, O2 should 

be lowered to 1% or less. However, excessively low O2 concentrations 

result in anaerobic respiration and the accumulation of alcohols and 

aldehydes in the tissues, which leads to the development of off-flavors 

and irreparable damage to the tissues (Sommer, 1982). Similarly, 

elevating the CO2 level of the atmosphere above 5% suppresses fruit 

respiration, but has almost no direct effect on fungal growth. In order to 

inhibit fungal growth significantly, the level of CO2 must be increased to 

about 10% or more (Wells and Uota, 1970) (see Fig. 23B). However, only 

a few fruits can tolerate very high CO2 levels for extended storage or 

transit periods. Most fruits are injured under these conditions and 

develop off-flavors. 




Evaluation of CA Efficiency 

Apples are an example of fruits for which CA is aimed primarily at 

retardation of ripening (Blanpied, 1990). Low O2 and high CO2, in 

combination with a low temperature, delay the climacteric of the fruit 

more than low temperature alone. Striking benefits were obtained with 

such combinations, especially in apple cultivars that suffer from chilling 

injury at temperatures near 0°C, and those which have a short storage 

life (Salunkhe and Desai, 1982; Sommer, 1985). A considerable 

improvement in the storage quality of several apple cultivars occurs 

following a reduction of the O2 concentration in the atmosphere to 

1-1.5%, immediately after harvest (Couey and Williams, 1982). Later 

findings showed the advantage of CA in which O2 at less than 1% was 

combined with ethylene removal. Under these conditions apple fruits 

remained firmer and their resistance to pathogens, such as Penicillium 

expansum and Monilinia fructicola, was increased (Stow and Geng, 

1990). Studying the effect of decreased O2 concentrations combined with 

ethylene removal on Cox's Orange Pippin apples, Johnson et al. (1993) 

found that fruits stored in 0.75% O2 were firmer and showed lower 

incidence of Penicillium and Monilinia rots than those stored in 1.0 or 

1.25% O2. This was achieved through the reductions both in primary 

infection and in secondary infection by contact between infected and 

sound fruit. The development of Botrytis and Nectria rots, however, was 

unaffected by the lower oxygen concentrations. 

Low O2 storage (1%) also considerably reduced Botrytis development in 

Kiwifruits at 0°C, along with the delay of the fruit softening and ethylene 

production, which are tsîcal with infected fruit (Niklis et al., 1992). 

Sitton and Patterson (1992) found that CA storage of several apple 

cultivars with CO2 concentrations above 2.8% was more effective than 

I0W-O2 oxygen atmospheres, in reducing decay development incited by 

Botrytis cinerea and P. expansum. However, the response to CO2 

treatment depended on the physiological age of the fruit. Apples that 

had been stored in air at 0°C for 8 months, prior to the CO2 treatment 

showed increased skin discoloration, whereas younger apples, even 

under higher CO2 did not show skin darkening. It was suggested that 

high-C02 CAs could be beneficially used to control postharvest disease in 

freshly harvested apples (Sitton and Patterson, 1992). High CO2 (25%) 

was also found beneficial for cherry fruits by totally preventing decay 

development during both refrigeration and shelf life (Brash et al., 1992). 

Avocados benefit greatly from CA storage. Early studies had already 

shown that the climacteric respiration peak of avocado fruits was delayed 




by decreasing the O2 concentration from 21% to 2.5% or raising the CO2 

level from 0.03% to 10% (Biale, 1946; Young et al., 1962). Storing Florida 

avocados for 45-60 days at lO'^C in an atmosphere of 2% O2 and 10% CO2 

resulted in a remarkable reduction in anthracnose (Colletotrichum 

gloeosporioides), the main cause of spoilage during storage and 

marketing, and at the same time prevented chilling injury, which is 

generally accompanied by increased susceptibility to fungal attack 

(Spalding and Reeder, 1975). Since such concentrations of O2 and CO2 

have only a slight inhibitory effect on the growth of C. gloeosporioides, 

decay control in this atmosphere was attributed to maintenance of the 

fruits in the firm mature-green stage and thus to the maintenance of the 

quiescence of the fungus. In contrast to avocados, CA was only slightly, 

or not at all beneficial to papayas, for which, similarly, decay is the chief 

limiting factor in storage extension (Spalding and Reeder, 1974). 

Atmospheres of 10-20% CO2 or more reduce the incidence of decay 

caused by S. cinerea and Rhizopus stolonifer in several cultivars of 

strawberries, as exhibited, not only during cold storage but also after 

removal of the fruit to shelf-life conditions, without causing objectionable 

off-flavors (Harris and Harvey, 1973). Under these conditions decay 

suppression may be attributed both to the direct effect of the CA on the 

pathogens and to its effect on the physiological deterioration of the fruit. 

The resistance of the fruit to Botrytis and Rhizopus decay in a 

C02-enriched atmosphere was hypothesized to be the result of the high 

levels of acetaldehyde and ethyl acetate produced by fruit under such 

conditions (Shaw, 1969). An atmosphere enriched with CO2, at 10, 15 or 

20%, retarded storage decay of blueberries by 1-2 days after the berries 

had been removed from the CA and held at 21°C, whereas an atmosphere 

of 2% O2 was ineffective on its own in suppressing decay, and did not 

enhance the effect of the CO2 atmosphere (Ceponis and Cappellini, 1985). 

Decay suppression by high CO2 (20%) has also been recorded in 

muskmelons after 14 days at 5°C followed by shelf-life conditions (Stewart, 

1979), while CA (10-20% CO2 + 10% O2) along with ethylene absorbance 

prolonged the storage life of Galia melons, through the maintenance of 

tissue firmness and decay suppression (Aharoni et al., 1993a). 

CA had considerable benefits over air for the long-term refrigerated 

storage of white cabbage (Geeson and Browne, 1980): storing cabbage in 

an atmosphere of 5-6% CO2 + 3% O2 at 0-2''C resulted in the retardation 

of general senescence effects, such as yellowing, toughening and loss of 

flavor, and in the inhibition of physiological disorders and the repression 

of decay development during prolonged storage. 




One of the beneficial effects of CA storage is its ability to reduce 

chilling injury in certain sensitive crops. Several studies indicated that 

prestorage treatments of grapefruits with high concentrations of CO2 

were effective in inhibiting the development of pitting in cold storage 

(Hatton and Cubbedge, 1982). However, no reduction in chilling injury 

was induced by CA in other citrus fruits, such as limes and lemons 

(Spalding and Reeder, 1983; McDonald, 1986). Intermittent exposure of 

unripe avocado fruits to 20% CO2 reduced chilling injury during storage 

at 4°C (Marcellin and Chaves, 1983). An atmosphere of 2% O2 and 10% 

CO2 prevented the development of anthracnose {Colletotrichum 

gloeosporioides) at 7.5°C and reduced chilling injury at 4.4°C in Fuchs 

avocados (Spalding and Reeder, 1975). Low-oxygen atmospheres were 

found to delay the development of chilling injury symptoms while 

reducing their severity, in peaches stored at 5°C (Ke et al., 1991), and to 

ameliorate chilling-induced pitting in zucchini squash (Mencarelli et al., 

1983; Wang and Ji, 1989). 

2. CONTROLLED ATMOSPHERE WITH CARBON MONOXIDE 

The addition of CO to the atmosphere results in the suppression of 

various fungi sensitive to the gas, such as Penicillium digitatum, 

P. italicum and Monilinia fructicola. The effectiveness of this fungistatic 

gas is pathogen dependent and is greatly enhanced in combination with a 

I0W-O2 atmosphere (2.3%). The addition of high CO2 to this atmosphere 

further enhanced fungal growth inhibition, because of the additive 

effects of CO and CO2 (El Goorani and Sommer, 1979, 1981). 

The possibility of using CO atmosphere for decay suppression during 

storage has been studied for various commodities. Kader et al. (1978) 

showed that the addition of CO (5 or 10%) to a I0W-O2 atmosphere (4%) 

reduced the incidence and severity of the gray mold decay in Botrytis 

cmerea-inoculated tomatoes at their mature-green or pink stage. When 

added to air, CO increased the CO2 and ethylene production rates and, in 

parallel, hastened the ripening of mature-green tomatoes, while the 

addition of CO to a I0W-O2 atmosphere had no or very little effect on 

these physiological responses. Studying the effects of various 

atmospheres on the severity of watery soft rot of celery caused by 

Sclerotinia sclerotiorium, Reyes (1988) found that whereas a I0W-O2 CA 

(1.5%), with or without high CO2 (16%), reduced decay development only 

slightly, a combination of 7.5% CO + 1.5% O2 suppressed disease 

significantly without causing undesirable effects on celery quality. 

Applying mixtures of O2 and CO to B. cmerea-inoculated apples inhibited 




lesion development, and the inhibition was closely related to the decrease 

in the O2 level from 8% to 2% (Sommer et al., 1981). The development of 

the CO-sensitive Monilinia fructicola in peaches was completely inhibited 

in cold storage by the addition of CO to a I0W-O2 atmosphere (4%); 

however, a normal rate of rot development was resumed once the fruits 

were transferred to air at 20°C (Kader et al., 1982). 

In spite of the efficiency of CO combined with I0W-O2 atmospheres in 

decay suppression, the use of this gas is limited, mainly because of its 

high toxicity to humans and the fear of inhalation of the poisonous gas by 

workers. The use of CO is additionally limited by its tendency to mimic 

ethylene effects, as has been found in tomatoes, strawberries, sweet 

cherries and peaches (Barkai-Golan, 1990). 

3. MODIFIED ATMOSPHERE PACKAGING 

Sealing certain commodities in polymeric film packages promotes the 

extension of their storage life as a result of the formation of a modified 

atmosphere (MA) within the package. This practice is referred to as 

modified atmosphere packaging (MAP). The method is based on the 

alteration in the composition of the gases around the commodity within 

the sealed package. Elevation in the CO2 level along with the decrease in 

the O2 level in the package result in a decreased respiration rate of the 

stored fruits or vegetables, and the extension of their physiological life. 

The levels of the gases accumulated in the sealed package are 

determined by the respiration rate of the commodity, the storage 

temperature, the permeability of the film material to the atmospheric 

gases, the ratio of the produce weight or surface area to the area of 

semi-permeable film, among other factors (Smock, 1979). 

In looking for suitable wrapping for strawberries exported from Israel 

to Europe by sea, it was found that polyethylene or polyvinyl chloride 

(PVC) film enabled fruit to be stored at 0-2°C for 10 days, followed by two 

additional days at 20°C (Barkai-Golan, 1990). The prolongation of the 

storage life was evident in the firmness and the fresh appearance of the 

fruit and the calyx, and the considerable reduction of decay, caused 

mainly by Botrytis cinerea. The incidence of decay was correlated with 

the level of CO2 accumulated in the atmosphere around the fruit. During 

cold storage the level of CO2 ranged between 1.5 and 2.5%, but the low 

temperature controlled decay. Following transfer to 20°C, CO2 levels rose 

up to 7.5-12%; under these conditions, the suppressive effect of CO2 on 

decay development is attributed mainly to its effect of delaying the 




senescence processes of the fruit and maintaining host resistance. Decay 

during prolonged storage can be almost prevented by application of 

fungicidal sprays during the flowering season preceding polymeric film 

packaging (Aharoni and Barkai-Golan, 1987). 

A reduction in the incidence of brown rot caused by Monilinia 

fructicola was recorded in sweet cherries packaged in polyethylene bags 

and stored at 0.5°C for 42 days. Disease reduction under these conditions 

was attributed to the modified atmosphere of 5.1% O2 and 11.4% CO2 

produced within the package during storage (Spotts et al., 1998). A 

further reduction in disease incidence was achieved when a preharvest 

fungicide treatment had been applied prior to packaging, or when the 

fruit was treated with a postharvest antagonistic yeast (see the chapter 

on Biological Control). The disease-suppression effect of CO2 in sweet 

cherries was regarded as fungistatic, since decay developed normally 

when the fruit was returned to air (De Vries-Paterson et al., 1991). In a 

study with d'Anjou pear fruits, the combination of MAP (2-6% O2 and 

0.5-1% CO2) with an antagonistic yeast as a biological control agent was 

found to reduce the blue mold {Penicillium expansum) better than the 

modified atmosphere without the yeast or the yeast application followed 

by storage in air (Miller and Sigar, 1997). 

MAP has been found to improve the storability of perishable 

commodities such as fresh herbs, broccoli and green onions. Packaging 

broccoli heads in sealed polyethylene bags inhibited both senescence and 

decay development in the flower buds, and kept the produce green and 

fresh during prolonged cold storage (0.5°C) followed by 2 days of shelf-life 

conditions (20°C) (Aharoni et al., 1986). The extension of the useful life of 

broccoli was attributed to the high respiration rate of this vegetable, 

which resulted in CO2 accumulation from 2-3% in cold storage to 6-8% 

after transfer to shelf-life conditions (20°C); O2 concentrations decreased 

to about 6% at shelf life. Decay suppression under these conditions was 

attributed to the delay in host senescence by the CO2, because of the 

ability of the gas to curb the action of the endogenic ethylene, which plays 

a primary role in the control of aging of broccoli. The improvement of 

storability of broccoli heads in sealed PVC film is illustrated in Photo 1. 

Green onions were found to resist a relatively high concentration of 

CO2 (more than 15% for a short period) produced in sealed 

microperforated film (Aharoni et al., 1996b). The elevated level of CO2, 

combined with the decreased level of O2 inside the packaging reduced 

growth, yellowing and decay of the green leaves (Photo 2). 




Photo 1. Delay of aging and decay in broccoli packaged in sealed polyethylene 

bag during storage and after bag opening. 

Photo 2. The effect of modified atmosphere packaging with a microperforated 

film on green onion. 




The retardation of yellowing and decay in CA storage has been 

reported for various leafy vegetables, for which CO2 and O2 

concentrations are controlled during storage (Lipton, 1987). MAP has 

proved effective for extending the postharvest lives of green herbs, 

including dill, parsley, coriander, chives, chervil, watercress and sorrel. 

Such an atmosphere was achieved by packaging fresh herbs in 

non-perforated polyethylene-lined cartons (Aharoni et al., 1989). 

Retardation of senescence processes in these yellowing-susceptible herbs 

was again attributed to the high CO2 levels accumulated within the 

package and the ethylene-antagonistic action. However, only little 

commercial success was reported for the use of this procedure with green 

herbs, because of the difficulty in controlling the O2 and CO2 variations 

within the package. The drastic decrease in the O2 level, combined with 

the increased CO2 level, may lead to anaerobic respiration, fermentation 

and development of an off-flavor (Cantwell and Reid, 1993). 

In order to obtain beneficial fungistatic concentrations of CO2 within 

polymeric packages of sweet corn, retail packages were stored at 2°C 

within additional plastic liners (Rodov et al., 2000). The MA generated in 

these packages by the corn respiration complied with the recommended 

range of 5-10 kPa CO2, and inhibited mold development. However, when 

the produce is transferred to shelf life, the outer layer should be opened 

to counterbalance the enhancement of the corn respiration at the higher 

temperature and to prevent O2 depletion and off-flavor development. 

Further advances in modified atmosphere packaging involve the 

development of 'active' or *smart' films which, in addition to their ability 

to generate the conventional modified atmosphere around the 

commodity, are also capable of actively improving the atmosphere 

(Church, 1994). These properties have been imparted by introducing into 

the package, or into the packaging materials, compounds capable of 

altering the atmosphere composition by absorbing atmospheric gases, 

vapors, wetness and by-products. Ethylene, which is produced by fruits 

and vegetables during storage, may accumulate in the package and 

stimulate their ripening and senescence processes that may lead to 

shortening their postharvest life. By the use of ethylene-scavenging 

materials, the smart packaging is capable of retarding these processes, so 

providing a significant shelf-life extension. As a matter of fact, packaging 

broccoli in such a film doubled the shelf life of the fresh produce (Church, 

1994). The shelf life of banana fruits sealed in bags made of polyethylene 

treated with the anti-ethylene compound, 1-methylcylclopropane, was 

extended from 16 days in non-treated polyethylene bags up to 58 days in 



Means for Maintaining Host Resistance 13 5 

the anti-ethylene treated bags. Ethylene-mediated ripening changes were 

markedly delayed, suggesting that MAP, in combination with the potent 

antagonist of ethylene action, may serve as a technology for long-distance 

transport of green bananas without refrigeration (Jiang et al., 1999). 

For food products other than fruits and vegetables, developments in 

'smart' packaging include films impregnated with anti-microbial 

compounds that actively retard the development of harmful 

microorganisms during storage. Church (1994) indicates that changes in 

the gas composition within the package during storage may provide an 

indirect indication of the condition of the product. Therefore, labels which 

change color in response to the concentrations of CO2, O2 and other gases 

in the atmosphere could serve as indicators to detect damage in the 

packaged product. 

With the development of polymeric films suitable for the providing the 

advantages of MAP, and the increased choice of packaging materials, the 

number of products that can benefit has increased. Today, thanks to the 

introduction of the 'smart packages', the in-package atmosphere offers 

the potential to extend the technology to a wider range of products and 

markets. 

4. HYPOBARIC PRESSURE 

Hypobaric or low-pressure (LP) storage, similarly to modified or 

controlled atmosphere storage, is designed to delay ripening and 

senescence processes in fruits and vegetables, and to extend their 

postharvest life. In this procedure, the commodity is maintained in a 

vacuum-tight and refrigerated container, held at subatmospheric 

pressure and continually ventilated with humidified air at relative 

humidity (RH) of 80-100% (Burg, 1990; Dilley et al., 1982). With the 

reduction in the atmospheric pressure beneath 760 mm Hg, the oxygen 

level in the atmosphere is reduced. Under continuously ventilated partial 

pressure, CO2, ethylene and various volatile by-products of metabolism 

rapidly diffuse out of the commodity and are flushed from the storage 

chamber. As a consequence of the low partial pressure of O2 and the low 

levels of ethylene in the atmosphere, ripening and senescence of fresh 

fruits and vegetables are delayed and storage life is extended. 

Following the disclosure of hypobaric storage technology by Burg and 

Burg in 1966, extensive investigations have been conducted with a wide 

range of fruits and vegetables (Laugheed et al., 1978). Although the 

high-RH conditions under LP storage appear ideal for fungal growth and 

decay development, several studies report on the beneficial effects of the 




combination of these conditions on decay control. Hypobaric pressure has 

direct fungistatic effects on fungal spore germination and mycelium 

growth of various storage fungi. Apelbaum and Barkai-Golan (1977) 

showed that the degree of inhibition increased with the reduction in 

pressure below 150 mm Hg. Reducing the pressure from 760 to 100 mm 

Hg inhibited spore germination oi Penicillium digitatum, while inhibition 

of spore germination of Botrytis cinerea and Alternaria alternata occurred 

only at 50 mm Hg. Such a level had no effect on germination of 

Geotrichum candidum spores. Reducing the pressure to 25 mm Hg totally 

prevented spore germination of the first three fungi mentioned but had 

almost no effect on the germination of Geotrichum spores (Fig. 24). 

Colony growth and the consequent fungal sporulation of the four fungi 

were markedly reduced at 50 mm Hg, while at 25 mm, Geotrichum was 

the only fungus exhibiting some growth. Transfer of inhibited cultures 

from hypobaric to atmospheric pressure resulted in renewed growth, 

suggesting that there was no irreversible damage to the fungi. 

150 100 50 

Pressure(mm Hg) 

Fig. 24. Hypobaric pressure effects on spore germination of Penicillium 

digitatum (n), Botrytis cinerea (o), Alternaria alternata (A) and Geotrichum 

candidum var. citri-curantii (•). (Reproduced from Apelbaum and Barkai-Golan, 



25



The reduction in fungal growth under hypobaric pressure can be 

attributed to the reduction in the partial O2 tension, which decreases 

gradually with the decrease in the total pressure to 100 or 50 mm Hg. 

However, growth retardation may be due, at least in part, to other 

factors, such as reduction in the CO2 partial pressure or the accelerated 

escape of volatile compounds from the tissues (Apelbaum and 

Barkai-Golan, 1977). 

Beneficial effects of hypobaric storage have been reported for various 

fruits. Storing apples under LP conditions resulted in delayed softening, 

control of physiological disorders and reduced decay development, which 

resulted in extended shelf life after removal from storage (Dilley et al., 

1982; Laugheed et al., 1978). Several Florida cultivars of mango which 

ripened in air at shelf-life temperature, showed less anthracnose 

(Colletotrichum gloeosporioides) and stem-end rot (Diplodia natalensis) 

when they had previously been stored under LP at 13°C. The reduction 

in decay coincided with a retardation in fruit ripening, permitting a 

prolonged storage at 13°C (Spalding and Reeder, 1977). Similarly, 

exposure of papayas to LP at 10°C during shipment, inhibited both 

ripening and disease development as compared with fruit stored in 

refrigerated containers at normal atmospheric pressure (Alvarez, 1980). 

Studies on LP storage of avocados suggested that atmospheres both low 

in O2 and high in CO2 are required for successful suppression of 

anthracnose development and for increasing the percentage of acceptable 

fruit retained after softening in air under normal pressure (Spalding and 

Reeder, 1976). Studies with various Yruit-vegetables', such as tomatoes, 

peppers and cucumbers, found a high incidence of decay after storage at 

subatmospheric pressure; in order to suppress decay, the combination of 

LP with postharvest fungicidal treatments was required (Bangerth, 1974). 

C. GROWTH REGULATORS 

Plant growth regulators, or plant hormones, may - similarly to low 

temperatures - suppress decay development indirectly, by retarding 

ripening and senescence processes in fruits and vegetables, and 

maintaining the natural resistance of young tissue. 

Prolonged storage of citrus fruits generally results in the development 

of stem-end fungi, mainly Diplodia natalensis, Phomopsis citri, 

Alternaria citri and Fusarium spp. Applying the synthetic auxin 

2,4-dichlorophenoxy acetic acid (2,4-D) to citrus fruits prior to storage 



13 8 Postharvest Diseases of Fruits and Vegetables 

delayed aging of the stem-end button, which is the usual point of attack 

by the stem-end fungi, resulting in considerable reduction of stem-end rot 

(Schiffmann-Nadel et al., 1972). The pathogenic fungi lodge in the 

button area in a quiescent state, and the delayed dropping of the button 

from the fruit which results from 2,4-D treatment delays their transition 

to the active state, so retarding decay development. The effectiveness of 

2,4-D in preserving green buttons and suppressing decay development, 

has been proven both by using preharvest spraying in the orchards and 

by postharvest treatment by dipping the fruits in aqueous solutions of 

2,4-D or of waxes containing 2,4-D (Stewart, 1949; Schiffmann-Nadel et 

al., 1972). Most lemons grown in California are treated with 2,4-D before 

storage to delay senescence of the button, and a wax formation 

containing 2,4-D is applied to various citrus cultivars in South Africa and 

Israel, to control stem-end rot during long-distance shipment and 

prolonged storage (Ben-Arie and Lurie, 1986). 

Gibberelins, which are hormones found naturally in plant tissue, may 

also function in a similar way. The increased susceptibility of citrus fruits 

to Penicillium digitatum and P. italicum, as they become physiologically 

older, may be due to the reduction in resistance to fungal entry as well as 

to an improvement of the rind as a medium for the pathogen growth. 

Gibberelic acid (GA3) application delayed the senescence of the rind of 

Navel oranges, as exhibited by softening, the appearance of intercellular 

spaces, weakened or broken cell walls and increased fruit resistance to 

infection (Coggins and Lewis, 1965). 

A more recent study (Ben-Yehoshua et al., 1995) indicated that 

dipping lemon fruits in GA3 (50 or 100 ppm) or in 2,4-D (200 ppm) prior 

to storage retards the decrease in the antifungal citral compound in the 

fruit rind (the flavedo), suppresses the antifungal activity of the rind and 

results in reduced decay during storage. The inhibition of the 

decomposition of the antifungal compounds in the rind is the basis of the 

mode of action of the growth regulators in decreasing decay in the 

harvested lemon fruit. 

Pre-harvest sprays with GA3 (20 mg ml^) applied during the 

development of persimmon fruits were effective in reducing decay caused 

by the black spot disease (Alternaria alternata)^ which typically appears 

in the high humidity environment beneath the calyx in stored persimmon 

fruits (Perez et al., 1995). As a result of GAs treatment, the calyx of the 

fruit remained erect until harvest and fruit softening was inhibited, and 

in parallel, the fruit area covered with the black spot was markedly 

reduced during 3 months of storage at O'^C. The plant hormone had no 




effect on fungal development in vitro, or in vivo on inoculated fruits, and 

its effect on decay development was attributed to the enhancement of 

fruit resistance. A pre-harvest combination of GA3 and iprodione 

treatments may further reduce the percentage of fruits rendered 

unmarketable by the black spot disease (Perez et al., 1995). 

Retardation of senescence by growth regulators has also been reported 

for leafy vegetables. Fresh herbs are prone to accelerated senescence 

because of their high rate of metabolism, which is further increased 

following harvesting and handling procedures. Their accelerated 

senescence is expressed in decreased levels of RNA, proteins, fats, 

carbohydrates, organic acids and vitamins, and increased enzymatic 

activity of RNase, protease, lypase and other hydrolytic enzymes, as well 

as those associated with fat oxidation. These changes are accompanied by 

chloroplast destruction and chlorophyll decomposition, resulting in a 

rapid yellowing of the leaves (Aharoni, 1994). During harvesting, grading 

and packaging, when the leaves lose water because of evaporation, and 

suffer from mechanical injury, the senescence process is further 

enhanced and is accompanied by increased ethylene and respiration 

levels. The endogenous ethylene continues to accelerate chlorophyll 

destruction and leaf senescence (Meir et al., 1992). Along with their 

aging, the leaves lose their natural resistance to pathogens and tend to 

rot. In many cases, the senescence process is controlled by plant 

hormones; it is accompanied by decreases in the levels of 

growth-stimulating hormones - gibberelin, cytokinin and auxin, and 

increases in those of growth-inhibiting hormones - ethylene and abscissic 

acid (ABA). It is not surprising that application of synthetic hormones - 

gibberelin, cytokinin and a very low concentration of auxin - was very 

efficient in delaying leaf senescence, while application of ABA or ethylene 

accelerated it (Aharoni and Richmond, 1978). Delay of senescence by 

gibberelin and cytokinin is attributed to their antagonistic effects on 

ethylene and to their ability to neutralize the stimulation of chlorophyll 

destruction caused by ethylene (Aharoni, 1989). 

The delay of aging conferred by GA3 has also been found in several 

fresh herbs which tend to yellow, such as chives, dill, chervil, coriander 

and parsley (Aharoni, 1994). In a laboratory experiment with chives, a 

dip in GA3 solution (10 ppm) was found to prevent the accelerated 

respiration caused by the cutting stress, and then to prevent the 

climacteric respiration associated with leaf senescence. A marked effect 

on respiration was achieved by spraying chives in the greenhouse, close 

to harvesting. Such a treatment delays chlorophyll destruction, retards 




the climacteric respiration and maintains the level of climacteric 

ethylene (Aharoni, 1994) (Fig. 25). Along with the delay in the senescence 

process, there was a reduction in leaf decay during storage. 

Experiments conducted in Israel with Romaine lettuce showed that a 

single spray in the field with GA3 (25 ppm), several hours prior to 

harvest, delayed deterioration during storage by delaying leaf yellowing. 

In parallel, there was a reduction in the rate of decay caused mainly by 

Sclerotinia sclerotiorum and soft rot bacteria (N. Aharoni and R. 

Barkai-Golan, unpublished data). 

The addition of GA3 (10 ppm) to a disinfecting solution containing 

thiabenolazole (TBZ) and chlorine resulted in considerable delay in celery 

senescence as expressed by the delayed yellowing of the petioles and 

leaflets. Along with the maintenance of celery freshness, fungal soft rots 

caused by S. sclerotiorum and Botrytis cinerea, and bacterial soft rot 

caused by Erwinia carotovora were reduced (Barkai-Golan and Aharoni, 

1980). It is interesting to note that the decrease in decay following GA3 

treatment is greater than the decrease caused by the chemicals alone, 

and that is manifested mainly in the prevention of decay progress along 

the petiole during storage. TBZ itself exhibits activity similar to that of 

the plant hormones, and it has a synergistic effect with GA3 in the delay 

of senescence processes in celery leaves and petioles. No direct effect of 

7S 80 

c 

o 

CD 

CL 

60-h 

-£ 40 

0) 

O 

20 

ot 

12.5 25.0 37.5 

GA3 concentration (ppm) 

Chlorophyll _L i n 

Respiration 

Ethylene 

50.0 

-f 0.8 = CT 

-f 0.6 P •*- 

I ^ ? 

-f 0.4 O ^ 

Fig. 25. Effect of GA3 concentrations applied to chives immediately before harvest, 

on levels of chlorophyll, respiration (CO2 emanation) and ethylene production rates 

after 7 days at 20°C in darkness (Reproduced from Aharoni, 1994). 


0.2 

0



GAs on fungal and bacterial growth in vitro has been recorded, and its 

effect on the development of pathogens has been attributed to 

maintaining the decay resistance of the fresh celery heads. 

Dipping harvested celery heads in GA3 solution with TBZ and chlorine 

served for many years in Israel as a standard treatment in celery 

intended for export. A recent study showed that spraying the fields with 

GA3 before harvest delayed the natural decrease in the antifungal 

compound marmesin (Afek et al., 1995a). Maintenance of the high level 

of marmesin in the petiole tissues after harvest was suggested to be the 

mode of action by which the plant hormones inhibit decay development in 

stored celery. 

Delay of deterioration by the synthetic auxin, h-naphthalene acetic 

acid (NAA) was studied in eggplants (Temkin-Gorodeiski et al., 1993). 

Eggplant fruits deteriorate during prolonged storage, mainly because of 

accelerated senescence of the calyx, which precedes deterioration of the 

fruit. Dipping the calyx in NAA solution (200 ppm) maintained calyx 

freshness, which is an important aspect of the appearance of the fruit, 

but it did not prevent decay development. The addition of the fungicide 

prochloraz (900 ppm) retarded both calyx senescence and decay 

development, caused mainly by A. alternata, and kept the fruit firm. 

Dipping the whole fruit gave even better results and the residual level of 

prochloraz in such fruits was less than 0.34 mg T^, which is lower than 

the maximal level approved by the authorities in Europe. A triple 

combination of NAA, fungicide solution (Prochloraz, 900 mg l-i active 

ingredient) and bulk packaging in polyethylene-lined bags with 

water-absorbent materials, prevented water condensation and 

maintained eggplant quality along with reduced incidence of decayed 

fruit during storage (Fallik et al., 1994b). The higher levels of CO2 

obtained within the bags (1.2%) may have partially contributed indirectly 

to the inhibition of decay development, by delaying fruit senescence. Such 

a combination could considerably expand the export potential of 

high-quality fruit (Fallik et al., 1994b). 

The use of auxins is limited since, in high concentrations, these 

compounds might accelerate ethylene production and thus enhance the 

aging process. In general, plant growth regulators may be of special 

importance mainly for fruits and vegetables that cannot be stored at low 

temperatures because of their cold sensitivity, as well as in developing 

countries where cold storage cannot be applied. 




D. CALCIUM APPLICATION 

Another means of suppressing storage disease by maintaining or 

enhancing the natural resistance of fruits and vegetables to pathogens, is 

to increase the calcium content of the various plant organs (Conway et 

al., 1994b). Calcium is an essential element which influences the growth 

and fruiting of plants and contributes to preserving the structural 

integrity and functionality of membranes and the cell wall during fruit 

ripening and senescence. 

Calcium treatments applied to improve fruit and vegetable quality 

have primarily addressed the association of low calcium content in plant 

tissue with the development of physiological disorders. Calcium 

treatment may reduce storage disorders, such as bitter pit and internal 

breakdown in apples (Bangerth et al., 1972; Reid and Padfield, 1975) or 

the internal brown spot in potato tubers (Tzeng et al., 1986). However, 

many reports have indicated that an increase in tissue calcium content 

also led to reductions in fungal and bacterial decay. It was thus found 

that pre-harvest calcium sprays reduced the rate of storage losses caused 

by Gloeosporium spp. in apples (Sharpies and Johnson, 1977), or Botrytis 

and Geotrichum rots in stored grapes (Miceli et al., 1999), while 

postharvest calcium treatments reduced the rate of the blue mold disease 

caused by Penicillium expansum in this fruit (Conway and Sams, 1983). 

Similarly, bacterial soft rot caused by Erwinia carotovora pv. atroseptica 

in potato tubers decreased as tissue calcium increased (McGuire and 

Kelman, 1984). 

Various methods for increasing the calcium concentration in storage 

organs have been investigated. Applying Ca to harvested fruit seems to 

be the best method for increasing the calcium content in apples; active 

infiltration procedures with CaCb solution, such as vacuum or pressure 

that force solutions into the fruits, were more effective in reducing decay 

following inoculation with P, expansum, than dipping into the solution 

(Conway et al., 1994a). Vacuum infiltration of Ca(N03)2 solutions also 

increased potato tuber calcium and was efficient in reducing soft rot in 

tubers inoculated with E, carotovora pv. atroseptica, although an increase 

in the calcium content of potato tubers could have been achieved by 

calcium fertilization during the growth period (McGuire and Kelman, 

1984). 

Calcium enters the tissues through lenticels (Betts and Bramlage, 

1977), but cracks in the cuticle and epidermis may also provide 

important points of entry. The extent of cracking may, therefore, play a 




significant role in calcium intake and affect the efficiency of the 

treatment. However, the amount of calcium taken in during treatment 

also depends on the growing conditions and environmental factors, and 

can vary with the cultivar and with the state of maturity of a given 

cultivar. Experiments with 'Golden Delicious' apples showed that calcium 

absorption in fruits picked too early (2 weeks before the prime harvest 

period) and treated with CaCb solution was low and did not inhibit 

decay. However, fruit picked 2 weeks after prime harvest absorbed large 

quantities of calcium that markedly reduced decay development, but 

caused severe calcium injury. The optimum calcium treatment at prime 

harvest reduced decay 40% with no injury (Conway and Sams, 1985). 

Most of the calcium that penetrates into the host tissue seems to 

accumulate in the middle lamella region of the cell wall. The 

calcium-induced resistance of storage organs (fruits or tubers) to 

postharvest pathogens has been attributed to an interaction between the 

cell wall pectins and Ca ions. By binding to pectins in the cell wall, Ca 

ions contribute to maintaining the structural integrity of the cell wall. 

Pectins are chains of polygalacturonic acid residues into which rhamnose 

is inserted. The rhamnose insertion causes kinks in the chain, which 

allow the attachment of Ca ions. The cations form bonds between 

adjacent pectic acids or between pectic acids and other polysaccharides, 

forming cross bridges which make the cell walls less accessible to the 

action of pectolytic enzymes of the pathogen (Preston, 1979). When 

apples were infiltrated with Ca, or potato tubers were fertilized with high 

Ca or infiltrated with Ca, the quantity of Ca bound to the cell walls 

increased (Conway et al., 1987; McGuire and Kelman, 1984), and cell 

walls became less vulnerable to the pectolytic enzymes of the pathogen. 

The reduction in decay caused by P. expansum in apple fruits has thus 

been correlated, at least in part, to the improved structural integrity of 

the tissues imparted by an increase in the Ca content of the cell walls 

(Conway et al., 1987). 

Analysis of the pectic fractions of grape berries cv. Sauvignon, known 

to be resistant to Botrytis cinerea, showed that the content of host 

cell-wall pectins decreased following infection with this pathogen. At the 

same time, the proportions of water and a chelator-soluble protein 

increased. The changes in the pectin composition of the berries were 

markedly less pronounced when the berries were infiltrated with calcium 

prior to infection (Chardonnet et al., 1997). It was suggested that 

dimethylated proteins produced by B. cinerea are probably protected by a 

rapid calcium chelator. 




Applying CaCl2 solution to developing table grapes cv. Italia during 

the season increased calcium content in the whole berries and in the peel, 

resulting in reduced decay incidence during cold storage and the 

subsequent shelf life (Miceli et al., 1999). In this case, calcium-treated 

berries infected by B, cinerea showed an increase in the cellulose content 

of cell walls, but no changes in the water-soluble pectin or hemicellulose 

contents. However, a higher protopectin content was recorded in the cell 

walls of Ca-treated berries after infection, indicating the involvement of 

Ca ions in the stabilization of the cell-wall structure that led to increased 

resistance to the pathogen (Miceli et al., 1999). 

Although prolongation of storage life as a result of calcium application 

is thought to be due mainly to the role of calcium in ameliorating 

physiological disorders and thus indirectly reducing pathogen activity, 

direct effects of calcium on the pathogen have also been recognized. 

Calcium interfered with spore germination and germ tube elongation of 

P. expansum and B. cinerea (Conway et al., 1994a). At low concentrations, 

calcium may also directly inhibit polygalacturonase activity. Conway et 

al. (1994a), who studied the commercial potential of Ca for maintaining 

the quality of apples in storage, found that the Ca content of fruits from 

trees that had been sprayed with CaCb solutions throughout the growing 

season was significantly increased but not enough to be expected to 

retard pathogens infecting wounded fruits. Pressure-infiltrating fruits 

after harvest with high CaCl2 concentrations (4-8%) resulted in a tissue 

Ca level that would potentially maintain fruit firmness and retard decay, 

but it was suggested that proper sanitation, to reduce fungal infection 

that could arise from the pressure-infiltration procedure, should also be 

implemented before this technique could be seriously considered for 

commercial use. Furthermore, such concentrations stimulate pectate lyase 

activity; in order to inhibit this activity, higher calcium concentrations 

would be required (Conway et al., 1994a). 

Biggs (1999), who studied the potential role of preharvest calcium 

supplementation of apples in reducing pathological diseases, found that 

calcium salts directly suppress bitter rot caused by Colletotrichum 

gloeosporioides and C. acutatum. Calcium chloride and calcium 

propionate at 1000|ag of calcium per milliliter had no effect on conidial 

germination, but markedly inhibited germ-tube growth of these 

pathogens. These two salts, as well as calcium silicate, also inhibited 

mycelial growth: fungal dry weight in liquid culture media was reduced. 

When calcium salts were applied to wounded apples prior to inoculation 




with Colletotrichum spp., fruits treated with calcium chloride and 

calcium propionate exhibited delayed formation of acervuli (aggregated 

conidial structure of Colletotrichum) by the fungi, and markedly 

reduced lesion development. Reduced rates of infection were 

demonstrated in field trials, when fruits treated with weakly diluted 

applications of calcium salts were inoculated with conidia of either 

C. gloeosporioides or C. acutatum. 

Biggs et al. (1997) demonstrated the direct effect of 18 calcium salts 

(at 600 mg calcium per liter) on the colony growth of Monilinia fructicola, 

the cause of brown rot, mainly on stone fruits. All the salts, except 

calcium formate, calcium pantothenate and dibasic calcium phosphate, 

reduced fungal growth on amended potato-dextrose agar after 7 days at 

20°C. Calcium propionate was the most inhibitory compound, reducing 

growth by 90%. Calcium hydroxide, calcium oxide, calcium silicate and 

calcium pyrophosphate reduced growth by approximately 65% compared 

with the control, and were not significantly different from each other. In 

general, salts that were inhibitory on amended agar medium were also 

inhibitory in a liquid culture (potato-dextrose broth). However, the effect 

of calcium salts on fungal growth in the liquid culture, but not on the 

agar medium, was correlated with the incidence and severity of disease 

in detached non-wounded fruit inoculated by spraying with Monilinia 

spore suspension. It was suggested that calcium salts are more likely to 

remain in continuous contact with fungal mycelium in the liquid culture 

system and that future tests should utilize growth in a liquid medium for 

preliminary determination of toxicity levels (Biggs et al., 1997). All the 

salts examined, except dibasic calcium phosphate and calcium tartrate, 

inhibited polygalacturonase (PG) activity of M fructicola. The greatest 

inhibition of PG was caused by calcium propionate (85%), followed by 

calcium sulfate, tribaric calcium phosphate, calcium gluconate and 

calcium succinate. These four salts reduced PG activity by approximately 

75%. Regarding these results, it is interesting to note that disease 

incidence and severity following calcium treatment were correlated with 

PG activity. The ability of calcium salts to reduce fungal growth and PG 

activity directly suggested the possibility that the effects of calcium on 

the incidence of brown rot and lesion diameter may result partly from 

suppressed pathogen activity. 

Calcium propionate, which proved to be the most active salt inhibitor 

of fungal PG, has been extensively used as a food additive, and is known 

as an inhibitor of molds and bacteria in stored grain (Raeker et al., 1992). 

Furthermore, a substance such as calcium propionate, that has no 




activity against yeasts, could be used to supplement biological control by 

yeasts (Biggs et al., 1997). As a matter of fact, calcium salts were 

actually found to improve the efficacy of yeast biocontrol against various 

postharvest pathogens (McLaughlin et al., 1990; Droby et al., 1997). 

Combined Treatments 

Calcium application can be combined with postharvest fungicides, heat 

treatment or biological control, allowing a lower concentration or level of 

either of the components while maintaining the effectiveness of the 

treatment in decay suppression. For combined treatments comprising 

heating or biological control with Ca application, see the chapter on 

Physical Means - Heat Treatments, or that on Biological Control - 

Combined Treatments and Integration into Postharvest Strategies. 

New combinations which have recently been described (Saftner et al., 

1997) include CaCl2 application plus polyamine biosynthesis inhibitors. 

Three different polyamine biosynthesis inhibitors have each been found 

to reduce the in vitro growth of B, cinerea and P. expansum, either alone 

or in combination with CaCh, whereas CaCb applied alone had no effect 

on fungal growth. However, experiments in vivo showed that pressure 

infiltration of each of the polyamine inhibitors or of CaCb solution into 

apples, reduced soft rot development by the two pathogens. A 

combination of Ca with a polyamine inhibitor enhanced the decay 

reducing effect of either of the separate treatments. It was, therefore, 

concluded that a combination of specific polyamine inhibitors with CaCb 

pressure infiltration is capable of effective decay suppression, although 

fruits treated with such a combination were less firm than fruits treated 

with Ca alone (Saftner et al., 1997). 



CHAPTER 9 

CHEMICAL CONTROL 

Chemical substances intended for controlling postharvest diseases 

may be fungicides and bactericides (lethal to fungi and bacteria), or 

fungistats and bacteristats (inhibiting fungal and bacterial development). 

To serve its purpose, the chemical should come into contact with the 

pathogen; the minimal effective concentration is pathogen dependent. 

Chemical treatments can be applied under various strategies, with 

various timings: (1) preharvest application to prevent infection in the 

field; (2) sanitation procedure to reduce the level of inoculum in the 

environment to which injured fruits or vegetable are liable to be 

exposed; (3) postharvest application to prevent infection through wounds 

and to eradicate or attenuate established infections, so as to prevent 

their development and spread during storage. In order to choose the 

appropriate strategy for decay control, we have to understand the mode 

of infection of the pathogen, its time of infection, and the environmental 

factors that may affect disease development. 

A. PREHARVEST CHEMICAL TREATMENTS 

The possibility of controlling well established pathogens by 

postharvest disinfection is very small since most fungicides are unable to 

penetrate deeply into the tissues, and effective concentrations of the 

fungicide would not reach deep-seated infections. The effective way to 

reduce infections initiated in the field, including quiescent infections, is 

the application of broad-spectrum protective fungicides to the developing 

fruit on the plant, in order to prevent spore germination or infection 

establishment in the lenticels or in floral remnants of the fruit. In 

practice, preharvest fungicides are combined with other preharvest 

treatments applied against pests in the field. 

A classic example of an effective preharvest treatment is the 

preventive spraying of citrus fruit in the grove with fixed copper 

compounds, to inhibit incipient infections of brown rot {Phytophthora 

citrophthora) in the fruit peel. In this case, the preventive sprays are 

essential for disease control, since penetration of the fungal zoospores 




into the fruit occurs on the tree. Furthermore, the success of the 

preventive sprays depends on the time of infection: since germination of 

the zoospores depends on water, the sprays should be apphed prior to the 

rains (Timmer and Fucik, 1975). Oranges are sprayed with benomyl 

before harvest, to prevent the development of stem-end rot, which arises 

from infections of Diplodia natalensis and Phomopsis citri initiated on 

the tree in the button of the fruit where they remain quiescent until the 

buttons start to separate from the fruit (Brown, G.E. and Albrigo, 1972). 

Protective sprays in the plantation have been widely used to prevent 

anthracnose (Colletotrichum gloeosporioides) in various tropical and 

sub-tropical fruits, since this fungus penetrates the young fruit on the 

tree, and establishes a quiescent infection. The developing fruits are 

sprayed to prevent spore germination and subsequent formation of 

appressoria and infective hyphae, which are the quiescent stages of the 

fungus. Plantation spraying every 7-14 days successfully prevents 

anthracnose on mangoes (Prusky et al., 1983), avocados (Darvis, 1982; 

Muirhead, 1981a), papayas (Alvarez et al., 1977) and bananas (Griffee 

and Burden, 1974; Slabaugh and Grove, 1982). In addition to the 

reducing the incidence of anthracnose, preharvest spraying of papaya 

fruits reduces the infectivity of Phytophthora and Alternaria and the 

inoculum levels of the stem-end fungi, Botryodiplodia, Mycosphaerella 

(Ascochyta), and other fungi. These sprays reduce the 'infection pressure' 

on the fruit at the time of harvest and enable hot water treatment, 

applied after harvest, to complete the control of fruit decay (Aragaki et 

al., 1981). Furthermore, field sprays of mancozeb on papaya were also 

found to reduce postharvest Rhizopus soft rot, probably by reducing 

field-initiated fruit diseases caused by Colletotrichum and Phomopsis 

species; lesions caused by these fungi may serve as courts of infection for 

Rhizopus stolonifer, which requires 'wounds' to penetrate the host 

(Nishijima et al., 1990). 

The gray mold (Botrytis cinerea), which is the major decay in stored 

strawberries, results in part from fungal penetration through the aging 

floral parts at the base of the fruit, and the formation of quiescent 

infections in the young, developing fruits while still in the field. Upon 

harvest, no initial infections are visible, but the fungus begins active 

development during marketing, producing typical rot 'nests'. Therefore, a 

good control should both reduce the spore contamination or the inoculum 

level on the flowers, and prevent the formation of quiescent infections in 

the young fruits (Sommer et al., 1973). Fungicidal sprays with the 

systemic benzimidazole compounds, during the flowering period. 



Chemical Control 149 

successfully controlled both preharvest and postharvest diseases in the 

early 1970s (Jordan, 1973). Following the emergence of benzimidazoleresistant 

B, cinerea strains because of the very specific site of action of 

the chemicals (Dekker, 1977), dicarboximide fungicides (including 

iprodione and vinclozolin), which have a different mode of action, were 

adopted for preharvest application to soft fruits. Sprays with iprodione 

several times during the flowering season have provided good control of 

Botrytis rot during subsequent storage (Aharoni and Barkai-Golan, 1987; 

Dennis, 1983a). In this case, the use of preharvest treatments not only 

matches the mode of fungal penetration, but is probably the only way to 

apply chemicals for decay control because of the great sensitivity of the 

fruit to wetting. However, dicarboximide-resistant strains have also been 

found on soft fruits (Hunter et al., 1987). Furthermore, similarly to the 

benzimidazole compounds, the dicarboximides are also ineffective against 

Mucor and Rhizopus species, and their use may result in increased 

incidence of postharvest diseases caused by these fungi (Davis and 

Dennis, 1979). Following the discovery of dicarboximide-tolerant 

B. cinerea strains in strawberry fields and in cucumber greenhouses, 

control strategies, including the use of dicarboximides in combination 

with other fungicides or in rotation with an unrelated fungicide, were 

developed (Katan, 1982, Katan and Ovadia, 1985; Creemers, 1992). 

Field treatments are generally less effective than postharvest 

treatments in controlling wound infections, since only part of the 

field-sprayed fungicides will remain attached to the product to protect 

wounds that occur later. In addition, fungicides that do remain on the 

surface of the produce may be removed by washing and waxing (Eckert, 

1978). However, several investigations have indicated that in some cases 

field sprays may also be effective in reducing wound decay or lenticel 

decay, because of the sedimentation of the fungicide in the infection site 

and its maintenance in appropriate levels. It was thus possible to reduce 

wound infection in i?/ii2:opus-inoculated peaches by sprays of dicloran 

applied in the orchard 1 week before harvest, while orchard sprays with 

benomyl reduced lenticel infection by Gloeosporium spp. in apples, and 

orchard sprays with benomyl, thiabendazole and carbendazim in the 

1970s controlled the blue mold (Penicillium expansum) and the gray 

mold {B. cinerea) in stored pears (Eckert and Ogawa, 1988). Similarly, 

sprays of benomyl and thiabendazole, applied in the grove 30 days before 

harvest, markedly reduced the green mold (Penicillium digitatum) in 

stored oranges (Brown and Albrigo, 1972; Gutter and Yanko, 1971). 




Although a postharvest treatment is usually more effective in cases 

where wound infection is involved, preharvest sprays may be a suitable 

control means when considerable harvest injury is involved and handling 

practices make postharvest treatment difficult to apply soon after 

harvest. Thus, orchard sprays may be the best means for reducing decay 

in peaches that will be subjected to controlled ripening after harvest, and 

in oranges that will be subjected to degreening, since both of these 

practices often increase decay by wound pathogens (Eckert and Ogawa, 

1985). 

When dealing with preharvest sprays, it is important to emphasize the 

need for careful selection of the fungicides. With the emergence of fungal 

strains resistant to chemicals, we frequently need to replace a chemical, 

previously proven to be effective, with another chemical or with two or 

several different compounds. Furthermore, there is a significant risk that 

residues from preharvest treatments will encourage the buildup of 

fungicide-resistant strains of the pathogens, and these will not allow us 

to benefit from postharvest treatment with the same fungicide or with 

related fungicides with similar chemical structures (Eckert and Ogawa, 

1988). 

B. SANITATION 

Fruits and vegetables that have been injured during harvest or 

shipping, and have succeeded in avoiding infection by wound pathogens, 

are still liable to come into contact with the pathogens during packing or 

storage. Since disease development requires the presence of a given 

pathogen along with an available wound for penetration, a reduction in 

either of these factors will lead to the suppression of disease 

development. 

Wounding can be minimized by careful harvesting, sorting, packaging 

and transportation, including preventing the fruit from falling at all 

stages. Regarding the avoidance of wounds one should remember that 

physiological injuries caused by cold, heat, oxygen deficiency, and other 

environmental stresses, also predispose the commodity to attack by 

wound pathogens. A general reduction in wound formation should also 

take such factors into consideration, even when no external symptoms 

can be distinguished. 

The level of inoculum may be reduced by careful and strict sanitation 

procedures. The air of the packinghouse permanently carries an 




abundance of pathogenic spores, which may originate from infected 

fruits and vegetables covered with spores, or from plant remnants in 

the packinghouse or its surroundings, which serve as substrates for 

many pathogenic fungi. During the citrus packing season, spores of 

Penicillium digitatum and P. italicum form the most conspicuous 

components of the spore population in packing houses and their vicinity. 

Fruit containers, the equipment in the packinghouse, as well as the 

workers' hands and tools, all bear pathogenic fungal spores during this 

season. Fresh fruits arriving at the packinghouse may, therefore, come 

into contact with pathogenic spores from any of these sources 

(Barkai-Golan, 1966). 

Many fruits and vegetables have to be cleaned and washed on arrival 

at the packinghouse, in order to remove soil particles, dust or other 

contaminants. Water that does not contain disinfectants becomes heavily 

contaminated by fungal spores and bacterial cells and may infect 

products treated with non-recirculated water. Chlorine is the principal 

disinfectant used to sanitize wash water in packinghouses. Solutions of 

hypochlorous acid and its salts (sodium or calcium hypochlorite) are the 

most effective and economical agents available for destroying 

microorganisms in water and they have been applied widely to reduce 

bacterial contamination of the water used to wash or to hydrocool fruits 

and vegetables (Eckert and Sommer, 1967). Elemental chlorine (CI2) 

reacts with water to form hypochlorous acid (HOCI), hydrogen ion, and 

chloride ion: 

CI2+H2O '^—^ HOCl + C1-+H+ 

Hypochlorous acid is a weak acid (pka=7.46), which dissociates to 

hypochlorite ion (OCl") and H+: 

HOCl ^—^ 0C1-+H+ 

Hypochlorous acid and OCl" are in equilibrium in the solution, but the 

antimicrobial effectiveness of the chlorine is correlated with the 

concentration of the hypochlorous acid in the solution (Bartz and Eckert, 

1987). The relative concentration of the hypochlorous acid and its 

antimicrobial activity depend on the pH of the solution (Fig. 26). 

If the initial concentration of hypochlorous acid is not sufficient to 

kill microbes on contact, the hypochlorous acid formed by conversion 

from hypochlorite ion should become involved, provided the exposure 

period is long enough and the pH is not too high to inhibit the 




TD 

O 

CO 

C/> 

ZJ 

O 

u. 

i2 

JZ 

o CL 

>» 

X 

100 

80 t 

60 

40 

20 

0 

Fig. 26. Relationship between pH and the relative concentration of undissociated 

hypochlorous acid (HOCl) in a solution of chlorine. (Reproduced from Bartz and 

Eckert, 1987 with permission of Marcel Dekker Inc.). 

conversion (Robbs et al., 1995). Examining the effect of chlorine solutions 

on cells of the bacterium, Erwinia caratovora subsp. carotovora and the 

fungus, Geotrichum candidum suspended in water, Robbs et al. (1995) 

showed that the chlorine solution toxicity was correlated with free 

chlorine concentrations, the pH of the solution and the 

oxidation-reduction potential of the solutions. The oxidation-reduction 

potential was directly correlated with logic of the chlorine concentration 

at each pH. It was also found that cells of JE. carotovora subsp. carotovora 

were 50 times more sensitive to chlorine than were conidia of G. 

candidum, with populations of 10'^ cells or 10'^ conidia per milliliter, 

respectively. Populations of Erwinia were reduced below detectable 

levels (> 102 cells ml-i) by approximately 0.5, 0.5 or 0.75 mg l-i of free 

chlorine at pH 6.0, 7.0 or 8.0, respectively. With conidia of Geotrichum, 

25, 25 and more than 3 mg l-i, respectively, were required to produce 

similar levels of efficacy. 

In addition, the hypochlorous acid attacks organic matter, and its 

concentration in the solution should always be appropriately determined 

(Bartz and Eckert, 1987). Although a few ppm active chlorine may be 




sufficient to destroy bacterial cells suspended in clean water within 

seconds, 50-500 ppm at pH 7.5-8.5 is frequently recommended for 

commercial conditions, necessitating killing bacteria in packinghouse 

water containing a considerable amount of soil and organic matter. 

Sulfamic acid and other amines have been added to the water to form 

N-chloramines, in order to stabilize the concentration of the active 

chlorine in the solution. Surfactants are sometimes added to the water to 

improve the wetting of vegetable surfaces and to enhance the cleaning 

capability of the chlorinated water (Bartz and Kelman, 1984). The use of 

chlorine dioxide (CIO2) as a disinfectant has been evaluated for water 

containing high levels of organic matter. This compound may be effective 

for packinghouse water systems since its efficiency is not significantly 

influenced by pH in the presence of organic matter and it does not react 

with amines or ammonia, while reacting slowly with organic matter 

(Bartz and Eckert, 1987). 

Because of its instability in the presence of organic matter, chlorine is 

not effective in killing microorganisms embedded within injured tissue 

or in natural openings of the host, such as stomata and lenticels. The 

effectiveness of chlorine lies, therefore, in its ability to reduce the level of 

the inoculum or to eliminate most of the waterborne pathogenic 

microorganisms that might inoculate the product during the treatment. 

In other words, the main contribution of chlorine solutions to decay 

control is the prevention of the buildup of the pathogenic microbial 

population in the water used for washing and hydrocooling the harvested 

produce. The effectiveness of chlorine on the human pathogen 

Escherichia coli, which is responsible for food poisoning outbreaks linked 

to the consumption of contaminated vegetables, has recently been 

studied. Dipping inoculated Cos lettuce leaves or inoculated broccoli 

florets in hypochlorite solutions was found to reduce E. coli counts 

significantly more than water alone, but did not eliminate the bacterium 

population. The reduction in E, coli cells was dependent on the time of 

exposure and the concentration of free chlorine (Behrsing et al., 2000). 

Immersing fruits and vegetables in appropriate disinfectants may 

actually remove most of the fungal spores from their surfaces. However, 

a product that has undergone disinfection may be rapidly reinfested with 

pathogenic spores, either directly by the continuous fall of airborne 

spores on their surfaces, or by contact with infested equipment in the 

packinghouse. An appropriate sanitation system should therefore ensure 

both the removal of pathogen sources from every corner and the 




disinfection of the atmosphere and the equipment in the packinghouse 

and storage rooms. 

Removal of pathogen sources may be achieved through the immediate 

disposal of every rotted fruit or vegetable, or by immersing it in a 

disinfectant solution in a special container, which would eradicate fungal 

spores and prevent their dispersion. For disinfecting packinghouses or 

store rooms, including their equipment, it is possible to use one of the 

following disinfectants: formaldehyde, isopropyl alcohol, quadronic 

ammonium compounds, captan or other chemicals. A standard 

commercial practice in fruit packinghouses is to disinfect the atmosphere 

with formaldehyde, boxes and other equipment with quaternary 

ammonium compounds and the surface of the fruit with a solution of 

active chlorine (Eckert, 1990). Steam may also be used to sanitize citrus 

boxes (Klotz and DeWolfe, 1952). 

C. POSTHARVEST CHEMICAL TREATMENTS 

Since open wounds, created during harvesting, handling and 

packaging, are the major sites of invasion by postharvest wound 

pathogens, the protection of wounds by chemicals will considerably 

decrease decay in storage. Among the various types of 'wounds' we 

should also include injuries created in severing the crop from the plant or 

cuts created deliberately during handling procedures, such as stem cuts 

in banana hands or petiole cuts in celery intended for export. Other 

potential sites of infection are the natural openings in the host surface, 

such as lenticels and stomata, whose sensitivity to infection is increased 

by wounding or after washing the commodity in water. An efficient 

disinfection process should reach the pathogenic microorganisms 

accumulated in all these sites. 

Disinfection should be applied as close as possible to the time of 

exposure to infection, since a fungal spore located in the wound may, 

under appropriate conditions, germinate in several hours and initiate its 

establishment in the tissue. The allowable time between inoculation 

(harvesting) and the chemical application, if the treatment is not to lose 

its effectiveness, depends on several factors: the rate of spore 

germination and mycelial growth of the pathogen, the level of host 

resistance, the prevailing environmental conditions, and the 

penetrability of the tissue to the chemical. 




In some instances, however, several hours delay between the picking 

and the chemical treatment will not reduce the effectiveness of the 

treatment, and may even increase it. The increased efficiency of the 

delayed treatment may be related to the fact that germinating spores are 

more sensitive to the treatment than dormant spores (Eckert, 1978). 

Since, in practice, a few hours will generally elapse between harvest and 

treatment, application of the chemical should take place as soon as 

possible after harvest in order to avoid further development of the 

germinating spores in the host tissue. In the case of quiescent infections, 

such as anthracnose (Colletotrichum gloeosporioides) in tropical and 

sub-tropical fruits, and lenticel rotting (Gloeosporium spp.) in apples, in 

which the pathogen remains confined to the tissue beneath the peel, 

because of host resistance, it is permissible to delay the systemic 

fungicide application until several weeks after inoculation (Eckert, 1978). 

During the last 50 years, more than 30 organic compounds have been 

introduced for controlling decay by postharvest application. The selection 

of the appropriate compound depends on: (a) the sensitivity of the 

pathogen to the chemical substance; (b) the ability of the substance to 

penetrate through surface barriers into the infection site; and (c) the 

tolerance of the host, as expressed both by injury and other phytotoxic 

effects, and by any adverse effect upon the quality of the product (Eckert 

and Ogawa, 1985). The 'first generation' of postharvest fungicides 

includes diphenyl, sodium orit/io-phenylphenate, dicloran and 

sec-butylamine. These compounds are effective in preventing decay 

caused by wound pathogens, such as species of Penicillium and Rhizopus, 

but they have little effect on quiescent infections, or other infections 

situated within the host tissue (Eckert, 1977). 

Biphenyl (diphenyl) is a fungistat that has been used extensively by 

citrus exporters for more than five decades. It played a most important 

role in the development of distant markets for citrus fruits and of the 

world trade in these fruits. The fungistat may be impregnated into paper 

wraps on each individual fruit, or into paper sheets placed beneath and 

above the fruits within the container. It sublimes slowly into the 

atmosphere and protects the fruits during the entire period of shipping to 

distant markets. The main function of biphenyl is the inhibition of 

sporulation of Penicillium spp. (P. digitatum and P. italicum) on 

decaying fruits. Because of this action, biphenyl prevents contact 

infection of adjacent sound fruits by fungal spores which normally cover 

the surface of decayed fruit. However, this compound is only weakly 




effective against stem-end rot fungi, such as Diplodia and Phomopsis, 

and does not protect the fruit against initiation of infection by either 

Penicillium spp. or stem-end pathogens. Biphenyl is not active against 

Geotrichum, Alternaria or Phytophthora (Eckert and Eaks, 1989). 

Although biphenyl is still sometimes used in export shipments, mainly 

by impregnation into paper sheets covering the fruit in the container, its 

commercial utilization is discouraged by consumer resistance to the 

characteristic odor of the treated fruit and by undesired residues on the 

fruit. 

Sodium or^/io-phenylphenate (SOPP) is an example of a broadspectrum 

fungicide, effective against many postharvest pathogens. It is 

used mostly against fungal pathogens but is also known for its 

antibacterial properties. The undissociated phenol (or^/io-phenylphenol) 

penetrates with comparative ease into the surfaces of various fruits and 

vegetables, causing phytotoxicity and leaving considerable residues on 

the fruit. However, a solution of SOPP containing excess alkali to 

suppress hydrolysis of the salt, with a pH around 11.5, is both effective 

and quite safe for treatment of several fresh fruits and vegetables. These 

include citrus fruits, apples, pears, peaches, tomatoes, peppers, 

cucumbers, carrots and sweet potatoes. 

Although the intact cuticle of the fruit is not permeable to aqueous 

solutions of the fungicide, the crop is generally rinsed lightly with fresh 

water to remove most of the fungicide from the surface of the product, 

leaving a significant residue at the injured site. The fungicide has, 

therefore, a double function: (1) it eradicates fungal spores and bacteria 

cells infesting the surface of the treated commodity; and (2) through 

accumulation in the wound site, it prevents infection via wounds during 

storage and marketing (Eckert, 1978). 

The intensive and continuous use of biphenyl and SOPP has led to the 

development of Penicillium digitatum and P. italicum strains resistant to 

these chemicals (Dave et al., 1980; Eckert and Wild, 1983; Houck, 1977). 

Furthermore, Penicillium isolates that are resistant to SOPP show 

cross-resistance to biphenyl, which is structurally related to it. 

Sec-butylamine (amino-butane) was developed as a postharvest 

fungicide for citrus fruits in the mid 1960s (Eckert and Kolbezen, 1970). 

It has a relatively narrow spectrum of antifungal activity and its main 

function is the prevention of wound infection by Penicillium digitatum 

and P. italicum; it does not, however, suppress their sporulation on 




decaying fruit. The compound has some activity against stem-end rot but 

has no effect on Geotrichum, Alternaria or Phytophthora. Since 

sec-butylamine has low phytotoxicity and toxicity to mammals, the fruit 

does not have to be rinsed after treatment (Eckert and Ogawa, 1985). 

Sec-butylamine can be applied to the harvested fruit as a salt solution 

to protect oranges during the period of ethylene degreening; or it can be 

added to wax formulations applied to lemons before storage. The 

fungicide may also be volatilized and applied as a fumigation treatment 

which inhibits the development of Penicillium after the injuries on the 

surface of the fruit become alkaline by absorption of the amine vapor. In 

addition, residues of neutral sec-butylamine salts persist in the injuries 

following the fumigation treatment (Eckert and Kolbezen, 1970). 

Captan N-trichloromethylmercapto-4-cyclohexene-l,2-dicarboximide), 

which is a bicarboximide fungicide, has been proven effective as a 

postharvest dip against decay development in various fruits and 

vegetables, such as strawberries, peaches, cherries, pears, figs and 

potatoes (Eckert and Sommer, 1967). However, captan is not well suited 

to postharvest treatment because, when applied as a wettable powder 

suspension at effective concentrations, it may leave visible residues of 

powder on the surface of the fruit. 

Dicloran (2,6-dichloro-4-nitroaniline, DCNA, botran) is effective 

against several postharvest fungi. It is particularly efficient in 

controlling soft-watery rot caused by Rhizopus stolonifer in stone fruits 

and sweet potatoes, for which Rhizopus is the main postharvest 

pathogen. The capability of dicloran to penetrate to a depth of 11 mm 

into peaches (Ravetto and Ogawa, 1972) explains the inhibitory action of 

dicloran treatment against established lesions of Rhizopus on peaches. 

However, this treatment is less effective against other decays of stone 

fruits, such as the brown rot caused by Monilinia fructicola, and the blue 

mold caused by Penicillium expansum (Daines, 1970), and 

dicloran-resistant strains of R. stolonifer have also been reported 

(Webster et al., 1968). The effectiveness of dicloran against postharvest 

pathogens depends on the level of the fungicide persisting on the fruit 

after treatment, and the optimum deposit of dicloran for brown rot 

control is much greater than that required for adequate control of 

Rhizopus rot. 

The effect of dicloran against M fructicola can be greatly improved by 

heating the fungicide suspension or combining it with another fungicide. 




such as benomyl or iprodione, which are active against Monilinia (Eckert 

and Ogawa, 1988). 

Sulfur dioxide (SO2) is appHed as a postharvest fumigation to grapes 

in order to eradicate spores of Botrytis cinerea, to inhibit very superficial 

infections on the fruit and, above all, to prevent the contact spread or 

'nesting' of the fungus on grapes during storage. The tolerance of grapes 

to SO2 is higher than that of all other fruits and vegetables; however, 

even for grapes an effective combination of concentration and time has to 

be carefully chosen in order to compromise between effective decay 

control and minimum fruit injury as exhibited by bleaching and stem 

drying (Eckert and Ogawa, 1988). In a recent study Coertze and Holz 

(1999) found that exposure of grapes to SO2, similarly to prolonged cold 

storage, reduced the resistance of the berry to infection by individual 

conidia of B. cinerea, and could result in an increased rate of lesions. 

Individual conidia did not induce disease on the fresh, resistant berries. 

A common fumigation procedure consists of an initial application of 

0.5% SO2 (v/v) for 20 min, immediately after harvest, followed by 

low-concentration (0.1-0.2%) fumigation for 30-60 min every 7-10 days 

during the storage period. Another procedure involves applications of 

reduced levels of sulfur dioxide (0.2% SO2) three times weekly, which 

results in less residual bisulfite in the grapes, with less bleaching than 

the traditional high dosage treatment (Marois et al., 1986). In California, 

Australia, South Africa, Chile and Israel, the exposure of grapes to SO2 

during export shipments is carried out by adding a bisulfite or 

metabisulfite salt to the packaging material (Nelson, 1985; Guelfat-Reich 

and Safran, 1973). The SO2 is released into the atmosphere around the 

grapes and prevents infection with S. cinerea when in contact with 

infected berries. Liquid or solid SO2 formulations which have been 

developed (Combrink and Truter, 1979; Guelfat-Reich et al., 1975; 

Kokkalos, 1986), release the SO2 after the package has been closed, and 

maintain efficient concentrations (5-10 ppm) of SO2 in the atmosphere, 

which prevent contact-infection during prolonged storage. Studies of the 

efficacy of a range of S02-generating pads in stored grapes showed that 

some properly regulated pads were capable of controlling Botrytis rot in 

table grapes even when moderate levels of initial inoculum were present 

(Mustonen, 1992). 

SO2 fumigation at 1600 ppm for 20-30 min or 3200 ppm for 5 min gave 

almost complete control of Botrytis storage rot of kiwifruit in New 

Zealand (Cheah et al., 1993). Absorption of SO2 by the fruit gradually 




increased as the duration of fumigation increased from 5 to 30 min; and 

total SO2 residues were proportional to exposure time and treatment 

rate, and declined sharply after fumigation. No SO2 injury was observed 

on the fruit at the concentration-time combinations tested. 

The possibility of using SO2 solutions against the green mold in citrus 

fruits has been examined in Penicillium digitatum-inoculaited lemons 

(Smilanick et al., 1995). Immersion of inoculated fruits in 2% SO2 

solutions reduced green mold incidence without injuring the fruit, but 

heating of the solution was needed to attain acceptable efficacy (see 

Combined Applications in the chapter on Physical Means - Heat 

Treatments). 

The benzimidazole compounds - thiabendazole (TBZ), benomyl, 

carbendazim (methyl-2-benzimidazole carbamate - MBC) and 

thiophanate-methyl ~ were introduced as postharvest fungicides in the 

late 1960s. The two systemic fungicides, TBZ and benomyl, have 

undoubtedly added a new dimension to the control of postharvest 

diseases. These compounds, whose action is associated with the 

inhibition of mitosis (Davidse, 1988), are active against a broad spectrum 

of pathogenic fungi. They have been used all over the world to control: 

decay of citrus fruits caused by the two wound molds, Penicillium 

digitatum and P. italicum and by the stem-end fungi, Diplodia natalensis 

and Phomopsis citri; the brown rot caused by Monilinia fructicola in 

stone fruits; blue mold (Penicillium expansum), gray mold (Botrytis 

cinerea) and lenticel rot (Gloeosporium spp.) in apples; anthracnose 

(Colletotrichum gloeosporioides) in banana, papaya, mango and other 

tropical fruits (Eckert, 1977, 1990); and black rot (Ceratocystis paradoxa) 

in pineapple (Eckert and Ogawa, 1985). 

The strong inhibitory effects of the thiabendazole compounds, 

especially benomyl, have been attributed to their systemic property and 

their ability to penetrate the wax and the cuticle of the host surface to 

reach and inhibit the pathogen beneath them (Ben-Arie, 1975; Phillips, 

1975). This property enables the fungicides to act also against quiescent 

infections, such as anthracnose in tropical fruits, or stem-end rot at the 

button of citrus fruits. Both TBZ and benomyl act as antisporulants of 

Penicillium on decaying citrus fruit and, therefore, inhibit infection of 

adjacent fruits during storage and shipment. Benomyl at equivalent 

concentrations is usually more effective in controlling fruit decay than 

the other benzimidazole fungicides. This is in accordance with its ability 

to penetrate into the cuticle of the fruit and the stem button. In providing 




control of banana crown rot caused by several fungi (including 

Colletotrichum, Fusarium, Verticillium, Botryodiplodia and Cephalosporium) 

and anthracnose {Colletotrichum), carbendanzim and thiophanatemethyl 

were superior to TBZ, but both were slightly inferior to benomyl 

(Eckert, 1990). 

The benzimidazole compounds are insoluble in water but are fairly 

soluble in dilute acids and alkalis. They are generally applied to fruits as 

water suspensions or in wax emulsions, but may also be applied as 

solutions in a hydrocarbon solvent used in the formulation of fruit 

coating (Eckert, 1975). TBZ is stable under postharvest application, 

whereas benomyl is unstable and decomposes slowly in water to 

methyl-2-benzimidazole carbamate (MBC). MBC is only slightly less 

active than benomyl and probably plays a major role in disease control. 

However, its penetration ability into the tissue is very small (Eckert, 

1978). 

One of the limitations of the benzimidazole compounds is that they are 

not active against a number of important postharvest pathogens, such as 

Rhizopus, Mucor, Phytophthora, Alternaria, Geotrichum and soft rot 

bacteria (Eckert and Ogawa, 1988). Because of its lack of activity against 

these pathogens, benzimidazole cannot be used alone to control total 

decay in storage. Furthermore, the resistance of these pathogens towards 

the benzimidazole compounds may result in changes in the normal 

balance that naturally exists within the pathogenic population of each 

product. Thus, diseases of relatively little importance, incited by 

Alternaria and Geotrichum in citrus fruit or by Alternaria in white 

cabbage and in zucchini squashes, may become a limiting factor in 

storage of these products (Albrigo and Brown, 1977; Temkin-Gorodeiski 

and Katchanski, 1974; Wale and Epton, 1981). Furthermore, of the two 

species of Penicillium which attack citrus fruits, P. digitatum has proven 

to be more sensitive to the benzimidazoles; continuous treatment of 

citrus fruits with these fungicides resulted in a remarkable increase in 

decay by P. italicum, the less sensitive fungus of the two (Gutter, 1975). 

A very serious problem arising from long and continuous treatment 

with the benzimidazole compounds, is the development of fungal strains 

resistant to these fungicides, as a result of the heavy selection pressure 

exerted by the chemicals on the pathogen populations, (Georgopoulus, 

1977). Common examples are the resistant strains of P. digitatum and 

P. italicum in citrus fruit, of P. expansum in pome and stone fruits, and 

of B. cinerea in various crops, that have frequently been reported within 

the natural sensitive population of these species. Recently the appearance 




of thiabendazole-resistant Colletotrichum musae isolates has been 

reported in banana plantations that have commonly received pre- and 

postharvest thiabendazole treatments (de Lapeyre de Bellaire and 

Dubois, 1997). The ability of the benzimidazoles to persist in the fruit 

throughout the storage period ensures continuous contact with the 

pathogen, which exerts continuous pressure for selection of 

benzimidazole-resistant strains with various degrees of resistance. In 

addition, pathogens which develop resistance toward one benzimidazole 

compound, show a *cross-resistance' to other compounds with similar 

chemical compositions and structures (Eckert and Ogawa, 1985). 

The emergence of fungal strains resistant to benzimidazoles, and the 

resulting decrease in the efficacy of the postharvest benzimidazole 

treatments, raise questions about the future use of these compounds in 

the control of Penicillium rots in citrus fruits and apples and of Botrytis 

rot in various fruits and vegetables. The risk of the emergence of such 

resistant strains significantly increases when these compounds are 

applied as both pre- and postharvest treatments in certain crops. 

In order to broaden the scope of benzimidazole activity, as well as to 

solve the problem of the emergence of resistant strains, their 

combination with a fungicide of a different chemical structure and a 

different mode of action, was suggested. It is thus possible to use a 

combination of benomyl and prochloraz in order to control both 

Penicillium and Alternaria in pears stored at 0.5°C (Sitton and Pierson, 

1983). A combination of benomyl and dicloran is effective in controlling 

the two main decays in stone fruits: the brown rot caused by Monilinia, 

and the watery rot caused by Rhizopus (Wade and Gipps, 1973). 

Similarly, a combination of TBZ and iprodione controls both 

TBZ-resistant and TBZ-sensitive Botrytis strains in stored celery 

(Barkai-Golan et al., 1993a), and is effective against both Botrytis and 

Alternaria in white cabbage, although it is ineffective against bacterial 

soft rots in this crop (Geeson and Browne, 1979). Thiabendazole and 

dicloran dust, applied to white cabbage in alternate years, was 

recommended by Brown and collaborators (Brown, A.C. et al., 1975) to 

minimize the development of benzimidazole-resistant strains of 

B. cinerea in storage. For solving the problems associated with the 

emergence of benzimidazole-resistant strains of P. expansum in pears, 

TBZ was combined with captan, and this combination was alternated 

with another fungicide (imazalil, etaconazole or iprodione). Such a 




strategy considerably decreased the incidence of P, expansum strains 

resistant to TBZ during storage (Prusky et al., 1985a). 

Following the development of benomyl-resistant strains of B. cinerea, 

one of the most important pre- and postharvest pathogens of grapes, 

Chiba and Northover (1988) studied the fungitoxicity of four synthesized 

alkyl isocyanate homologues of benomyl towards both benomyl-resistant 

and benomyl-sensitive isolates of the fungus. On the basis of spore 

germination as a criterion for fungal response, the benomyl-resistant 

isolates were found to be more sensitive than the benomyl-sensitive 

isolates to ethyl isocyanate and propyl isocyanate. This response was 

inverse to the relative response of sensitive and resistant isolates to 

benomyl and constituted an example of 'negative cross resistance' (Chiba 

and Northover, 1988). When the germ-tube length of Botrytis 

germinating spores was used as a criterion, the four isocyanate 

homologues of benomyl - methyl, ethyl, propyl and hexyl - were found to 

be as effective against sensitive isolates as benomyl, but more effective 

against the resistant isolates. The hexyl isocyanate homologue was less 

effective than the other three compounds. Studies with wounded apples 

clearly demonstrated that, while benomyl gave better protection against 

sensitive isolates of JB. cinerea, the ethyl isocyanate homologue of benomyl 

was effective against both benomyl-sensitive and benomyl-resistant 

isolates. 

Iprodione and vinclozolin, which are dicarboximide fungicides, have 

been represented as alternative treatments to TBZ in many commodities, 

such as cucumbers, tomatoes, strawberries, eggplants and grapes 

(Lorenz, 1988). These fungicides are characterized by a different 

mechanism of action from that of the benzimidazoles, although this 

mechanism is not yet clear. Iprodione retards the development of 

Botrytis, Penicillium, Monilinia and Rhizopus and it is, therefore, 

capable of suppressing the main storage diseases of stone and pome 

fruits (Bompeix and Morgat, 1977; Heaton, 1980). However, the 

emergence of Penicillium expansum strains resistant to both iprodione 

and vinclozolin, may prevent their use for decay control on these fruits 

(Rosenberger and Meyer, 1981). In contrast to TBZ, iprodione is also 

effective against Alternaria, and considerably reduces the incidence of 

the black spots caused by this fungus on mangoes (Prusky et al., 1983) 

and on potato tubers in Israel (Droby et al., 1984). However, 

iprodione-tolerant isolates of Alternaria alternata pv. citri have already 

been found in Mineola tangelo orchards in Israel and Florida, where the 




fungicide had previously been applied successfully for several years 

(Solel et al., 1996). A combination of iprodione and TBZ was found to be 

advantageous in controlling the gray mold decay of celery caused by a 

heterogenic spore population of B, cinerea, which comprised spores 

sensitive to TBZ and/or to iprodione, along with spores resistant to these 

fungicides (Barkai-Golan et al., 1993a). 

Imazalil is a systemic fungicide, which was introduced as a 

postharvest treatment in the early 1970s. It belongs to a group of 

fungicides that act by inhibiting the biosynthesis of ergosterol, an 

essential component in the membrane of fungal cells, and it was the first 

ergosterol biosynthesis inhibitor to be used as a postharvest fungicide. 

The fungicide is very efficient in controlling Penicillium digitatum and 

P. italicum in citrus fruits, including isolates that are resistant to TBZ, 

benomyl, SOPP and sec-butylamine (Harding, 1976; Eckert and Ogawa, 

1985). Thanks to these properties, imazalil has become the most popular 

postharvest fungicide for controlling decay in citrus fruits. As a result of 

the enhanced resistance of P, digitatum and P. italicum to TBZ, imazalil 

was registered in the United States in 1984 as a postharvest treatment of 

citrus fruits and has been commercially used in several citrus-producing 

areas of the world. Water solutions of imazalil act on the two Penicillia, 

as both protectants and anti-sporulants (Laville et al., 1977). When 

applied in a water-wax formulation, its effectiveness is reduced and the 

concentration of the fungicide must be doubled to achieve an effect 

similar to that in water solutions (Brown, G.E., 1984). It is generally 

applied one or more times at relatively high dosages (2 to 4 g l-i) in a wax 

formulation that covers the surface of the fruit. Following its application, 

substantial antifungal residues of imazalil persist for the postharvest life 

of the fruit. These conditions produce intense pressure for selection of 

imazalil-resistant biotypes in the P. digitatum population and several 

years after imazalil was accepted commercially for citrus decay control, 

biotypes of P. digitatum with reduced sensitivity to the fungicide were 

reported and their potential impact on decay control has been 

demonstrated (Eckert et al., 1994). 

Wild (1994) reported on differential sensitivity of P. digitatum isolates 

to imazalil: while isolates obtained from Australian citrus fruits were 

defined as sensitive to the fungicide, those imported from the USA were 

much more tolerant, and an imazalil concentration of 0.1 mg ml^ in the 

growth media was established as suitable for differentiating 

imazalil-tolerant strains. 




The relative fitness of imazalil-resistant and imazalil-sensitive wild 

biotypes of P. digitatum was evaluated by Holmes and Eckert (1995) in 

Californian citrus groves and packinghouses. Both resistant and 

sensitive biotypes of the fungus were found to be stable with respect to 

their responses to imazalil over several disease cycles in non-treated 

lemon fruit and over several generations on culture medium. According 

to Holmes and Eckert (1995), the fact that imazalil-resistant biotypes 

have reached high frequency in packinghouses with a history of heavy 

imazalil application, could have indicated that the resistant biotypes 

have an advantage in relative fitness over the sensitive wild biotype in 

imazalil-treated fruit. On the other hand, the absence of resistant 

biotypes from groves where imazalil has never been applied, despite the 

regular introduction of resistant spores on picking boxes, suggested that 

resistant biotypes may be less fit than sensitive biotypes of P. digitatum 

in non-treated fruit. Upon studying the competition between sensitive 

and resistant biotypes of P. digitatum in resistant/sensitive mixtures 

(1:1), it was found that both on non-treated fruit and in culture medium 

without imazalil, resistant biotypes were generally less competitive than 

sensitive biotypes. Holmes and Eckert (1995) considered that such 

results should encourage an evaluation of resistance-management 

programs involving the rotation of imazalil with a non-selective 

fungicide. However, the continued intensive use of imazalil could 

adversely affect this strategy by the development of increased fitness 

among fungicide-resistant fungi (Holmes and Eckert, 1995). 

In most packinghouses in California, citrus fruits are routinely treated 

with sodium o-phenylphenate, imazalil and TBZ, to control Penicillium 

decay. A recent study on the sensitivity o{ Penicillium spp. to postharvest 

citrus fungicides indicated that the intensive use of the three chemically 

unrelated fungicides has resulted in the proliferation of triple-resistant 

biotypes of P. digitatum (Holmes and Eckert, 1999). This did not lead, 

however, to an increase in the level of resistance to any of the fungicides 

in the various isolates of Penicillium collected during several years. On 

the other hand, the proportion of isolates that were resistant to all three 

fungicides increased from 43% in 1948 to 74% in 1994. It was also found 

that while imazalil-resistant biotypes of P. digitatum were frequently 

isolated in packinghouses, resistant P. italicum was rare. It was 

suggested that a fundamental difference in the biology of P. italicum and 

P. digitatum might be involved. The difference could result from smaller 

populations of P. italicum available for selection in the packinghouse, 

greater imazalil sensitivity of resistant mutants, and the lower parasitic 




fitness and reproductive capacity of this species compared with P. 

digitatum (Holmes and Eckert, 1999). 

The antifungal spectrum of imazalil is similar to that of the 

benzimidazoles, but it is also active against Alternaria. The fungicide 

was also found active against stem-end rots caused by Diplodia 

natalensis and Phomopsis citri, although it was less effective than 

benomyl (Brown, G.E., 1984); no activity was recorded against sour rot 

caused by Geotrichum candidum, Postharvest treatments with imazalil 

have also been effective against benzimidazole-resistant isolates of 

Colletotrichum, Diplodia and Phomopsis from mango fruits (Spalding, 

1982). However, good control of both stem-end rot and anthracnose and 

considerably more fruit acceptable for marketing were recorded when 

imazalil was applied in hot water at 53°C (Spalding and Reeder, 1986b). 

Imazalil inhibits both spore germination and mycelial growth of 

Alternaria and suppresses decay development caused by this fungus in 

apples, pears, persimmons, tomatoes and bell peppers (Miller et al., 1984; 

Prusky and Ben-Arie, 1981; Spalding, 1980). Postharvest treatment of 

tomatoes also controls the gray mold caused by Botrytis cinerea during 

storage (Manji and Ogawa, 1985). Imazalil is also effective against the 

black rot incited by Ceratocystis paradoxa in pineapple, when applied 

within 6 to 12 hours after harvest (Eckert, 1990). 

Prochloraz, which also acts by inhibiting the synthesis of ergosterol, 

is an imidazole with a spectrum of activity similar to that of imazalil. It 

eradicates initial infections by the two Penicillium species in citrus fruit, 

including strains resistant to benomyl and TBZ. Its efficacy in 

suppressing the blue and green molds is just as good as that of benomyl 

or imazalil (Tuset et al., 1981), and similarly to these fungicides, it also 

has a marked antisporulant activity. This fungicide was also effective in 

controlling anthracnose and stem-end rot on papayas (Muirhead, 1981a), 

and virtually eliminated anthracnose from mango lots with up to 72% 

diseased fruit in the untreated control (Knights, 1986). Prochloraz was 

found to be efficacious against both Fusarium rots and soft rots caused 

by Geotrichum and Rhizopus in muskmelon fruits (Wade and Morris, 

1983). 

Etaconazole is a triazole with a mode of action similar to that of 

imazalil and prochloraz. It controls initial infections of the two species of 

Penicillium in citrus fruits, including infections by benzimidazoleresistant 

isolates, and it is also characterized by its ability to suppress 




sporulation of these fungi on diseased fruits and protect them from 

infections through new wounds sustained after treatment (Brown, G.E., 

1983). The fungicide is markedly effective against sour rot (Geotrichum 

candidum) but provides only moderate protection against the stem-end 

fungi, Alternaria citri, Phomopsis citri and Diplodia natalensis. The 

ability of etaconazole to control the major pathogens of harvested citrus 

fruits gives this fungicide a very important role as a postharvest 

fungicide for these fruits. Trials with Fuerta avocados have shown that 

etaconazole controls stem-end rot but is not effective against anthracnose 

(Muirhead et al., 1982). However, both anthracnose and stem-end rot in 

mangoes have been controlled by this compound during storage and 

ripening of the fruit (Spalding, 1982). 

Guazatine is a fungicide with a wide spectrum of activity, capable of 

eradicating incipient infections by the two Penicillia, including 

benzimidazole-resistant isolates, as well as infections by Geotrichum in 

citrus fruit (Brown, G.E., 1983). The fungicide is also efficient in 

controlling Geotrichum and Alternaria in melons (Wade and Morris, 

1983). 

Metalaxyl (ridomil), which is an acylalanine fungicide, acts as a 

strong inhibitor of the various developmental stages oi Phytophthora spp. 

(Bruck et al., 1980; Farih et al., 1981). In addition to the inhibition of the 

mycelial growth of the fungus, the fungicide generally inhibits the 

formation of sporangia, chlamidospores and oospores at low 

concentrations. This systemic fungicide uniquely arrests incipient 

infections of Phytophthora in citrus fruits and prevents contact spread of 

brown rot during prolonged storage (Cohen, 1981, 1982). Application of 

metalaxyl, in a water-wax formulation, to Phytophthora citrophthorainoculated 

Shamouti oranges, followed by the standard packinghouse 

procedure (including washing, immersion in a 0.5% sodium ortho 

phenylphenate for 3 min at 36°C, washing in tap water and drying), has 

shown that the fungicide considerably delays or prevents the 

development of brown rot in storage. The antifungal effect on 

Phytophthora is much more pronounced than that of TBZ or imazalil, but 

it has no influence on the development of other postharvest pathogens. 

However, the combination of metalaxyl with etaconazole, in a water-wax 

formulation, broadens the scope of the antifungal activity and controls 

Penicillium rots, sour rot {Geotrichum) and brown rot {Phytophthora) in 

citrus fruits (Cohen, 1981). 




Studies carried out 10 years later (Ferrin and Kabashinna, 1991), 

described the development of metalaxyl-resistant isolates of 

Phytophthora parasitica, with EC50 values for inhibition of linear 

mycelial growth exceeding 700 \xg ml-i, as compared with values of 

0.25-3.08 |Lig ml-i for the sensitive isolates. However, the resistances to 

the mycelium growth may differ from that for the formation of sporangia 

or chlamidospores. 

Metalaxyl-resistant genotypes emerged rapidly worldwide and are 

responsible for a lack of Phytophthora infestans control in potatoes and 

tomatoes. A cross-resistance was reported for this class of fungicides, and 

the metalaxyl-resistant isolates were also resistant to oxadixyl, while 

the metalaxyl-sensitive isolates were also sensitive to oxadixyl; both of 

these are systemic fungicides (Sedegui et al., 1999). Resistance to 

metalaxyl, which is mediated by a single gene (Cooke, 1991), has evolved 

where this fungicide was used as a single product for disease control. A 

coordinated strategy of using the systemic fungicide, metalaxyl in 

mixtures with protective multisite fungicides, such as chlorothalonil, 

has been effective in slowing the development of resistance (Cooke, 1991; 

Kadish et al., 1990). 

Fosetyl al (Fosetyl aluminum) is another selectively active fungicide 

against incipient infections of Phytophthora (GauUiard and Pelossier, 

1983). It is applied to harvested citrus fruits to protect against and to 

control Phytophthora spp., but it also reduces the incidence of green mold 

(Penicillium digitatum) in storage (Eckert and Ogawa, 1985). 

Among the fungicides that have proven efficient in controlling citrus 

fruit decay, including infections by Geotrichum candidum, we have to 

mention fenpropimorph and flutriafol. These compounds considerably 

reduce the rate of rot, both in artificially inoculated lemons and in those 

under natural infection conditions in the packinghouse. Their major 

action is the suppression of contact-spread of the sour rot and the green 

and blue molds following close contact with diseased fruit in the 

container (Cohen, 1989). 

Fungicides and fungistats, used for postharvest decay control, and 

their chemical structures are given in Fig. 27. Detailed descriptions of 

chemical compounds used for the control of postharvest diseases, and 

their effect on disease development, are given in two comprehensive 

reviews by Eckert and Ogawa (1985, 1988). 



168 

o : ^ 


OH 

/ ^ 

BIPHENYL 0-PHENYLPHENOL 

O 

CAPTAN 

-S~CGU 

CHo 

CH3CH2C-NH2 

H 

NHn 

NO2 

DICLORAN 

NH2 NH2 

^^;CNH(CH2)8NH(CH2)8NHC:^^ 

SEC . BUTYLAMINE GUAZATINE 

CI 

^flo. 

CH=CHn 

^x R ,,/CONHCH 

CI O 

VINCLOZOLIN IPRODIONE 

a>v 

H 

.^ 

CH3 

C-NC4H9 

H O 

x>-N-C0CH3 

THIABENDAZOLE BENQMYL 

Fig. 27. Postharvest fungicides. (Reproduced from Eckert and Ogawa, 1985, 

1988 with permission of the Annual Review of Phytopathology). 




^ 

Qi;1^NHC0CH3 

CARBENDA2IM 

DtPHENYLAMINE 

CH2 CHOCH2 ^^ = ^^2 

.CI 

CI 

IMA2ALIL 

OH 

CI-^^0-CH-C-C(CH3)3 

a 

TRIADIMENOL 

CH3 ^^Ô 

/=



The Use and Development of Fungicides - Perspectives 

It should be emphasized that the use of a fungicide or bactericide is 

not a substitute for appropriate storage conditions, since these 

compounds only seldom affect the physiological deterioration of the 

product. Moreover, chemical compounds are more effective when the 

fruit or vegetable is held under storage conditions which maintain the 

natural infection resistance of the produce, but are not good for the 

growth of the pathogen. However, when appropriate refrigeration or a 

controlled atmosphere cannot be installed, the chemical treatment may 

be the only means available to prolong the postharvest life of fruits and 

vegetables. 

In the past, the selection of fungicides for development was largely 

based on their fungicidal performances and safety to the consumer and 

operator. However, there is now a greater awareness of the need to 

define other aspects of new fungicides early in the development process. 

According to Knight et al. (1997) these include, in addition to 

cost-effective fungicidal activity: low toxicity to humans and wildlife, low 

environmental impact, low residues in food, and the ability to integrate 

with other disease control technologies. Examples of substances with low 

environmental impact and low residues in the treated commodity are 

compounds that are 'generally recognized as safe' (GRAS) and naturally 

occurring plant products, as given below. 

D. GENERALLY RECOGNIZED AS SAFE (GRAS) 

COMPOUNDS 

Fungicides that leave low or non-detectable residues in the commodity 

are actively sought in research programs. Compounds are selected that 

rapidly degrade on the host surface or metabolize quickly in the tissue. 

Hydrogen peroxide is such a compound; it degrades into O2 and H2O 

leaving no harmful residue, and is considered a generallyrecognized-as-safe 

(GRAS) compound by the Food and Drug 

Administration of the USA. Vapor phase hydrogen peroxide treatment 

was found significantly to reduce the number of germinable Botrytis 

cinerea spores on grapes. It also resulted in reduced decay in 

non-inoculated 'Thompson Seedless' and 'Red Globe' grapes after 12 days 

of storage at 10°C, without affecting grape color or soluble solids content 

(Forney et al., 1991). 




Sanosil-25, a disinfectant containing 48% hydrogen peroxide, and 

silver salts as stabilizing agents, inhibits spore germination and mycelial 

growth of Alternaria alternata, Fusarium solani and B, cinerea, and 

markedly decreases decay in melons at 5000 ^11^ when incorporated into 

a wax treatment, without causing any phytotoxic effects (Aharoni et al., 

1994). Dipping commercially harvested eggplants and red peppers in 

0.5% Sanosil-25 reduced decay development by A alternata and 

B. cinerea after storage and shelf life, to a commercially accepted level 

(Fallik et al., 1994a). On the other hand, immersion of Penicillium 

digitatum-m.OQX)ldiiedi lemons in hydrogen peroxide at 5-15% did not 

effectively control the green mold and caused unacceptable injury to the 

fruit when the immersion period was increased to 90 s (Smilanick et al., 

1995). 

Acetic acid and other short-chain organic acids, such as propionic 

acid, are commonly used by food manufacturers as antimicrobial 

preservatives or acidulants in a variety of food products (Davidson and 

Juneja, 1990). The possibility of using vaporized acetic acid against 

postharvest decay has been studied in various fruits (Sholberg, 1998; 

Sholberg and Gaunce 1995, 1996; Sholberg et al., 1996). Sholberg and 

Gaunce (1995) found acetic acid, applied as a vapor at low concentrations 

in air (2-4 mg l-i), to be extremely effective in reducing or preventing 

decay in: various cultivars of apples and Anjou pears inoculated with 

B. cinerea and Penicillium expansum conidia; tomatoes, grapes and 

kiwifruit inoculated with B, cinerea; and Navel oranges inoculated with 

Penicillium italicum. The same authors (Sholberg and Gaunce, 1996) 

found acetic acid to be similarly effective as a postharvest fumigant on 

stone fruits, controlling decay by Monilinia fructicola and Rhizopus 

stolonifer at concentrations in air as low as 1.4 mgl-^. On table grapes, 

fumigation with 0.27% (vol/vol) acetic acid controlled Botrytis and 

Penicillium decay as effectively as sulfur dioxide applied at commercial 

rates (Sholberg et al., 1996). This treatment has been suggested as an 

alternative to SO2 fumigation, that could provide the table grape 

industry with many benefits. While SO2 treatment leaves sulfite residues 

on the surface of the berries, acetic acid does not leave any toxic residues 

on grapes, and no differences from SO2 in terms of external fruit quality 

and fruit composition have been recorded. Furthermore, wine grapes 

could also benefit from fumigation with acetic acid (Sholberg et al., 1996). 

Sholberg (1998) found that fumigation with acetic acid (1.9 nl l-i), and 

other closely related short-chain organic acids, formic (1.2 |LI1 l-i) and 




propionic (2.5 |LI1 l-i), significantly reduced decay in eight cherry 

cultivars inoculated with fungal spores (10^ spores/ml) of M fructicola, P. 

expansum and R, stolonifer. However, phytotoxicity, expressed as 

black-end stems and pitting of the fruit surface, occurred on fumigated 

cherries. On the other hand, decay in P, expansum-inoculated pome 

fruits was reduced from 98% to 16, 4 and 8% by acetic, formic and 

propionic acids, respectively, without injury to the fruit. 

Limited data are available on the effects of peracetic acid on 

postharvest fruit decay. Evaluating its effectiveness on the control of the 

brown rot caused by Monilinia laxa on stone fruits, Mari et al. (1999) 

indicated that the chemical acted directly on the fungal spores, its effect 

being related to the chemical concentrations and duration of treatment. 

Complete inhibition of spore germination was observed with peracetic 

acid at 500 fig/ml after 5 min of contact with Monilinia conidia. The 

inhibitory effect on the pathogen spores was confirmed in vivo in 

inoculated plums, for which a 1000 p



carbonate, potassium carbonate, sodium bicarbonate, ammonium 

bicarbonate and potassium bicarbonate were 5.0, 6.2, 14.1, 16.4 and 

33.4 mM, respectively. All these compounds were fungistatic: spores 

removed from the solutions resumed germination when incubated in 

potato dextrose broth. In spite of the differences between the effects of 

sodium bicarbonate and sodium carbonate on spore germination, the two 

solutions were similarly effective in controlling the green mold in lemon 

and orange fruits inoculated 24 h before treatment. Therefore, the in 

vitro toxicity of the solutions does not indicate their efficacy in controlling 

decay. Since bicarbonate anion concentration is related to pH, this 

parameter was examined in combination with several salts to separate 

pH effects from the bicarbonate effects on B. cinerea growth (Palmer et 

al., 1997). It was found that as the pH increased from 7.0 to 8.5, colony 

growth on media supplemented with bicarbonates and phosphates 

decreased more than could be accounted for from pH alone. 

Potassium bicarbonate was found to inhibit spore germination, 

germ-tube elongation and mycelial growth of B, cinerea and A, alternata, 

the main postharvest pathogens of bell pepper fruits. Its action was 

fungistatic rather than fungicidic: mycelial plugs of both fungi, when 

transferred from potassium bicarbonate-amended PDA to unamended 

medium, grew similarly to unamended controls (Fallik et al., 1997). At 

concentrations greater than 1% for Botrytis or 2% for Alternaria, mycelial 

mats remained white because of the inability of the fungi to sporulate. 

Scanning electron microscopy analysis revealed that the fungistat caused 

shrinkage and collapse of hyphae and spores, resulting in the prevention 

of normal sporulation. A 2-min pre-storage dip of red sweet peppers in 1 

or 2% potassium bicarbonate reduced decay incidence to a commercially 

acceptable level during a storage and marketing simulation. However, a 

longer dipping time, especially at higher salt concentrations (3%), 

reduced fruit quality, as indicated by decreased firmness and further 

decay development (Fallik et al., 1997). Collapse of hyphal walls and 

shrinkage of B, cinerea conidia were suggested to be partly due to the 

reduction in fungal cell turgor pressure or the increase of the 

permeability of the fungal cell membrane, caused by the bicarbonate ion 

(Palmer et al., 1997; Fallik et al., 1997). 

A direct inhibitory effect of sodium bicarbonate on in vitro mycelium 

growth was similarly recorded for K stolonifer, A. alternata and 

Fusarium spp., the major pathogens of stored melons (Aharoni et al., 

1997). Coating harvested melons with wax containing 2% sodium 

bicarbonate resulted in a marked reduction in decay incidence after 




storage, while the fresh appearance of the fruit was maintained. A trial 

shipment of 'Galia' melons by sea transport from Israel to Europe 

demonstrated that sodium bicarbonate incorporated into a wax coating 

maintained the marketability of the fruit. It was suggested that this 

biocide could serve as an alternative to imazalil, which had been 

incorporated into the wax until recently, to control decay during 

prolonged storage (Aharoni et al., 1997). 

Since Helminthosporium solani, the causal agent of silver scurf in 

potato tubers, developed resistance to benzimidazole compounds, 

postharvest losses attributed to this disease have increased and no 

fungicide is currently registered for its control in the United States 

(Olivier et al., 1998). Looking for alternative antifungal compounds for 

suppression of silver scurf on potatoes, Olivier et al. (1998) evaluated the 

effectiveness of several organic and inorganic salts on H. solani 

development; they found that potassium sorbate, calcium propionate, 

sodium carbonate, sodium bicarbonate, potassium carbonate, potassium 

bicarbonate and ammonium bicarbonate directly reduced radial growth of 

the fungus in vitro at 0.06-0.2 M, and that no lesions developed on 

H. soZani-inoculated tubers treated with 0.2 M solutions of each of the salts 

during 6 weeks of incubation at 22-24°C. Postharvest applications of these 

compounds (at 0.2 M) on naturally infected field-grown tubers also 

significantly reduced disease severity and Helminthosporium sporulation 

after 15 weeks of storage at 10°C. In this case, radial growth inhibition of 

H, solani on salt-amended media was generally predictive of the capability 

of the salts to suppress lesion development and sporulation on potato 

tubers. Similarly to the effect of the salts on P. digitatum, the citrus 

pathogen (Smilanick et al., 1999), treatments with sodium and potassium 

carbonate were also more effective than those with the respective 

bicarbonate salts against H, solani growth (Olivier et al., 1998). 

Several studies with carbonate and bicarbonate salts have confirmed 

that the anions are primarily responsible for pathogen suppression by 

inorganic salts, while the cations play only a minor role. However, 

ammonium bicarbonate is an exception since, in addition to the anion, 

ammonium too may contribute to the toxicity of the salt (DePasquale and 

Montville, 1990). 

Chlorine (as hsôchlorous acid) is an effective and economical biocide 

that has long been accepted as a potent disinfectant for sanitation 

purposes and is recognized as safe in many countries (Eckert and Ogawa, 

1988). Chlorination of the washing water used in tanks and hydrocoolers, 




with calcium hypochlorite or sodium hypochlorite, is a standard 

procedure to disinfect various fruits and vegetables in the packinghouse. 

Its main effect in reducing decay development is via reduction of the level 

of inoculum in the treated water (see the chapter on Chemical Control - 

Sanitation). Chlorine acts on fungal propagules by direct contact and can 

inactivate spores which are suspended in water or located on the surface 

of fruits or vegetables. It does not act on pathogens under the fruit skin 

or after infection has occurred. Because of the rapid drop in the biocidic 

activity of chlorine in the presence of organic substances (that modify the 

pH of the solution), it has frequently been replaced by chlorine dioxide 

(CIO2), which is more stable than chlorine and is effective over a wide pH 

range (Bartz and Eckert, 1987). In addition, CIO2 is not corrosive to the 

packinghouse equipment. 

In a recent study, CIO2 has been evaluated for its effectiveness in 

controlling the brown rot caused by M laxa in stone fruits (Mari et al., 

1999). CIO2 was found to affect M laxa conidia directly, and at 100 |ig/ml for 

20 s or 50 |Lig/ml for 1 min, totally inhibited their germination. The direct 

effect of the chemical on fungal spores has also been exhibited in vivo, when 

decay development in nectarines and plums, wounded and inoculated with 

C102-treated conidia, was markedly suppressed. Under these conditions, 

however, the effects varied with the CIO2 concentration, the time of 

exposure of the conidia, and the species of fruit tested (Mari et al., 1999). 

For table grapes, that cannot be subjected to water immersion 

treatments without altering their quality, a modified chlorine 

atmosphere with a relatively long residual effect has been developed 

(Zoffoli et al., 1999). Chlorine gas (CI2) produced by a salt mixture of 

calcium hypochloride [Ca(0Cl)]2, sodium chloride (NaCl) and calcium 

chloride (CaCb) with citric acid, combined with cold storage (25 days at 

0°) significantly reduce Botrytis rot in both inoculated and field-infected 

grape cultivars. Infections by conidia or mycelium of J3. cinerea were 

suppressed for up to 45 days in cool storage and no deleterious effect of 

chlorine gas generation was detected. This procedure was suggested to be 

an alternative to SO2 treatment for controlling decay in table grapes, 

since the latter frequently leaves residues exceeding the tolerance limits 

registered in the US (Zoffoh et al., 1999). 

Sugar analogs have long been known to affect metabolic processes of 

fungi and plant cells (Moore, 1981). The compounds 2-deoxy-D-glucose 

and L-sorbose were found to interfere with the growth of several 

filamentous fungi and yeasts when used as a sole carbon source (Biely 




et al., 1971). The antifungal action is thought to be due to the 

interference with the cell-wall-forming enzymes. 

Studsdng the potential of various sugar analogs as antifungal agents 

for decay suppression, El Ghaouth et al. (1995) found that only 

2-deoxy-D-glucose was effective in controlling decay in apples inoculated 

with JB. cinerea and P. expansum (Table 10), or in peaches inoculated 

with M fructicola. This compound inhibited in vitro radial growth of 

P. expansum, B, cinerea, M. fructicola and even K stolonifer, which is 

known to be insensitive to most postharvest fungicides. At 0.1% it 

induced severe morphological alterations in the tested fungi, suggesting 

that growth inhibition is the result of direct antifungal properties of the 

sugar analog. It was also suggested that this sugar analog could serve as 

a useful additive to a biological control method, provided that the 

antagonist is resistant to its inhibitory action (El Ghaouth et al., 1995). 

TABLE 10 

Effect of sugar analogs on gray mold and blue mold of apples 

after 7 days at 24°Ci 

Treatment^ 

Control 

Glucose 

Mannose 

L-sorbose 

Rafinose 

Deoxy-D-ribose 

2-deoxy-D-glucose 

LSDos 

1 Reproduced from El Ghaouth et al. (1995) with permission of the 


2 Wounded fruits treated with 50 |LI1 of different sugar analogs or sterile 

water and 30 min later were inoculated with 30 \xl conidial 

suspensions oiBotrytis cinerea or Penicillium expansum. 

3 After 14 days' incubation. 

Infected fruit (%) 

Botrytis cinerea Penicillium 

expansum 

100 

100 

100 

100 

97.0 

95.2 

94.3 

93.1 

98.1 

100 

99.3 

96.4 

0 (21.3)3 

0 (22.4)3 

4.9 

2.9 




E. NATURAL CHEMICAL COMPOUNDS 

The use of synthetic fungicides has been the major commercial means 

of postharvest decay control for several decades. However, the chemical 

residues that are liable to remain on the fruit or within its tissues 

following fungicidal treatment, and the 1986 report from the US 

National Academy of Sciences (Research Council, Board of Agriculture, 

1987) indicating that fungicide residues on food pose a great health risk 

to the consumer, led to the search for safe alternatives to synthetic 

fungicides. The fact that the effectiveness of synthetic fungicides has 

been reduced by the frequent development of resistance by the 

pathogens, further highlighted the need for new substances and methods 

for the control of storage diseases. Naturally occurring plant products are 

important sources of antifungal compounds with low toxicity to mammals 

and safe to the environment, which may serve as substitutes for 

synthetically produced fungicides. It was later suggested that these 

compounds might be developed either as products per se or used as 

starting points for synthesis (Knight et al., 1997). 

Acetaldehyde, which is a natural volatile compound produced by 

various plant organs, and accumulates in fruits during ripening, has 

shown fungicidal properties against various postharvest pathogens. In 

sublethal concentration it is capable of inhibiting both spore germination 

and mycelial growth of common storage fungi (Aharoni and 

Stadelbacher, 1973; Prasad, 1975; Vaughn et al., 1993; Hamilton-Kemp 

et al., 1992), and the development of yeast species responsible for 

spoilage of concentrated fruit juices (Barkai-Golan and Aharoni, 1976). It 

has been reported to inactivate ribonuclease (Mauch et al., 1987) and to 

bind to other proteins (Perata et al., 1992), but the mechanism of 

aldehyde toxicity to fungal spores is still unknown. 

Fumigation of apples with acetaldehyde inhibits Penicillium 

expansum development in the fruit (Stadelbacher and Prasad, 1974), 

while fumigation of strawberries with acetaldehyde considerably reduces 

the incidence of postharvest decay caused by Rhizopus stolonifer and 

Botrytis cinerea (Prasad and Stadelbacher, 1974; Pesis and Avissar, 

1990). Treating ripe strawberries with 5000 |al l-i acetaldehyde for 1 h or 

with 1500 |Lil 1-1 for 4 h resulted in minimal fungal decay during storage 

for 4 days at 5°C following 1 day at 20°C. Such treatments also induced 

modest amounts of various fruit volatiles, such as ethanol, ethyl acetate 

and ethyl butsrrate, which increased fruit aroma and improved its taste 




(Pesis and Avissar, 1990). Trials carried out with table grapes under 

natural infection conditions (Avissar and Pesis, 1991) showed that 0.5% 

acetaldehyde for 24 h suppressed decay caused by B. cinerea, R, stolonifer, 

and Aspergillus niger, without injuring the fruit or changing its taste 

(Fig. 28). Furthermore, fumigating the fruit under these conditions 

prevents drying of the stems after removal of the grapes to shelf life. The 

effect of acetaldehyde on Rhizopus development is well exhibited after 

artificial inoculation of grape bunches with an inoculum rich in spores. 

This effect is of importance because SO2, which is still in use for decay 

control in some cases, is efficient mainly in preventing Botrytis 

development, but is ineffective against Rhizopus (Avissar and Pesis, 

1991). 

The efficacy of acetaldehyde vapors and of a number of other aliphatic 

aldehydes, produced naturally by sweet cherry (cv. Bing), was evaluated 

in P. expansum-inoculsited fruits (Mattheis and Roberts, 1993). High 

concentrations of acetaldehyde, propanal and butanal suppressed 

15-k 

10+ 

8 

0) 

Q 5 + 

0 

• 

D 

• 

CH3CHO 

% 

1 

0.5 

0.25 

0 

1 2 3 4 5 6 

Days at 20°C 

Fig. 28. Effect of acetaldehyde vapor application on the percentage of decay in 

'Sultanina' grapes. Vertical bars indicate standard error. (Reproduced from 

Avissar and Pesis, 1991 with permission of the Association of Applied Biologists). 




conidial germination but resulted in extensive stem browning and fruit 

phytoxicity, which increased with the aldehyde concentration. The degree 

of inhibition decreased with increasing aldehyde molecular weight. The 

incidence of P. expansum decay was significantly impacted by aldehyde 

identity and concentration and by the interaction of the two factors. 

Since stem quality of sweet cherries is a critical component in the 

evaluation of fresh fruit quality, the browning which follows aldehyde 

treatments will limit their commercial use. On the other hand, stem 

quality is less of a concern for fruits intended for processing, and for this 

purpose aldehyde fumigation may present an alternative to the use of 

synthetic fungicides. 

Injuries resulting from acetaldehyde vapors have been reported for 

various products, such as cultivars of apples (Stadelbacher and Prasad, 

1974), strawberries (Prasad and Stadelbacher, 1974), grapes (Pesis and 

Frenkel, 1989), lettuce (Stewart et al., 1980) and carrot tissue cultures 

(Perata and Alpi, 1991). 

Nine out of 16 volatile compounds occurring naturally in peach and 

plum fruits greatly inhibited spore germination of B. cinerea and 

Monilinia fructicola (Wilson et al., 1987a). The volatiles most effective in 

inhibiting spore germination were benzaldehyde, benzyl alcohol, 

y-caprolactone and y-valerolactone. Of these, benzaldehyde was active 

at the lowest concentrations tested and completely inhibited germination 

of B, cinerea spores at concentrations of 25 |il l^ and germination of M 

fructicola spores at 125 |il l^. It was clearly shown that the same 

compounds were effective on both fungi and that the relative inhibition 

by these compounds was similar against each pathogen. Various volatiles 

(benzaldehyde, methyl salicylate and ethyl benzoate) have been 

recorded as growth suppressors. It is of interest to note that while 

benzaldehyde and methyl salicylate were fungicidal against both 

pathogens, another volatile, ethyl benzoate was fungicidal against one 

pathogen (Monilinia) and fungistatic against the other pathogen 

(Botrytis). The most effective volatiles were found to be active at 

concentrations that should make them potential fumigants for 

postharvest disease control (Wilson et al., 1987a). 

Evaluation of 15 volatile odor compounds, released from raspberries 

and strawberries during ripening, for their ability to inhibit postharvest 

decay fungi, showed that five of them inhibited the growth of Alternaria 

alternata, B, cinerea and Colletotrichum gloeosporioides directly on the 

fruit at 0.4 [il ml-i (Vaughn et al., 1993). Among the five compounds, 

benzaldehyde was the most toxic to the fungi and completely inhibited 




cultures of the three fungal species at only 0.04 |il ml-i, while 1-hexanol, 

E-2-hexenal and 2-nonanone inhibited them at 0.1 |il ml-i and 

Z-3-hexen-l-ol did not completely suppress growth when applied at



60 /• 

50 

40+- 

^ 30 

CO 

o 

(D 

Q 20 

10 + 

14 days storage at 12^0 • • 

+ 4 days shelf life I I 

Hinokitol 

750 îl V 

L I' M \2zz V. 

Prochloraz 

2000 îl L' 

Control 

Fig. 29. Effect of hinokitiol and prochloraz on decay development in eggplant 

fruit after 14 days of storage (left) and an additional 4 days of shelf-life (right), 

compared with water dipped control. Letters represent significant differences 

according to Duncan's Multiple Range Test. (Reproduced from Fallik and 

Grinberg, 1992 with permission of Elsevier Science). 

(Aharoni at al., 1993b). Today, melons are coated with wax containing 

imazalil (200 ppm), and under the above conditions, this leaves 

fungicidal residue above the level approved in some countries (0.5 ppm). 

Introducing hinokitiol into the wax was also found to control decay 

during cold storage, caused mainly by A. alternata and Fusarium spp. - 

without any phytotoxic effects. In order to suppress decay at shelf life 

also, the concentration of the hinokitiol has to be increased, but such an 

increase causes peel browning. 

Essential oils and plant extracts are sources of antifungal activity 

against a wide range of fungi (Grange and Ahmed, 1988). A rapid assay to 

determine antifungal activity in both plant extracts and essential oils has 

recently been described by Wilson et al. (1997b). Among 345 plant extracts 

analyzed, 13 showed high levels of activity against B. cinerea, which 

served as a test fungus. The major plants showing the highest persistent 

antifungal activity were garlic (Allium sp.) and pepper (Capsicum sp.). 

The high antifungal activity recorded for garlic extracts is in accordance 




with an earlier study indicating the marked antifungal activity hidden in 

the tissues of this crop (Ark and Thompson, 1959). Among the 49 essential 

oils tested, those of palmarosa (Cymbopogon martini) and red thyme 

(Thymus zygis) showed the greatest inhibitory effect on J3. cinerea spore 

germination at the lowest concentration. The next best inhibitors were 

essential oils of clove buds (Eugenia caryophyllata) and cinnamon leaf 

(Cinnamomum zeylanicum). The most frequently occurring constituents in 

essential oils showing high antifungal activity were: Z)-limonene, cineole, 

a-pinene, P-pinene, P-myrcene and camphor. The fungicidal activity of the 

individual components, singly and in combination, is being studied (Wilson 

et al., 1997b). Essential oil derived from another species of Thymus, T 

capitatus, markedly reduced development of S. cinerea in inoculated 

mandarin fruit when applied as a vapor. Scanning electron microscope 

observations indicated a direct damaging effect of the thyme oil on fungal 

hyphae (Arras and Piga, 1994). 

Glucosinolates, a large class of compounds produced by plants of the 

Cruciferae (Mari and Guizzardi, 1998), are other natural substances with 

potential antimicrobial activity. When cells of plant tissues that 

metabolize glucosinolates are damaged, these compounds come into 

contact with the enzyme myrosinase, which catalyzes hydrolysis. The 

antifungal activity of six isothiocyanates, produced by the enzymatic 

hydrolysis of glucosinolates, has been tested on several postharvest 

pathogens in vitro (Mari et al., 1993) and in vivo on artificially-inoculated 

pears (Mari et al., 1996), with encouraging results. 

Chitosan is an animal-derived polymer formed by de-acetilation of 

chitin. It can serve as a coating for the fruit or vegetable and for regulating 

gas moisture and exchange around the product (Wilson et al., 1994). When 

applied as a coating, chitosan delayed the ripening of tomatoes and 

reduced decay incidence indirectly by modifying the internal atmosphere. 

A marked reduction of decay has been recorded in chitosan-treated bell 

peppers, cucumbers, strawberries and carrots (Cheah et al., 1997; Wilson 

et al., 1994). In litchi fruits, chitosan coating delayed browning of the 

pericarp, which is the main problem during prolonged storage (Zhang and 

Quantick, 1997). Application of a chitosan coating, in addition to reducing 

weight loss of the fruits, delayed changes in the content of antocyanin, 

flavonoid and total phenolics, delayed the increase in polyphenol oxidase 

activity and partially inhibited the increase in peroxidase activity, which 

are associated with tissue browning. This implies that a chitosan coating 




may form a protective barrier on the surface of the fruit and reduce the 

supply of oxygen for enzymatic oxidation of phenohcs (Zhang and 

Quantick, 1997). Apphcation of a chitosan coating also delayed, to some 

extent, the increase in decay of stored litchi. 

Chitosan application was also found to stimulate the formation of 

structural defense barriers in bell peppers and tomatoes. These included 

the induction of callose synthesis, thickening of host cell walls, formation 

of papillae and plugging of some intercellular spaces with fibrillar 

material, probably impregnated with antifungal phenolic-like compounds 

(Wilson et al., 1994). 

However, chitosan also exhibits a direct fungicidal activity and can 

affect postharvest pathogens by suppressing their growth: it inhibited 

spore germination, germ-tube elongation and radial growth of B, cinerea 

and K stolonifer, inducing cellular alterations and damage in culture (El 

Ghaouth et al., 1992). Similarly, exposure of Sclerotinia sclerotiorum to 

chitosan resulted in hyphal deformation and restricted mycelium growth 

(Cheah et al., 1997). Trials with chitosan-treated bell peppers indicated 

the damage caused to B. cinerea hyphae, exhibited as changes in the cell 

wall and by cytoplasm decomposition (El Ghaouth et al., 1994). 

Examination of sections from chitosan-treated tissue of bell pepper fruits 

by transmission electron microscope revealed that chitosan prevented the 

disintegration of host cell walls by the pectolytic enzymes of B. cinerea 

(El Ghaouth et al., 1997). This was indicated by the preservation of pectic 

binding sites and the regular intense cellulose labeling over host cell 

walls pressed against fungal cells. In addition to reducing the production 

of polygalacturonase by the pathogen, chitosan also caused severe 

cytological damage to invading hyphae. These phenomena may explain, 

at least in part, the limited ability of Botrytis to colonize the fruit tissues 

in the presence of chitosan (El Ghaouth et al., 1997). In parallel, chitosan 

may also induce the formation of antifungal hydrolases, such as chitinase 

and P-l,3-gluconase, which may lead to the reduction in the chitin 

content of fungal cell walls and the stimulation of various structural 

defense barriers in fruits such as bell peppers and tomatoes. Because of 

these characteristics, chitosan has been hypothesized to be an elicitor of 

resistance in harvested crops (see the chapter on Novel Approaches for 

Enhancing Host Resistance - Induced Resistance). 

Gel derived from Aloe vera plants has been found to have antifungal 

activity against four common postharvest pathogens: Penicillium 

digitatum, P. expansum, B. cinerea and A. alternata. The natural gel 




suppressed both germination and mycelial growth, with P, digitatum and 

A. alternata being the most sensitive species. The antifungal potential of 

the gel in decay suppression was exhibited on P. digitatum-inoculated 

grapefruit, and was expressed in a delay in lesion development as well as 

a significant reduction in infection incidence at shelf life conditions (Saks 

and Barkai-Golan, 1995). 

Latex, present in some fruits, is another natural fungicide or a source 

of fungicides, which is regarded as both safe and effective against various 

diseases of banana, papaya and other fruits (Adikaram et al., 1996). 

Papaya latex is a complex mixture of sugars with several enzymes, 

notably proteases, glucosidases, chitinases and lipases. The water-soluble 

fraction of papaya latex can completely digest the conidia of many fungi, 

including important postharvest pathogens (Indrakeerthi and Adikaram, 

1996). A small cystein-rich protein, he vein, was isolated from the latex of 

the rubber tree (Hevea brasiliensis). It showed a strong antifungal 

activity in vitro against several fungi such as B. cinerea and species of 

Fusarium and Trichoderma (van Parijs et al., 1991). Hevein, which is a 

chitin-binding protein, might interfere with fungal growth by binding or 

cross-linking newly synthesized chitin chains. Alternatively, the delicate 

balance between chitin synthesis and selective hydrolysis of preformed 

chitin chains in hyphal walls might be interrupted (Cabib, 1987; van 

Parijs et al., 1991). 

Antibiotic compounds secreted by antagonistic bacteria are also 

among the natural compounds which may suppress postharvest pathogen 

development. Such compounds are the iturins, antimicrobial substances 

produced by Bacillus subtilis strains or isolates. They have a wide 

antifungal spectrum and were found effectively to control M. fructicola 

development in cold-stored stone fruits (Pusey, 1991). Pyrrolnitrin is a 

natural antibiotic compound produced by Pseudomonas cepacia. This 

compound was found to delay JB. cinerea development in strawberries, 

although no reduction of the total incidence of decay was recorded 

(Takeda et al., 1990). 

F. LECTINS 

Lectins are a class of sugar-binding proteins that are widely 

distributed in nature and their occurrence in plants has been known 




since the end of the 20^^ century. However, the role of plant lectins is still 

not well defined and understood. One of the theories proposed 

hypothesizes that lectins act as recognition determinants in the 

formation of symbiotic relations between leguminous plants and 

nitrogen-fixing bacteria, a process of great importance in agriculture and 

in the nitrogen cycle of terrestrial life. According to a second hypothesis, 

these lectins can play a role in the defense of plants against various 

animals, as well as against phytopathogenic fungi (Sharon, 1997). 

Interactions of fungal hyphae with lectins were first demonstrated by 

Mirelman et al. (1975), who found that wheat germ agglutinin (WGA), 

which is specific to chitin oligosaccharides, binds to hyphal tips and 

hyphal septa of Trichoderma viride - a fungus with a chitinous hyphal 

surface. This interaction resulted in the inhibition of hyphal growth and 

spore germination. Based on these findings it was suggested that WGA 

has a role in the protection of wheat seedlings against chitin-containing 

fungi. 

To assess whether the binding of lectins to fungal surfaces and their 

inhibiting effects on fungal growth are general phenomena, Barkai-Golan 

et al. (1978) investigated the interaction of several lectins, characterized 

by differing sugar specificities, with various fungi belonging to different 

taxonomic groups. WGA was found to bind to young hyphal walls of all 

the fungi examined except for the chitinless Phytopthora citrophthora 

(Table 11). Soybean agglutinin (SBA), specific for D-galactose and 

N-acetyl-D-galactose, and peanut agglutinin (PNA), specific for 

D-galactose, were found to bind those fungi that contain galactose 

residues in their chitinous cell walls. Among these were several species of 

Penicillium and Aspergillus. Other fungi, such as Geotrichum candidum, 

Botrytis cinerea and Fusarium moniliforme, exhibited binding with SBA 

alone. The abilities of SBA and PNA to bind to fungal surfaces of 

Penicillium and Aspergillus species is in good agreement with the 

presence of galactose reported in some species of these genera, such as 

Penicillium digitatum and P. italicum (Grisaro et al., 1968) or 

Aspergillus niger (Bardalaye and Hordin, 1976). The binding of fungi to 

concanavalin A (Con A) is more difficult to account for since this lectin 

reacts only poorly with p-linked glucans and with chitin (Sharon and Lis, 

1989). It is possible, however, that a positive reaction with Con A 

indicates the presence of small quantities of a-linked D-glucose (or 

D-mannose) residues on the fungal surface (Barkai-Golan et al., 1978). 



186 


TABLE 11 

Binding of different lectins to fungal cell walls^ 

Fungus species and 

taxonomic group 

Phycomycetes 

Zygomycetes 

Rhizopus stolonifer Ehr. 

Mucor sp. 

Oomycetes 


(Sm. et Sm.) Leon. 

Ascomycetes 

Pleospora herbarum (Pers.) 

Raben. 

Sclerotinia sclerotiorum (Lib.) 

de By. 

Basidiomycetes 

Rhizoctonia solani Kuhn 

Sclerotium rolfsii Sacc. 

Deuteromycetes 

Penicillium italicum Wehm. 

P. expansum Link 

P. citrinum Thorn 

P. digitatum Sacc. 

Chemical Lectin^ 

category2 wGA SEA PNA ConA 

Chitosan-chitin 

Cellulose-P-glucan 

Chitin-P-glucan 



+ = marked binding; ± = moderate to weak binding; - = no binding 

1 Reproduced from Barkai-Golan et al. (1978) with permission of Springer- 

Verlag GmbH & Co. KG. 

2 As proposed by Bartnicki-Garcia, 1968 

3 WGA = wheat germ agglutininm; SBA = soybean agglutinin; 

PNA = peanut agglutinin; ConA = concanvalin A 

"^ Binding confined to thin hyaline hyphae 


+ 

+ 

- 

+ 

+ 

-H 

-H 

+ 

+ 

+ 

± 

- 

-1- 

± 

± 

± 

± 

- 

-H 

± 

± 

± 

± 

+ 

+ 

+ 

- 

+ 

+ 

+ 

4- 

±



Fungus species and 

taxonomic group 

Aspergillus niger v. Tiegh. 

A. flavus Link 

A. ochraceus Wilhelm 

Geotrichum candidum Link 

Botrytis cinerea Pers. 

Fusarium moniliforme Sheld. 

Trichoderma viride Pers. ex Fries 

Diplodia natalensis P.E. 

Stemphylium botryosum Wall. 

Alternaria tenuis Nees 

Cladosporium herbarum (Pres.) 

Link 

Chemical 

category^ 


WGA 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

Lectin^ 

SBA PNA ConA 

± 

± 

+ 

H- 

± 

± 

- 

- 

- 

- 

— 

± + 

± + 

+ + 

- + 

- + 

- + 

- + 

- ± 

- 

- 

— — 

The binding of PNA and SBA occurred typically along the hyphal 

walls, with no pronounced binding at the tips and septa, indicating that 

both lectins bind preferentially to relatively mature regions of the 

hyphae. The binding pattern of WGA, on the other hand, emphasized 

that this lectin binds to regions of the hyphae in which chitin is actively 

synthesized (Barkai-Golan et al., 1978). 

The three lectins were also found to inhibit the incorporation of 

acetate, N-acetyl-glucoseamine and galactose into young hyphae of 

Aspergillus ocharaceus, indicating the interference with normal fungal 

activity and growth (Barkai-Golan et al., 1978). 

These results suggest that lectins may be useful in studies of chemical 

composition of hyphal wall surfaces. Lectins with different sugar 

specificities have actually been efficient in studying hyphal cell walls of 

mycobionts isolated from different lichens (Galun et al., 1976) and were a 

useful aid for the study of cell wall composition of yeasts of different 

taxonomic groups (Barkai-Golan and Sharon, 1978). However, the 

findings also support the suggestion that one role of lectins in plants is 

protection against fungal pathogens. 

Additional support for the antifungal capacity of lectins was 

contributed by an extensive study with 11 purified lectins, representing 




all the major groups of lectin specificity. A growth disruption during 

germination of spores was reported for three fungal species, including the 

postharvest pathogen Botryodiplodia theobromae (Brambl and Gade, 

1985). According to a subsequent report, however, the antifungal activity 

of WGA was most likely due to contaminating chitinases in the lectin 

preparations used, which are known as potent inhibitors of fungal 

growth (Schlumbaum et al., 1986). However, this criticism does not 

invalidate the results obtained with lectins such as peanut and soybean 

agglutinins, which are specific for sugars other than oligomers of 

N-acetylglucosamine (Sharon and Lis, 1989). Furthermore, a 

chitinase-free lectin obtained from stinging nettle (Urtica diocia), also 

specific for chitin, was demonstrated to inhibit the growth of several 

phytopathogenic fungi (Broekaert et al., 1989). The nettle lectin probably 

acts by interfering with chitin synthesis by the fungi. 

It should be mentioned, however, that although plant lectins have 

shown antifungal activity towards various plant pathogenic fungi, no 

protective role of these lectins has been demonstrated in vivo. 



CHAPTER 10 

PHYSICAL MEANS 

There is increasing public awareness that many of the chemical 

treatments applied to fresh horticultural products to control decay are 

potentially harmful to consumers. This fact, along with the possible 

consequence that a number of chemicals may be withdrawn from use, 

has revived and increased interest in physical treatments that could 

serve as alternatives to fungicides. Cold storage and controlled or 

modified atmospheres are physical means discussed in the chapter on 

Means for Maintaining Host Resistance. The present chapter will 

discuss other physical means: heating, ionizing radiation and 

ultraviolet illumination. 

A. HEAT TREATMENTS 

Heat treatment may be applied to the commodity by means of hot 

water dips and sprays, hot vapor or dry air, or infrared or microwave 

radiation. However, practical systems have used mainly hot water or 

vapor (Couey, 1989). While hot water was originally used for fungal 

control and was extended to removal of insects from fresh commodities, 

vapor heating, developed for insect control may also serve to reduce 

fungal decay. Thus, the vapor heat treatment used for disinfestation of 

Carabao mangoes in the Philippines also significantly reduced the 

incidence of anthracnose and stem-end rot in fruit, although the onset of 

decay was not delayed by the treatment (Esquerra and Lizada, 1990). 

Similarly, the development of green mold on grapefruit, caused by 

Penicillium digitatum was inhibited by the 300-min treatment with 

moist forced air at 46°C used to provide quarantine security against the 

Mexican fruit fly (Shellie and Skaria, 1998). Hot humid air has also been 

useful in controlling decay in crops that would have been injured in hot 

water. For example, postharvest decay of strawberries caused by Botrytis 

cinerea and Rhizopus stolonifer was controlled by exposure of the fruit to 

humid air at 44°C for 40-60 min (Couey and FoUstad, 1966). 

The possibility of using hot water dips to control decay in citrus fruits 




had already been reported in the 1920s (Fawcett, 1922), and during the 

following decades, hot water treatments were tested on and applied to 

various fruits and vegetables, although the intensive use of chemicals 

pushed them aside, along with other means of control. Today, with the 

trend toward less reliance on chemical treatments, the interest in 

postharvest use of heat treatment has revived. A list of commodities 

heat-treated to control postharvest decay is presented in the reviews by 

Barkai-Golan and Phillips (1991) and Coates and Johnson (1993). 

Short-term and Long-term Heat Treatments 

Short-term heating, where the fruit or vegetable is dipped in hot water 

at temperatures above 40°C (generally 44-55°C) for a short time (from a 

few minutes to 1 h), has been the main heat treatment studied over the 

years. The principle is in the use of temperatures that are high enough to 

inactivate the pathogen without causing significant changes to the host. 

Early studies had already confirmed that fruits and vegetables commonly 

tolerate such temperatures for 5-10 min, and that even shorter exposures 

to these temperatures is sufficient to control many of the postharvest 

pathogens (Smith, W.L., Jr. et al., 1964). 

Recent studies reveal increased interest in long-term heat treatments, 

in which the commodity is exposed to temperatures lower than those 

mentioned above (usually 38-46°C), but for a longer duration (12 h to 4 

days) (Fallik et al., 1996; Klein et al., 1997). Both short-term and 

long-term heating, aimed at suppressing storage decay, could act directly 

by inactivating the pathogen, or indirectly via physiological and 

biochemical changes in the host, which enhance the resistance of the 

tissues to the pathogen. 

The Effect of Heat on the Pathogen 

Genetic differences among fungi are expressed in differences in their 

sensitivities to high temperatures and, therefore, in differences among 

levels which kill them or inhibit spore germination and hyphal growth 

(Sommer et al., 1967) (Fig. 30). For a given species, spore inactivation 

increases with both temperature and duration of treatment; conidia of 

Alternaria alternata may be inactivated equally by treatment for 2 min 

at 48°C or for 4 min at 46°C (Barkai-Golan, 1973) (Fig. 31). 

Spore sensitivity to heat is also dependent on their physiological state. 

Germinated fungal spores are markedly more sensitive to heat than 

non-germinated spores. It was found that water at 42°C does not affect 

dormant conidia of A, alternata but does inactivate many germinating 



Physical Means 191 

(0 

c 

100.0> 

10.0 

1.0 

0.1 f 

D M. fmcticoia 

A B. cinerea 

O C. herbarum 

A R. stolonifer 

• P. expansum 

-f h- 

40 45 50 55 60 

Temperature fC) 

Fig. 30. Survival of spores of Molinilia fructicola, Botrytis cinerea, Cladosporium 

herbarum, Rhizopus stolonifer and Penicillium expansum after 4 min heat at the 

indicated temperatures. Reproduced from Sommer et al., 1967 with permission 

of the American Phytopathological Society. 

(0 

> 

CO 

100 

24 42 44 46 48 

Temperature ("C) 

Fig. 31. Heat response curve for Alternaria alternata spores (2 and 4 min heat 

treatment). Reproduced from Barkai-Golan, 1973 with permission of the 

Mediterranean Phytopathological Union). 




conidia (Barkai-Golan, 1973) (Fig. 32). Similarly, the LD50 temperature 

for sporangiospores of Rhizopus stolonifer exposed to hot water for 4 min, 

was 49°C, whereas that for germinating spores was only 39°C (Eckert 

and Sommer, 1967). 

The moisture content of spores before or during exposure to heat can 

considerably influence heat transfer and spore survival (Edney and 

Burchill, 1967). Comparisons between the heat sensitivities of moist and 

dehydrated conidia of Penicillium digitatum revealed that exposure to 

70°C for 30 min killed 90% of the moist spore population but only 10% of 

the dry spore population. Dry spores that survived this treatment were 

capable of infecting citrus fruit, although the onset of symptoms was 

delayed 24 hours. The resistance of the dry spores to high temperatures 

may explain their ability to survive from season to season without losing 

their pathogenicity to citrus fruit (Barkai-Golan, 1972). 

Possible mechanisms of pathogen control by heating include: pectic 

enzyme inactivation or denaturation of other proteins; lipid liberation; 

destruction of hormones; depletion of food reserves; or metabolic injury, with 

or without accumulation of toxic intermediates. More than one of these 

mechanisms may act simultaneously (Barkai-Golan and Philhps, 1991). 

2 4 6 

Hours at 25°C 

Fig. 32. Sensitivity to heat (2 min at 42°C or 46T) of freshly harvested and 

germinating spores of Alternaria alternata. (Reproduced from Barkai-Golan, 

1973 with permission of the Mediterranean Phytopathological Union). 




Ultrastructural changes in heat-treated, non-germinated spores of 

Monilinia fructicola were seen as progressive destruction of the 

mitochondria, disruption of vacuolar membranes and formation of gaps 

in the conidial cytoplasm (Margosan and Phillips, 1990). The site most 

sensitive to heat in dormant conidia of M fructicola may be the 

mitochondria, probably in the inner membrane. In germinating conidia, 

exposure to heat results in changes in the nuclei, in the cell wall, or both 

(Baker and Smith, 1970). 

As can be seen from the in vitro studies (see Fig. 28), Botrytis cinerea 

is very sensitive to high temperatures, and heat treatments have found 

to control it on various fruits and vegetables, including apples (Fallik et 

al., 1995; Klein et al., 1997), sweet peppers (Fallik et al., 1996), tomatoes 

(Fallik et al., 1993) and strawberries (Garcia et al., 1996). 

Short-term, hot water dips (50°C for 3 min) completely inhibited, or 

significantly reduced, decay development in artificially inoculated or 

naturally infected sweet red peppers (Fallik et al., 1996). Higher 

temperatures or longer exposures to 50°C resulted in heat damage. The 

mode of action of hot water dips on decay development appears to be 

interaction with fungal pathogens, as was exhibited by the inhibition of 

spore germination and germ-tube elongation of B. cinerea and A. alternata, 

the two main fungi responsible for postharvest decay of peppers. 

Long-term, hot water immersion (38°C for 3 days) of Botrytisinoculated 

tomatoes totally prevented gray mold decay under shelf-life 

conditions, with no damage to the fruit (Fallik et al., 1993). It was shown 

that heat treatments at 38*^C directly suppressed spore germination of B. 

cinerea within 24 h following their exposure to heat stress. Longer heat 

treatments inhibited hyphal growth and prevented expansion of the 

colony. The direct effect of long-term heat treatment has been exhibited 

also for the more heat-resistant fungus, Penicillium expansum (Fallik et 

al., 1995). Heating to 38, 42 or 46°C directly arrested spore germination 

and mycelial growth in culture, the necessary duration of exposure being 

in inverse proportion to the temperature. However, the direct effect is not 

the only way in which the long-term heating (38°C, 96 h) inhibits decay 

development in P. expansum-inoculated apples: a similar inhibition of 

spore germination also occurs when the spores are incubated in peel 

extracts derived from fruits heated under the same conditions without 

any direct exposure of the spores to the heat. In this case, decay 

inhibition is the result of the indirect effect of heat on the pathogen, via 

the heat-treated host. 




The Effect of Heat on the Host 

Heat can affect many of the processes within the tissues, such as the 

rate of fruit ripening, the development of fruit color, sugar metabolism, 

electrolyte leakage, ethylene production, respiration rate, pectic enzyme 

activity, volatile production and susceptibility to pathogens 

(Barkai-Golan and Phillips, 1991). Decay control together with ripening 

inhibition was reported for plastic-wrapped nectarines treated by moist 

air heating (Anthony et al., 1989). The inhibition of ripening by heat may 

be mediated via its effect on the ripening hormone, ethylene, since heat 

treatment has been found to inhibit ethylene synthesis within hours, in 

both apples and tomatoes (Biggs et al., 1988; Klein, 1989). Elevated 

temperatures can lead to the accumulation of endogenous ACC 

(1-aminocyclopropane -1- carboxylic acid), the precursor of ethylene 

synthesis, in apple and tomato tissue, concomitantly with the decrease in 

ethylene. However, raising the temperature higher or extending the 

exposure of the fruit to it will cause the disappearance of ACC as well 

(Klein, 1989; Atta Aly, 1992). The inhibition of ethylene formation is 

reversed when the fruits are removed from heat; this recovery requires 

protein synthesis, and both mRNA and protein of ACC oxidase were 

found to accumulate during recovery from heat treatment (Lurie et al., 

1996). A relationship between the inhibition of both ACC synthase and 

ACC oxidase was detected in mango fruits during heat treatment (Ketsa 

et al., 1999): following heating, ACC oxidase recovered full activity while 

ACC synthase recovered only partially, but sufficiently to allow the 

previously heated fruits to achieve an ethylene peak. During the heating 

period, not only is endogenous ethylene production inhibited, but the 

fruits also do not respond to exogenous ethylene (Yang et al., 1990). 

Heated fruits often soften more slowly than non-heated fruits. Plums, 

pears, avocados, and tomatoes have all been found to soften more slowly 

when held at temperatures between 30 and 40°C than at 20''C. The rate 

of softening increased when the heated fruits were returned to 20°C, but 

it was still less than that of non-heated fruits (Klein and Lurie, 1991). 

Holding apples at 38°C for 3-4 days prior to removal to prolonged cold 

storage (0°C) resulted in maintenance of firmness even after 6 months of 

storage at 6°C, and 10 shelf-life days at 20°C. The retardation of 

softening was related to the high level of insoluble pectin in the heated 

fruit as compared with that in the non-heated fruit, because of the 

inhibition of the synthesis of cell-wall degrading enzymes. 

A heat treatment can change the climacteric respiration peak as well 

as advancing or delaying it after treatment; the extent to which it does so 




depends on the temperature and the length of exposure. The response of 

a particular fruit or vegetable is determined by a combination of factors: 

the physiological age of the commodity, the time and temperature of 

exposure, whether the commodity is removed from heat to storage or to 

ripening temperature, and whether the heat treatment causes damage 

(Lurie, 1998). 

Decay Suppression through Increased Host Resistance 

Along with the retardation of the ripening process which follows heat 

treatments, heating may lead to the maintenance of fruit quality during 

prolonged storage. In many cases no damage is caused to the fruit and, in 

some fruits, enhanced resistance to pathogens is induced in parallel with 

the maintenance of fruit firmness. In this respect, heat treatment could 

be included among the means that suppress decay by maintaining host 

resistance. 

Spotts and Chen (1987) described decay suppression in injured pears, 

obtained through enhanced resistance to postharvest diseases which 

followed heating at 37°C; decay caused by Mucor and Phialophora was 

reduced when the fruits were inoculated after heating - that is to say, 

without the direct exposure of the pathogen to heat. In this case, the 

heating was found to increase the resistance of the wound to fungal 

infection. The mechanism by which heat treatment causes changes in 

fruit ripening and induces host resistance may be tied to changes in gene 

expression and protein synthesis (Sachs and Ho, 1986). Various studies, 

however, related the increased infection resistance of the host, following 

heating, to the accumulation and enhanced activity of antifungal 

compounds. 

1. Enhanced antifungal activity of the fruit 

Heating green lemon fruits (36°C, 3 days) was found to inhibit the 

reduction of the antifungal citral in the rind, which occurs naturally 

during fruit ripening (Rodov et al., 1995b). This was suggested to be the 

reason why heating the fruit prolongs the antifungal activity of the rind 

and elicits a considerable reduction in decay development (Ben-Yehoshua 

et al., 1995). The hypothesis is, therefore, that decay suppression 

following heat treatments is related, at least in part, to the enhanced 

activity of the antifungal compounds located in the peel (preformed 

antifungal compounds). 

Heat stress can also induce the synthesis of the enzyme, phenylalanine 

ammonia lyase (PAL) in citrus fruit (Golomb et al., 1984). This is one of 




the key enzymes in the production of various antifungal compounds. PAL 

has also been suggested to be involved in the enhanced resistance of 

kiwifruits to infection, after curing; Ippolito et al. (1994) found that the 

gray mold rot in kiwifruits which had been inoculated with Botrytis 

cinerea at the pedicel scars or on artificial wounds, was significantly 

decreased after curing at 15°C and 95-98% RH for 48 h, before storage at 

0°C. The activity of PAL at the sites of infection was enhanced in cured 

fruits and, under blue light, an accumulation of phenolic compounds was 

recorded. 

The accumulation of a phytoalexinic compound in heat-treated citrus 

fruits was reported by Kim et al. (1991). Heat treatments applied to 

lemon fruits inoculated with Penicillium digitatum resulted in the 

accumulation of the phytoalexin scoparone (6, 7-dimethoxycoumarin) in 

the peel during heating and several days later. The increase in the 

scoparone concentration in the fruit was directly correlated to the 

increase in the antifungal activity of the fruit extracts, as exhibited in 

their ability to retard spore germination and germ-tube elongation. From 

these results arose the hypothesis that the phytoalexins have an 

important role in enhancing the disease resistance of the heated fruits 

(Kim et al., 1991). 

2. Induction of heat shock proteins 

Various varieties of fruits and vegetables may differ in their tolerance 

to high temperatures. In general, tropical fruits, such as mangoes and 

papayas, are more heat tolerant than fruits from temperate zones 

(Couey, 1989). Exposing the host to too high a temperature may result in 

heat injury - expressed in changes in color and texture, increased water 

loss and enhanced susceptibility to infesting microorganisms. 

The exposure of plant tissues to thermal stress was found to result in 

the rapid induction of a small set of specific proteins called heat shock 

proteins (HSPs) (Key et al., 1981). A correlation was found between the 

development of enhanced fruit thermotolerance and the synthesis of 

HSPs, as well as between the loss of thermotolerance and the 

disappearance of HSPs (Vierling, 1991). The development of 

thermotolerance depends on the temperature level: to initiate HSP 

synthesis, the temperature should be high enough (35-40'^C), but at 

higher temperatures (>42°C), however, HSP synthesis is attenuated and 

commodities are likely to suffer heat damage (Ferguson et al., 1994). The 

induction of HSPs of differing molecular weights has been described in 

various fruits, such as papaya, plum, apple and tomato, or in pear fruit 




cell culture (Lurie, 1998). By enhancing host thermotolerance, the HSP 

synthesis which follows heat treatment may contribute to the possibility 

of using higher, more effective heat exposures for heat-sensitive fruits as 

well. 

Induction of Resistance to Chilling Injury 

Immersing citrus fruits in hot water for short periods was found to 

reduce their sensitivity to cold-storage temperatures, thus reducing both 

chilling injury and decay incidence during storage (Rodov et al., 1995a; 

Wild and Hood, 1989). The addition of fungicides, such as thiabendazole 

or imazalil, to the hot water (53°C, 2 min) can enhance the tolerance of 

the fruit to low temperatures (McDonald et al., 1991; Wild and Hood, 

1989), but even when this combination does not reduce chilling injury, it 

enhances the decay control imparted by the hot water. 

Electron microscopic examination of grapefruits which have been 

immersed in hot water has revealed structural changes, and less 

cracking in the fruit cuticle than in those of fruits which developed 

chilling injury. Wrapping fruits which had previously been immersed in 

hot water, in sealed plastic film enhanced their tolerance to chilling, but 

was not essential for the success of the treatment (Rodov et al., 1995a). 

Similarly to the short-term heat treatments (involving high 

temperatures), long-term heat treatments (involving lower temperatures) 

can also induce resistance to chilling injury in cold-sensitive fruits. 

Mangoes, which are a tropical crop, are subject to chilling injury when 

stored below 10°C (Couey, 1986). The symptoms of injury include rind 

discoloration, pitting, uneven ripening, poor color and flavor, and 

increased susceptibility to decay. However, when the fruits were kept at 

38°C for 24 or 48 h before storage at 5°C for 11 days, chilling injury 

symptoms were reduced, and the reduction was enhanced with increased 

duration of the high-temperature treatment (McCoUum et al., 1993). The 

most pronounced effect was reduction of the rind pitting and 

discoloration that were apparent at the time of transfer from 5 to 21°C, 

and which persisted during ripening. 

Chilling injury is also characteristic of tomato fruits stored at 

temperatures lower than 10-12°C. However, tomatoes exposed to 36-40°C 

for 3 days did not develop chilling injury, and ripened normally following 

storage at 2°C for 3 weeks (Klein and Lurie, 1991; Sabehat et al., 1996). 

The resistance to low-temperature injury was found to be contingent on 

the presence of HSP (Sabehat et al., 1996), but it may not be due solely to 

the presence of HSP. Heat treatment may cause damage to the cell 




membrane and is known to cause a temporary increase in membrane 

leakage (Lurie and Klein, 1990; Salveit, 1991). The ability of heat 

treatments to enhance resistance to low-temperature injury is an 

example how one stress - heat stress - may protect the plant tissue 

against another stress - low-temperature stress (Lurie et al., 1994). One 

should remember that the defense of the fruit against chilling injury may 

result, in addition to the repair of the physiological damage, in a 

reduction in the decay that typically develops in chilled tissues. Thus, 

following heat treatment, it is possible to store sensitive fruits at low 

temperatures without inducing chilling injury and subsequently 

increased decay development. 

Efficiency of Heat Treatments 

Heat is delivered to the commodity by means of contact with hot air or 

water. The water content of the air greatly influences heat transfer, and 

greater pathogen inactivation is usually achieved by treatment with 

moist heated air than with dry air at the same temperature (Teitel et al., 

1989). The reason for this may be the more vigorous physiological 

activity of moist spores than of dry spores, or the fact that moist air 

transfers heat more effectively than dry air. When dry air is applied, 

condensation does not form on the target commodity, and the rate of heat 

transfer depends largely on the temperature of the air passing over the 

surface of the commodity and the heat conductivity of the commodity. 

When the air is saturated (vapor heat), condensation forms on surfaces 

that are cooler than the air and heat is transferred rapidly to the surface 

(Edney and Burchill, 1967). 

The complex structure of a given host may greatly influence the rate of 

heat transfer. A grape berry, for example, transfers heat faster than the 

tissues of an apple. Also, heat transfer from tissue to tissue can vary 

greatly within the leaf, stem, root or fruit. Furthermore, heat transfer 

may differ among different tissues of the fruit itself. The colored outer 

layer (flavedo) of the citrus rind, which may have few and small 

intercellular spaces, can transfer heat faster than the underlying white 

spongy tissue (albedo) (Barkai-Golan and Phillips, 1991). 

Since heat treatments may act directly on the spore population 

infesting the host surface, a short exposure to heat may sometimes be 

sufficient to reduce decay incidence markedly. However, heat treatment 

is also based on the gradual penetration of heat into the host tissues; 

thus the extent of pathogen progress within the tissues may determine 

the success or failure of the treatment. Immersion of lemons in hot water 




(46-49°C) for 4 min arrests development of Phytophthora citrophthora, 

the causal agent of brown rot, only if the fungus has not yet penetrated 

the outer layer of the rind. Furthermore, factors affecting the rate of 

fungal development and its progress in the fruits, prior to harvesting, 

such as the temperature prevailing in citrus groves during the rainy 

season, may also determine the efficacy of heat treatment in arresting 

decay (Schiffmann-Nadel and Cohen, 1966). 

The increased water loss that often follows hot water treatment can be 

reduced by applying wax, with or without fungicides (Wells, 1972), or by 

wrapping the produce in plastic film before or after heat treatment 

(Anthony et al., 1989; Teitel et al., 1989). Such a wrapping may protect 

the fruit, not only from water loss but also from recontamination and 

discoloration (Barkai-Golan and Phillips, 1991). The fruit may also be 

protected from heat injury by preconditioning. Lemons that were slightly 

wilted or had been kept for 2-8 days at 15.5°C prior to heating, were more 

tolerant to heat than those exposed to the high temperature immediately 

after picking (Houck, 1967). On the other hand, the benefits of heat 

treatment may be augmented by enhanced pathogen sensitivity to high 

temperatures. Alternaria rot was controlled more effectively in tomatoes 

heat-treated 8 h after inoculation than in those treated immediately after 

inoculation (Barkai-Golan, 1973); the spores that had germinated during 

that period, as well as the germ-tubes or the young hyphae, were more 

sensitive to heat than non-germinated spores. 

The efficacy of postharvest hot water dipping in decay suppression has 

been reported for many host-pathogen combinations. Among these are: 

P. citrophthora and Penicillium digitatum in citrus fruits; Colletotrichum 

gloeosporioides in mangoes; various fungi in papayas and melons; 

Monilinia fructicola in plums; M fructicola and Rhizopus stolonifer in 

peaches and nectarines; and Gloeosporium spp. and Penicillium 

expansum in apples. The efficacy of moist hot air treatment (43°C, 30 

min) in decay control was reported for species of Botrytis, Rhizopus, 

Alternaria and Cladosporium in strawberries (Barkai-Golan and Phillips, 

1991). Although hot water treatment of strawberries has generally 

resulted in fruit injury, a recent study with strawberries of cv. Tudela 

showed that dipping the fruit at 44 or 46°C for 15 min delayed B, cinerea 

development, with good retention of firmness and no development of 

off-color or off-flavor (Garcia et al., 1996). Of all the effective heat 

treatments, the one most commonly used commercially is that applied on 

mangoes. In this treatment, the fruits are immersed for 15 min in hot 

water (50-55°C) prior to storage, to prevent anthracnose development 




(Spalding and Reeder, 1986b). The traditional hot water treatment (48°C, 

20 min) has been used successfully in Hawaii to control C. gloeosporioides, 

stem-end rots and incipient Phytophthora infection in papaya (Aragaki et 

al., 1981). However, several problems, such as delayed color development 

and increase in Stemphylium rot, arose from the hot water application 

(Glazener and Couey, 1984). 

Following the withdrawal of ethylene dibromide as a pesticide against 

fruit flies on papaya in 1984, the double hot water dip (42°C for 30 min 

followed by 49°C for 20 min) has been adopted for fruit disinfestation in 

papaya shipments to fruit fly-free zones. The double-dip procedure also 

provides postharvest disease control when coupled with field fungicide 

sprays (Alvarez and Nishijima, 1987). However, various papaya cultivars 

are sensitive to heat, and the fruit becomes increasingly susceptible to 

heat injury as it ripens. A single hot water dip at 49°C for 15 min was 

reported by Nishijima (1995) to be the optimum treatment for disease 

control with minimum detrimental impact on fruit quality. 

The basic problem in short-term treatments is the high temperature 

needed for decay suppression. Such a temperature is often near the level 

injurious to the commodity and it must, therefore, be carefully measured 

and controlled. Furthermore, temperatures too high for a given host may 

increase the host susceptibility to pathogens, even when no visible 

damage develops (Phillips and Harris, 1979). 

Combined Applications 

Over the years, combined heat-plus-chemical treatments have been 

developed in order to achieve decay control by using lower temperatures 

and shortened exposure time on the one hand, and reduced fungicide 

concentration, on the other. The improvement may be due to the more 

effective infiltration of the fungicides into the wound sites on the fruit, 

that are exploited by the fungus (Brown, G.E., 1984). 

Hot water containing fungicides has been found more effective than 

either of the separate treatments alone in controlling Rhizopus stolonifer 

infection in peaches, plums and nectarines (Wells and Harvey, 1970). The 

addition of thiabendazole, benomyl, captan or botran (dicloran) to water 

heated to 52°C enabled the immersion time to be reduced from 15 to 0.5 

min, without affecting decay control. Hot suspensions of benomyl or 

botran were similarly more effective than unheated suspensions, in 

controlling brown rot (Monilinia fructicola) in peaches (Smith, W.L., 

1971). Combinations of fungicides at a quarter of the recommended rates 

with 1.5-2-min dips in hot water at 51.5 and 54.5°C were equally or more 




effective in controlling brown rot on sweet cherries, peaches and 

nectarines, than unheated water treatments containing higher 

concentrations (Jones and Burton, 1973). 

In heat-sensitive mango cultivars, the addition of fungicides to the hot 

water enabled the temperature needed for controlling anthracnose 

{Colletotrichum gloeosporioides) to be reduced below the level which 

causes heat injury (Spalding and Reeder, 1986b). Such a reduction 

obviates the need for pedantic temperature control during treatment. 

The mechanism of improved control with heated fungicide mixes may be 

related in part to the direct effect of heat or to increased chemical 

activity; however, improved control may also be attributed to increased 

penetration of the fungicides into the host tissues (Wells and Harvey, 

1970). Trials with mature guavas similarly indicated that the incidence 

of rots, including Colletotrichum and Pestalotia spp., was greatly reduced 

by a 5-min dip in benomyl (0.5-2.0g l-^) heated to 48-50°C, compared with 

the effect of each treatment alone (Wills et al., 1982). 

The fungicidal activity of imazalil, which is registered for postharvest 

application to citrus fruit, to reduce both the incidence and the 

sporulation of Penicillium digitatum, was considerably increased when 

the fungicide was applied in hot water. Studies with Redblush 

grapefruits (Schirra et al., 1995) showed that dipping the fruits for 3 min 

in 1500 ppm imazalil solution at 50°C considerably increased the effect of 

the chemical, compared with its use in cold water, on fruits stored for 16 

weeks at 8°C followed by shelf-life conditions. 

The chilling injury index under these conditions was threefold lower 

than in fruits dipped in water at 20''C. Smilanick et al. (1997) found that 

imazalil effectiveness on citrus fruit was substantially improved when the 

fruits were passed through an aqueous solution of the chemical heated to 

only 37.8°C, as compared with the current commercial practice, in some 

areas, of spra5dng wax containing imazalil at ambient temperatures. The 

improvement in imazalil efficacy in this case was due not only to the 

combination of the fungicide with heating, but also to its application in 

water, which is known to reduce the green mold more effectively than 

application in wax (Eckert et al., 1994). In addition, heating the fungicide 

solutions also accelerates the accumulation of fungicide residues in the 

fruit (Schirra et al., 1995), and the immersion of lemon or orange fruits for 

30 s in a 350-400 |ig ml-i fungicidal solution, instead of spraying the 

fungicide on the fruit, deposited sufficient residues (2-4 |ig gi) to control 

P. digitatum sporulation on the fruit. No rind injury was observed 

following these procedures (Smilanick et al., 1997). 




Immersion of P. digitatum-inocvloitedi lemons in 2% sulfur dioxide 

solutions reduced green mold incidence, without injury to the fruit; 

however, heating of the solutions was needed to attain acceptable efficacy 

(Smilanick et al., 1995). Heated solutions of sulfur dioxide were also 

superior to hot water alone for the control of green mold. Green mold 

incidence was reduced to less than 10% by sulfur dioxide treatment 

applied for 10 min at 30°C followed by two fresh water rinses, and no 

decay was recorded after storage for 1 week at 20°C, following 10-min 

treatments at 40 or 47°C (Fig. 33). No injury was observed after these 

treatments except in the case of immersion in sulfur dioxide solution 

heated to 47°C. 

Decay of lemon fruits inoculated with P. digitatum was effectively 

controlled by heated solutions of ethanol in water, at concentrations of 10 

to 20%, and control was superior to that of water alone at 32, 38 or 44°C 

(Smilanick et al., 1995). At 50°C the green mold was reduced to less than 

100 + 

80 

o 

CO 

+ol 60 

o 

0) 

Q 40 

20 + 

0+ 

T 

J I 

H 

Temperature (C) 

D 20 a 30 

• 40 g 47 

0 1 5 10 

Immersion time (min) in 2% sulfur dioxide 

Fig. 33. Incidence of postharvest green mold in Penicillium digitatuminoculated 

lemons followed 5h later by immersion for 1 to 10 min in 2% sulfur 

dioxide solutions at various temperatures followed by two fresh water rinses 

and storage for 1 week at 20°C. (Reproduced from Smilanick et al., 1995 with 

permission of the American Phytopathological Society). 


T 

00



5% even when low ethanol concentrations (2.5-5%) were used. Spore 

mortality of M fructicola and K stolonifer, the main postharvest 

pathogens of peaches and nectarines, occurred much more quickly in 

heated ethanol than in heated water (Margosan et al., 1997): a 10% 

ethanol solution at 46 or 50°C greatly reduced the LT95 (95% lethality) of 

both fungi (Table 12). 

TABLE 12 

Estimated time (seconds) to kill 95% of the spores of Monilinia 

fructicola or Rhizopus stolonifer in water, alone or containing 

10% ethanol, at 46 or 50°C* 

M. fructicola R. stolonifer 

Treatment 46^C 50^C 46^C 50°C 

Water 734.0a 206.3a >1000.0a 410.3a 

10% ethanol 57.4b 7.0b 94.4b 46.3b 

* Values followed by unlike letters are significantly different (P < 0.05). 

Reproduced from Margosan et al., 1997 with permission of the 


When peaches and nectarines, infected with M fructicola spores, were 

immersed in 10% ethanol at 46 or 50°C, or 20% ethanol at 46''C the 

incidence of decayed fruit was 83, 25 or 12%, respectively, i.e., similar or 

a better control was achieved than that following a 1-min dip in the 

fungicide, iprodione (1000 |ag ml^). No injury to the fruits occurred and 

no off-flavors were detected in the fruits. However, unlike fungicides, the 

hot ethanol treatments did not deposit persistent antifungal residues, so 

there was a lack of continuous protection of the fruits from 

recontamination. 

The increases in spore mortality and decay control - occurring when 

hot water and ethanol were combined - may result from their affecting 

the same sites in the spores. Since low concentrations of ethanol can 

lower the temperature at which phospholipids undergo a phase change 

(Rowe, 1983), the increases in spore mortality and decay control following 

the addition of ethanol may have resulted from a lowering of the 

phase-change temperature of the mitochondrial membranes of the spores 

under these conditions (Margosan et al., 1997). 




Non-pesticide chemicals can also improve the effectiveness of heat 

treatment. The addition of sucrose to hot water (52°C) reduced the 

external discoloration of peaches and nectarines, which usually appears 

under shelf-life conditions, following heat treatment. In fact, the addition 

of sucrose enabled heat to be used to control decay in these fruits 

(Barkai-Golan and Phillips, 1991). The action of sucrose is not yet clear, 

but it is possible that the sugar slows hydration of the fruit surface 

exposed to the hot water, or protects this surface in some other way. 

Enhanced efficiency of heat treatment in maintaining fruit firmness 

and reducing storage diseases can be achieved by combining heat and 

calcium treatments after harvest. Whereas immersing apples in water at 

45°C reduced Gloeosporium decay, but increased tissue breakdown, the 

addition of CaCh to the hot water controlled tissue breakdown and 

suppressed decay development (Sharpies and Johnson, 1976). Later 

studies with combined treatments showed that long-term heating (38°C, 

4 days) of Penicillium expansum- or Botrytis cmerea-inoculated Golden 

Delicious apples followed by pressure infiltration of 2 or 4% CaCb 

solution, elicited accumulative or synergistic effects in retarding both 

softening and decay development during cold storage and shelf-life 

conditions (Conway et al., 1994c; Sams et al., 1993). 

In another study with Golden Delicious apples, Klein et al. (1997) 

found that infections resulting from pre-storage inoculations of the fruit 

with B. cinerea spores were controlled only by a subsequent 4-day 

exposure to 38°C, and that neither 42°C for 1 day nor Ca treatment were 

effective in preventing decay during storage. Combining Ca infiltration 

(2% CaCl2) with 38°C heat treatment had no advantage over heating 

alone but resulted in increased decay development. It was suggested that 

the pressure infiltration with CaCk solution may have carried some 

B. cinerea spores further into the wound beyond the "zone of protection" 

afforded by the heat treatment (Klein et al., 1997). Furthermore, holding 

apples at 38°C for 4 days before Ca infiltration, decreased the amount of 

Ca taken in. The decreased intake of Ca may have been the result of a 

flow of epicuticular wax during heating, resulting in the sealing of 

surface cracks through which CaCh solution might have entered the fruit 

(Roy et al., 1994). 

A combination of heat with ionizing radiation has been found to be 

more effective than either means separately, in reducing various 

host-pathogen interactions, including oranges infected by Penicillium 

digitatum (Barkai-Golan et al., 1969), nectarines infected by Monilinia 

fructicola (Sommer et al., 1967), mangoes infected by Colletotrichum 




gloeosporioides (Spalding and Reeder, 1986b), and papayas infected by 

various postharvest pathogens (Brodrick et al., 1976). Heat and 

irradiation act synergistically to inactivate spore germination 

(Barkai-Golan et al., 1969, 1977a; Sommer et al., 1967), shortening the 

time of exposure needed for either treatment applied alone. The efficiency 

of the combined treatment is influenced by the sequence and is generally 

greater when heat precedes irradiation. The interval between heating 

and irradiation also affects the synergism; for good results, radiation 

should be applied within 24 h of the hot water treatment (Barkai-Golan 

et al., 1969; Spalding and Reeder, 1986b). 

A combination of heating and rinsing of fruits and vegetables has 

recently been achieved by the development of a fast hot water spray 

machine which simultaneously cleans the commodity and reduces the 

pathogen population on its surface (Fallik et al., 1999; 2000); the device 

is designed to be a part of a sorting line, and it exposes the commodity to 

hot water (50-70°C) for 10-60 s. This method significantly reduced decay 

incidence on sweet bell peppers and enabled fruit firmness to be 

maintained during prolonged storage and marketing. The optimal 

treatment conditions for this fruit were 55°C for about 12 s, while 59±1°C 

for 15 s was optimal for Galia melons. The low percentage of decayed 

fruits, together with the high level of cleanliness, were achieved by the 

combination of the hot water rinse with brushing that removes dirt, dust 

and fungal spores from the fruit, calyx and skin. The improved keeping 

quality of the rinsed and disinfected fruit has also been attributed to the 

melting of the wax layer, which seals small, almost invisible, cracks in 

the epidermis (Fallik et al., 1999; 2000). 

B. IONIZING RADIATION 

The possibility of utilizing ionizing radiation for extending the shelf 

life of fresh fruits and vegetables has been studied since the 1950s. These 

studies were mainly aimed at three directions: (a) control of postharvest 

diseases; (b) delay of the ripening and senescence processes; and 

(c) control of insect infestation for quarantine purposes. Most of the 

studies were carried out with CO^^ gamma rays. Here, we will focus on 

irradiation as a physical means for decay control and postharvest life 

extension of fruits and vegetables. This subject has been separately 

reviewed (Barkai-Golan, 1992). 




Radiation Effects on the Pathogen 

Ionizing radiation may directly harm the genetic material of the living 

cell, leading to mutagenesis and eventually to cell death. The nuclear 

DNA, which plays a central role in the cell, is the most important target 

molecule in microorganism irradiation, although radiation lesions in 

other components of the cell may also contribute to cell injury or even 

result in cell death (Grecz et al., 1983). Several factors affect the response 

of a pathogen to radiation, the first being the genetic resistance; fungal 

species may vary widely in their resistance to irradiation (Fig. 34). In 

general, multicellular spores, such as Alternaria or Stemphylium spores, 

or bicellular spores, such as those of the Cladosporium or Diplodia, are 

100 

100 150 0 50 100 150 200 250 300 350 400 

Radiation dose (krad) 

Fig. 34. Dose-response curves or inactivation of postharvest fungi by gamma 

radiation. 1 - Trichothecium roseum; 2 - Trichoderma viride; 3 - Phomopsis 

citri; 4 - Penicillium italicum; 5 - P. expansum; 6 - Aspergillus niger; 7 - P. 

digitatum; 8 - Geotrichum candidum; 9 - Monilinia fructicola; 10 - Botrytis 

cinerea; 11 - Diplodia natalensis; 12 - Stemphylium botryosum;13 - Rhizopus 

stolonifer; 14-Alternaria citri; 15-A. alternata; 16- Cladosporium herbarum; 

17 - Candida sp. (Reproduced from Barkai-Golan, 1992 with permission of 

Springer-Verlag GmbH & Co. KG). 




more resistant to gamma radiation than the unicellular spores of other 

fungal species (Sommer et al., 1964b). The multicellular or bicellular 

spore presumably exhibits resistance when one of the cells escapes injury 

and remains capable of forming a colony. 

For a given fungal species, the inhibiting effect of radiation increases 

with the radiation dose; however, for a certain dose, the rate of 

irradiation application may affect both spore survival and mycelial 

growth. It has generally been found that higher rates of irradiation 

increased the efficacy of the radiation, enabling the dose levels needed for 

pathogen inactivation to be reduced (Beraha, 1964). The greater 

radiobiological effect of a rapidly applied dose has been attributed to the 

lack of, or fewer opportunities for repair. 

The number of fungal spores or mycelial cells in the inoculum exposed to 

radiation may affect the radiation dose required for their inactivation. 

Increased spore density in the inoculum has generally necessitated an 

elevated radiation dose. In oranges inoculated with Penicillium digitatum 

and P. italicum, the reduction in spore concentration resulted in the 

prolongation of the incubation period of the disease (Barkai-Golan, 1992). 

The presence of oxygen in the atmosphere at the radiation site is an 

important factor in enhancing the effectiveness of a given dose. It was 

thus found that the dose required to reduce survival of Rhizopus 

stolonifer spores to 1% was much less in the presence of oxygen than that 

under anoxia (Sommer et al., 1964a). The increased antifungal effect was 

attributed, among other factors, to the formation of peroxides, which 

caused cell injury. 

Another factor affecting the radiation sensitivity of microorganisms is 

the water content of their cells. This may be the reason why vegetative 

cells are more sensitive to radiation than spores (Barkai-Golan, 1992). 

This is true for both fungi and spore-bearing bacteria. The high water 

content of bacterial vegetative cells may favor the production, within the 

cytoplasm, of a variety of harmful radicals that enhance the effects of 

radiation injury (Grecz et al., 1983). 

The timing of irradiation may also affect pathogen sensitivity: extending 

the time lag between inoculation and irradiation may increase dose 

requirements for pathogen suppression (Zegota, 1987). Since many of the 

postharvest pathogens enter the host via wounds incurred at harvest, the 

time lag between harvest and irradiation would parallel the time between 

inoculation and irradiation. During this period, the infection may be 

initiated and the size of population exposed to irradiation may increase. 

For quiescent fungi, an extension of the time interval between harvest and 




treatment may result in renewal of fungal development because of the 

progress of fruit ripening (Prusky et al., 1982; Droby et al., 1986). Under 

such conditions, decay cannot be suppressed by irradiation. 

Since radiation effects are associated with the size of the target 

population, one may presume that all environmental factors encouraging 

fungal growth, such as appropriate storage temperature or high 

humidity, will indirectly affect the dose required for pathogen control. 

Several studies indicate that the radiation dose required for pathogen 

suppression within host tissues is higher than under in vitro conditions. 

This is believed to be the consequence of the chemical protective effect 

afforded by the tissues when in contact with the pathogen. 

Irradiation may induce various degrees of injury and result in the 

formation of mutants among the survivors. Although mutants usually 

appear to be less pathogenic than the parent organism, mutants of wider 

virulence have also been observed in various plant pathogens (Sommer 

and Fortlage, 1966). An irradiated pathogen has the capacity to repair 

radiation damage under post-irradiation conditions. Environments found 

to be encouraging for recovery are characterized by their ability to slow 

down metabolism, such as sub-optimal temperatures, or starvation or 

anaerobic conditions (Barkai-Golan, 1992). 

Radiation effects on the pathogen may also be associated with the 

ability of the host tissue to induce antifungal phytoalexins in response to 

irradiation. Riov (1971) and Riov et al. (1971) reported on the 

accumulation of the stress metabolites, scopoletin and scopolin in the peel 

of mature grapefruits irradiated at 1-4 kGy, and of scoparone following 

irradiation at 3 and 4 kGy. These compounds were formed in the 

radiosensitive flavedo tissue of the peel in correlation with increased 

ethylene production, enhanced phenylalanine ammonia lyase activity 

(PAL), and the accumulation of phenolic compounds, which lead to cell 

death and peel pitting (Riov, 1975). More than 15 years after the first 

isolation of scoparone from irradiated grapefruits, and following isolation 

of this compound from other citrus cultivars (Valencia oranges and Eureka 

lemons), Dubery et al. (1988) showed its antifungal activity. Another 

irradiation-induced non-coumarin metabolite was extracted from damaged 

regions of citrus peel and was identified as 4-(3-methyl-2-butenoxy) 

isonitrosoacetophenone. This compound, which did not occur in extracts 

from non-irradiated fruits, has also been reported to have antifungal 

properties (Dubery et al., 1988). 

It should be mentioned, however, that in contrast to the radiationinduced 

phytoalexins in citrus fruits, El-Sayed (1978) reported that the 




phytoalexins, rishitin and lubimin formed in potato tubers and rishitin 

formed in tomato fruits, in response to infection, were reduced after 

gamma irradiation at doses required for potato tuber sprout inhibition 

(100 Gy) and for tomato fruit shelf-life extension (3 kGy). 

The formation of antifungal compounds in the host in response to 

radiation may be one factor in the complex of radiation effects on the 

pathogen, although its importance in disease suppression has not yet 

been evaluated. 

Radiation Effects on the Disease 

An important advantage of gamma radiation over most chemical 

treatments, which derives from its short wavelength, is its ability to 

penetrate into the tissues. This enables irradiation to reach not only 

microorganisms in wounds, but also those located within the host, as 

quiescent or active infections. Thus, we may also refer to irradiation as a 

therapeutic means, effective after infection has already started. However, 

the use of irradiation for decay suppression is basically determined by 

the tolerance of the host to radiation, rather than the fungicidal dose 

required for pathogen suppression; different host species, and even 

different cultivars of a given species, may differ in their tolerance to 

radiation. Furthermore, dose tolerance may be influenced by the state of 

fruit ripeness at the time of treatment and by the subsequent storage 

conditions (Maxie and Abdel-Kader, 1966). 

Early studies in the 1950s and 1960s had already established that 

fruits and vegetables are usually susceptible to radiation doses that are 

lethal to the common postharvest pathogens. However, the same studies 

also indicated that extension of the postharvest life of several 

commodities could be achieved by using sublethal doses, which could 

temporarily inhibit fungal growth and, therefore, prolong the incubation 

period of the disease. Such a prolongation could be sufficient to control 

decay in fruits with a short postharvest life, for which even a few 

additional lesion-free days are valuable. 

The best example of such a fruit is the strawberry, for which a dose of 2 

kGy may be sufficient to prevent decay for several days, without causing 

fruit injury or changes in the ascorbic acid content (Maxie et al., 1964; 

Barkai-Golan et al., 1971). However, one has to keep in mind that different 

cultivars of strawberries may respond differently to irradiation. Irradiating 

naturally infected strawberries of cv. Lassen with a 2-kGy dose, 

Barkai-Golan et al. (1971) found that the incubation period of gray mold at 

15°C was extended from 3 days to 10 days, and that the subsequent 




growth rate of the surviving fungal population was markedly reduced. 

Such a delay in the initiation of Botrytis infection may also indirectly 

reduce the levels of contact infection and "nesting^' during storage. 

A report from South Africa (Du Venage, 1985) indicates that 

irradiation of strawberries under commercial conditions prolonged the 

summer shelf life of the fruit from 3-12 days at ambient temperatures up 

to 50 days at 2'^C. However, for most commodities, even doses which are 

sublethal for the pathogen but are capable of retarding or temporarily 

halting its growth, may inflict radiation damage on the host. 

Radiation doses required for direct suppression of postharvest 

pathogens are generally above the tolerance level of the fruit and result 

in radiation damage (Barkai-Golan, 1992). Decay control by sublethal 

irradiation doses, which are beneath the damage threshold of the 

commodity, may be possible in two instances: a) when decay suppression 

is caused indirectly by delaying the ripening and senescence processes of 

the commodity; and b) when irradiation is combined with other physical 

or chemical treatments. 

Decay Suppression via Delay of Ripening and Senescence 

For several fruits a reduction in the incidence of fungal diseases has 

been recorded after exposure to relatively low radiation doses, that are 

incapable of directly suppressing, or even temporarily retarding, 

pathogen growth but are sufficient to delay the ripening and senescence 

of the commodity. Since the susceptibility of fruits and vegetables to 

postharvest infection increases as the ripening and senescence processes 

progress (see the chapter on Factors Affecting Disease Development - 

The Fruit Ripening Stage), one should not be surprised that radiation 

doses capable of retarding these processes may contribute towards 

disease suppression by maintaining the natural resistance to infection 

characteristic of younger tissues. It was thus found that doses of 50-850 

Gy are capable of inhibiting the ripening of mango, papaya, banana and 

other tropical and subtropical fruits (Thomas, 1985; Barkai-Golan, 1992). 

The success of the treatment in each of these fruits depends upon the 

balance between the dose needed to retard ripening and the dose 

tolerance of the fruit. Alabastro et al. (1978) found that irradiation of 

mature green Carabao mangoes with doses of up to 220 Gy delayed the 

appearance of anthracnose and stem-end rots by 3 to 6 days without any 

adverse effect on fruit appearance. The suppression of decay by such low 

doses suggests that no direct fungicidal effect on the pathogens is 

involved. Similarly, doses of 50-370 Gy on Cavendish bananas were 




sufficient to reduce Colletotrichum decay significantly, but not 

Thielaviopsis decay. The reduction of anthracnose was probably the 

result of ripening inhibition, while higher doses caused darkening of the 

peel, that increased with the dose (Alabastro et al., 1978). 

Irradiation is mostly effective in delaying senescence in potatoes and 

onions, the delay being typically expressed by the inhibition of sprouting 

during storage (photos 3 and 4). Because of its action on the meiosis in 

the meristematic area of these crops, irradiation may delay sprouting at 

doses as low as 50-150 Gy or even less (Thomas, 1983, 1984). Gamma 

irradiation at 60 Gy has similarly been found effective in preventing 

sprouting and rooting of fresh ginger rhizomes held at ambient 

temperatures (Mukherjee et al., 1995). 

However, in contrast to fruits, in which ripening and senescence delay is 

accompanied by decay suppression, potato or onion irradiation at 

sprout-inhibiting doses may result in enhanced susceptibility to storage 

pathogens. In order to prevent significantly increased decay in irradiated 

onions and garlic, healthy, good-quality bulbs should be used; they should 

be irradiated during the dormancy period; and good storage management 

should be practiced (Thomas, 1984). El-Sayed (1978) reported that the 

phs^toalexins, rishitin and lubimin formed in potato tubers, and rishitin 

formed in tomato fruits, in response to infection were reduced after gamma 

irradiation. In parallel, increased incidence of rotted tubers and fruits was 

frequently reported after exposure to the radiation doses required for 

potato sprout inhibition (100 Gy) and for tomato fruit shelf-life extension 

(3 kGy). Post-irradiation treatment of tubers and fruits with phytoalexins 

produced by members of the Solanaceae, contributed to the control of 

microbial spoilage (El-Sayed, 1978). 

Irradiating carrots at a sprout-inhibiting dose (120 Gy) generally 

results in increased decay during prolonged storage (6 months at 2°C and 

95-100% relative humidity). However, irradiation seems to contribute to 

decay suppression for short-term storage of carrots (Skou, 1977). Electron 

irradiation (P rays) apphed only to the top ends of carrots prevented 

sprouting for 6 months, without increasing the incidence of rot (Skou, 1977). 

Combined Treatments 

Treatments that combine radiation with other control means, physical, 

chemical or biological, were developed in an attempt to reduce the 

radiation doses required for disease control to beneath the damage 

threshold of the fruit or vegetable. Among the combined treatments, 

special attention has been drawn to heat-plus-radiation combinations. 




Hot water dips and gamma radiation were found to react synergistically 

on spore inactivation of various postharvest fungi (Barkai-Golan et al., 

1977a) permitting the use of a lower level of each of the treatments than 

when it was applied separately. In most cases the combined treatment is 

more effective when the hot water dip proceeds the irradiation (Sommer 

et al., 1967) (Fig. 35); the exposure of spores to high temperatures 

sensitizes them to radiation. As well as conferring the advantage of the 

synergistic effect of the two physical means, the combined treatment 

markedly delays decay development in many fruits, such as citrus, 

papaya, mango, plum and nectarine (Barkai-Golan, 1992). 

1 2 3 4 1 2 3 4 

The treatment 

Fig. 35. Survival of conidia after heat treatment and irradiation. 1 = irradiation 

alone, 2 = heat only, 3 = irradiation followed by heat, 4 = heat followed by 

irradiation: (A) Botrytis cinerea subjected to 44°C for 4 min or a 75-krad dose, or 

both; (B) Penicillium expansum subjected to 56°C for 4 min or a 20-krad dose, or 

both. Vertical lines indicate standard deviations (Reproduced from Sommer et al., 





In Penicillium digitatum-mocnlsited citrus fruit, a combined treatment 

of hot water (52°C, 5 min) and gamma irradiation (0.5 kGy) delayed the 

appearance of green rot by 33-40 days (Fig. 36) (Barkai-Golan et al., 

1969). Combining irradiation (0.75 kGy) with the conventional hot water 

treatment (50°C, 10 min) prolonged the shelf life of papayas by 9 days 

beyond the prolongation achieved by heating alone, facilitating the 

distribution of South African fruits within that country and making 

large-scale export by sea possible (Brodrick and Thomas, 1978). Hot 

water (55°C, 5 min) plus irradiation (0.75 kGy) also act synergistically in 

controlling anthracnose (Colletotrichum gloeosporioides) in mangoes. 

This treatment is commercially used for treating mangoes in South 

Africa, both for decay control and for quarantine control of the mango 

seed weevil (Sternochetus mangiferae) (Brodrick and Thomas, 1978). 

A study on South African cultivars of plums and nectarines has shown 

that a hot water dip (46°C, 10 min) enabled the suppression of postharvest 

100 

•5 60 + 

c 

o 

a: 

30 40 50 0 20 


Control #- 

Irradiation only A k 

Heat only (3 min) Irradiation + 3 min. Heat A A 

Heatonly (5min)o----0 Irradiation + 5 min. Heat A A 

40 50 

Fig. 36. Effects of combined treatments of heat and gamma radiation upon 

inactivation of Penicillium digitatum in inoculated Shamouti oranges. 

(Reproduced from Barkai-Golan et al., 1969 with permission of the American 





Photo 3. Sprouting inhibition of potatoes by gamma irradiation. 

Photo 4. Sprouting inhibition of onions by gamma irradiation. 




K/C RADUfi/# 

Photo 5. Decay inhibition in strawberries by gamma irradiation. 

::^^^m\ 

Photo 6. Effect of gamma irradiation combined with heating on mango fruits 

as compared to heating alone. 




pathogens but resulted in fruit damage (Brodrick et al., 1985); whereas 

irradiation alone (2 kGy) controlled fungal development but resulted in 

fruit softening. A combination of mild heat treatment (42°C, 10 min) with 

low-dose irradiation (0.75-1.5 kGy) effectively controlled development of 

Monilinia fructicola, Rhizopus stolonifer and Botrytis cinerea, with no 

significant changes in fruit texture, aroma or taste. 

Treating tomatoes with the combination of heating (50°C, 2 min) and 

low-dose irradiation (0.5 kGy) totally eliminated Alternaria rot under 

shelf-life conditions, but each of these treatments resulted in accelerated 

softening of the fruit (Barkai-Golan et al., 1993b). Gamma irradiation 

without hot water treatment reduced the incidence of bacterial soft rot 

caused by Erwinia and Pseudomonas species (Spalding and Reeder, 

1986a). In this case, the combination of heating and irradiation provided 

no better rot control than irradiation alone. 

By combining irradiation with fungicidal or fungistatic treatment it 

was possible to reduce both the irradiation dose and the concentration of 

the chemical compound. Moreover, since the suppressive effects of 

irradiation and the chemical treatments may differ greatly among fungal 

species, their combination might broaden the operational range against a 

wide variety of microorganisms (Barkai-Golan, 1992). 

Several studies have shown advantages in combining low-dose 

radiation, mild heat treatment and chemicals, compared with the 

double-component combined treatments, in controlling storage decay. Such 

an advantage was recorded in apples for which the association of heating 

(50°C, 10 min), irradiation (1.5 kGy) and benomyl (250 ppm), in this 

sequence, inhibited development of the blue mold (Penicillium expansum) 

under shelf-life conditions (Roy, 1975). For P. digitatum-inoculated 

Shamouti oranges, the combination of radiation (200 Gy), diphenyl (15 mg 

per fruit) and hot water dip (52°C, 5 min) extended the incubation period of 

the green mold beyond the extension caused by heat and radiation or by 

diphenyl and radiation (Barkai-Golan et al., 1977a). For Kensington Pride 

mangoes, a hot benomyl dip prior to low-dose irradiation (0.3-1.2 kGy) 

resulted in an additive effect which markedly improved the partial control 

achieved by irradiation alone (Johnson et al., 1990). 

A combination of gamma and ultraviolet radiation was found to be 

effective against fungi sensitive to low gamma doses, such as 

Phytophthora and Colletotrichum. In such cases, the combined treatment 

facilitates the reduction of the ionizing radiation dose needed for fungal 

inactivation (Moy et al., 1978). 




The feasibility of a combination process involving gamma irradiation, 

packing in closed polyethylene bags and biological control of rot-causing 

fungi was evaluated as a means of extending the shelf life of fresh ginger 

rhizomes at ambient temperatures (25-30°C) in India (Mukherjee et al., 

1995). The recommended procedure consisted of dipping rhizomes, which 

had previously been washed and dried, in a spore suspension of a 

Trichoderma sp. isolated from sclerotia of Sclerotium rolfsii (the major 

pathogen during extended storage) along with other antagonistic 

microorganisms. The next steps included air-drsdng, packing in chosen 

low-density polyethylene bags and irradiation at 60 Gy. The treated 

ginger rhizomes remained in good marketable condition for up to 2 

months at ambient temperatures without sprouting (thanks to 

irradiation at a sprout-inhibiting dose), showed suppressed decay 

development (thanks to the protection of the rhizomes or the cut surface 

of sliced rhizomes by the antagonistic Trichoderma isolate) and reduced 

weight loss (thanks to storage in closed polyethylene bags). 

Radiation Approval 

Following extensive studies in recent decades of the "wholesomeness" 

of irradiated food, the list of commodities approved for radiation by 

health authorities in various countries has been considerably lengthened. 

An important event took place in 1986 when gamma irradiation up to a 

dosage of 1 kGy was approved by the Food and Drug Administration of 

the United States, for treatment of fruits and vegetables. This has 

accelerated both research and development in the use of ionizing 

radiation as a physical means for postharvest life extension. 

C. ULTRAVIOLET ILLUMINATION 

Ultraviolet (UV) illumination is known to damage plant DNA and to 

affect several physiological processes (Stapleton, 1992). However, a 

special interest has recently been drawn to the ability of low doses of 

UV-C light (wavelength of 190-280 nm) to induce disease resistance in a 

wide range of fruits and vegetables. 

A number of lines of evidence from various fruits and vegetables 

(Wilson et al., 1994) indicate that the effect of UV-C is not solely due to 

its germicidal activity: (1) tissue inoculated after UV treatment was more 

resistant to invasion by the pathogen; (2) the UV effect was not always 

correlated with increased UV doses. In addition, UV treatments have 




also been found to retard ripening of several harvested commodities, such 

as tomato and peach (Liu et al., 1993; Lu et al., 1991), thus leading 

indirectly to reduction in their susceptibility to infection. 

Optimum UV-C doses for induced resistance in the various 

commodities fall within narrow ranges, specific for each commodity. Also 

the maximum protection provided by UV treatments occurs at varying 

times after treatment and depends on the commodity. Studies with 

UV-treated grapefruits showed that fruits picked at different times 

during the growing season responded differently to UV treatments. The 

temperature at which the fruit was stored following treatment was 

another factor affecting resistance development (Droby et al., 1993a). 

Maximum resistance of peaches against Monilinia fructicola and 

tomatoes against Rhizopus stolonifer was observed 48-72 h after UV 

treatment, whereas sweet potatoes exhibited maximum protection 

against Diplodia tubericola 1-7 days after treatment (Wilson et al., 

1997a). 

Ultraviolet Effects 

In UV-illuminated apples and peaches the induced resistance to decay 

has been attributed to the inhibition of ripening and the maintenance of 

the natural resistance of the young fruits to infection (Lu et al., 1991). 

Studying the changes found in UV-treated citrus fruits indicated that 

induced resistance occurs in concomitance with the induced activity of 

the enzymes phenylalanine ammonia lyase (PAL) and peroxidase 

(Chalutz et al., 1992; Droby et al., 1993a). These findings led to the 

hypothesis that induced activity of the two enzymes plays a role in the 

enhanced resistance to decay that follows UV treatment. 

Several investigations indicated the ability of UV illumination to 

induce phytoalexin production in various host tissues. UV illumination 

(254 nm) of citrus fruits was found to induce the phytoalexin scoparon 

(Rodov et al., 1995b; Rodov et al., 1992), the production of which is 

enhanced both by increasing the UV dose and by raising the storage 

temperature (Rodov et al., 1992). Its accumulation in kumquat fruit 

reached its peak 11 days after illumination, but the amount declined 

rapidly, returning to trace levels, typical of non-illuminated fruit, 1 

month after treatment. The accumulation of the phytoalexin was 

correlated with an increase in the antifungal activity of the flavedo which 

led to improved resistance of the fruit to infection by Penicillium 

digitatum. Such a defense against infection was achieved when 




illumination was applied before inoculation with P. digitatum, without 

direct contact of the pathogen with the UV light. Moreover, illumination 

of previously inoculated fruit failed to prevent decay development (see 

Fig. 14). 

The experiments with kumquats clarified that the fruit/pathogen 

interaction depends on the rate of fungal growth against resistance 

development. Inoculation of the fruits before UV treatment gives an 

advantage to the pathogen, whereas illumination applied 2 days before 

inoculation was sufficient to improve fruit resistance. Host resistance is 

thus dependent on the existence of a lag period between illumination and 

inoculation, during which phytoalexin can be accumulated and can 

suppress pathogen development (Rodov et al., 1992). Induced resistance 

in UV-treated grapefruits was found to reach its maximum level 24-28 h 

after the exposure to UV light, after which the level decreased (Droby et 

al., 1993a). Resistance was affected by the temperature at which the fruit 

was stored 24 h after UV treatment and before P. digitatum infection. 

Fungal development in the UV-treated fruit was characterized by 

sporadic mycelium and a marked inhibition of sporulation. All these 

findings in citrus fruits emphasize the fact that decay suppression 

following UV treatment is not connected with its germicidal properties 

alone, but also with its ability to induce biochemical and physiological 

changes in the tissues. This is especially marked when low doses of UV 

are involved. 

Further studies with UV-illuminated grapefruits (Porat et al., 1999) 

indicated that UV-treatment induced the accumulation of chitinase 

protein, while the combination of UV and wounding induced both 

chitinase and P-l,3-endoglucanase. These two enzymes, which are 

included among the 'pathogenesis-related' (PR) proteins, are believed to 

be involved in plant defense mechanisms against fungal infection (El 

Ghaouth, 1994). 

UV illumination of carrots was found to induce the accumulation of the 

antifungal phytoalexin, 6-methoxymellein, and to enhance root 

resistance to infection by Botrytis cinerea and Sclerotinia sclerotiorum 

(Mercier et al., 1993). The direct relationship between the accumulation 

of methoxymellein and the development of resistance to infection by B, 

cinerea, has previously been exhibited in freshly cut slices of carrot roots 

treated with heat-killed conidia of 5. cinerea (Harding and Heale, 1980). 

A recent study on table grapes (Nigro et al., 1998) found a significantly 

lower incidence of infected berries and a reduced diameter of S. cinerea 

lesions among artificially inoculated berries treated with UV-C doses of 




0.125-0.5 kJ m-2. These results were obtained both for berries wounded 

and inoculated just after the UV treatment, and for berries inoculated 

24-48 h after the treatment. Similarly to the effect of UV-C light on citrus 

fruit (Rodov et al., 1992), an interaction between the UV-C dose and the 

time of inoculation was also found in grapes. A significantly lower level 

of disease was found, however, in berries inoculated 24-48 h after 

illumination than in those inoculated just after the treatment. To check 

the possible influence of UV-C illumination on the wound-healing 

processes, berries were wounded before illumination and inoculated at 

different times (hours) later. Such berries showed lower infection levels 

than those wounded after the UV treatment. These results could be due 

to a wound-type response since, during the period between irradiation 

and inoculation, the wounded berries were kept at 15°C under high-RH, 

conditions reported to be suitable for the induction of wound-healing 

processes in several fruits and vegetables (Ben-Yehoshua et al., 1988; 

Ippolito et al., 1994). However, a disease reduction recorded in grape 

berries illuminated after the inoculation of freshly cut wounds led to the 

suggestion that the effect of UV-C light was independent of a 

wound-healing reaction and could be attributed to UV-C-induced 

resistance alone (Nigro et al., 1998). Induced resistance was exhibited 

within 24-48 h of irradiation, and increasing the time until inoculation 

resulted in increased disease, probably because of a decline in the 

UV-C-induced resistance. These results suggest a temporary effect of 

UV-C treatments, as was also found in citrus fruits inoculated with P. 

digitatum (Droby et al., 1993a). 



CHAPTER 11 

BIOLOGICAL CONTROL 

Increased official and public concern about the presence of fungicide 

residues in foods, and the development by pathogens of resistance to 

major fungicides, are two important reasons for the enhanced interest in 

the possibility of using biological control as an alternative, non-chemical 

means of decay suppression. 

The term 'TDiological control" or îDiocontrol" refers to the use of 

naturally found microorganisms which antagonize the postharvest 

pathogens we wish to suppress. Antagonism between microorganisms is 

a ubiquitous phenomenon involving fungi (including yeasts) and bacteria 

which naturally inhabit the soil and the surfaces of various plant organs 

(Blakeman and Fokkema, 1982; Fokkema and van den Heuvel, 1986; 

Andrews, 1992). It is assumed that biocontrol of plant diseases occurs 

naturally on aerial plant surfaces and may be one of the main reasons 

why crops are protected to some extent during their cultivation (Droby et 

al., 1996). 

One of the approaches to the isolation of antagonistic microorganisms 

for controlling postharvest diseases is through the promotion and 

management of natural epiphytic antagonists, already present on fruit 

and vegetable surfaces. Preharvest pesticide application and various 

postharvest treatments, such as fungicide and wax sprays, washes and 

dips, can greatly affect the resident microflora, both qualitatively and 

quantitatively. It has been commonly observed that the common practice 

of postharvest washing of harvested fruits and vegetables to obtain clean 

produce, may affect the microbial populations by removal of naturally 

occurring microorganisms. Fungicides may change the epiphytic 

microflora by affecting microorganisms other than the pathogens against 

which they are directed. These changes can, in turn, affect the pathogen 

resistance of the host. Such practices could possibly be modified to 

promote beneficial antagonistic microflora (Wilson and Wisniewski, 

1989). To manipulate epiphytic microbial populations of fruits and 

vegetables effectively, in order to control decay, information is needed on 

their ecology. Spurr et al. (1991) suggested that emphasis be placed on 

studying the impact of the environment on the microflora and that of 

preharvest, harvest and postharvest activities on disease development. 




In accordance with the results of these steps, models to guide 

management should be constructed. 

Essentially, a great amount of research over the years has been 

focused on developing biological control procedures based mainly on 

artificially introduced selective antagonists against postharvest 

pathogens. Considerable success has been achieved in this field, and a 

large amount of information on the subject has been accumulated 

(Wilson and Wisniewski, 1994). 

A. ISOLATION AND SELECTION OF ANTAGONISTS 

The production of antibiotics by the antagonist was long considered to 

be the only mechanism of antagonism. In accordance with this 

hypothesis, screening of antagonistic microorganisms has generally been 

carried out in vitro, based on encouraging the selection of organisms that 

produce growth-free antibiotic zones in Petri dishes when challenged 

with the pathogen. However, such screening is likely to overlook many of 

the microorganisms which may serve as potential antagonists in vivo, in 

spite of failing to produce antibiotic compounds in culture (Wilson and 

Wisniewski, 1989). Recent screening procedures, therefore, emphasize 

the isolation of antagonists which do not necessarily produce antibiotic 

compounds during their life cycle (Droby et al., 1992; Wilson et al., 1993). 

Most investigators have usually preferred to study antagonistic 

microorganisms which occur naturally on fruit and vegetable surfaces 

and, in fact, these have proved to be a reliable source of such 

microorganisms. Much attention has been given to naturally occurring 

yeast species, which do not rely on the production of antibiotic 

substances. They can colonize a wound for long periods, produce 

extracellular polysaccharides that enhance their survival, can proliferate 

rapidly by using available nutrients and are minimally affected by 

pesticides (Janisiewicz, 1988). 

Looking for alternative non-chemical methods to control postharvest 

diseases of avocados in South Africa, Korsten et al. (1995) evaluated the 

inhibitory effects of 33 bacteria isolated from avocado leaf and fruit 

surfaces, against the major fungal pathogens of avocado fruits. These 

pathogens included Colletotrichum gloeosporioides, Dothiorella 

aromatica and species of Thyronectria, Phomopsis, Pestalotiopsis and 

Fusarium. The antagonistic bacteria included various Bacillus species, 

which comprise a major component of the microflora tested, and species 



Biological Control 223 

of Corynebacterium, Erwinia, Pseudomonas, Staphylococcus, Vibrio and 

Xanthomonas. Of these, B, subtilis was the most effective antagonist. 

The search for antagonistic fungi against Colletotrichum musae, the 

causal agent of anthracnose of banana, revealed that almost all the fungi 

isolated from banana fruit surfaces had at least some antagonistic effect 

against the pathogen. The greatest antagonistic activity was recorded for 

Alternaria tenuissima and Acremonium strictum, that effectively 

inhibited Colletotrichum growth, both in vitro and on banana fruits 

(Ragazzi and Turco, 1997). Searching for bacteria antagonistic to Botrytis 

cinerea, the main pathogen of strawberry fruits, Moline et al. (1999) 

showed that most of the 52 bacterial isolates that were obtained from the 

surface of 'organically grown' strawberry fruits had some ability to 

inhibit the growth of the fungus. Eleven of these, which were found 

capable of interfering with conidial germination or fungal growth in 

vitro, were selected for additional screening tests on JB. cmerea-inoculated 

strawberry fruit. These isolates were identified as strains of 

Pseudomonas putida and Chryseobacterium indologenes. Three of the 

best isolates (two Pseudomonas and one Chryseobacterium) were also 

found to reduce the incidence of gray mold rot on fruits under field 

conditions (Moline et al., 1999). 

Wilson et al. (1993) developed a method for the isolation and screening 

of yeast antagonists. The method is based on the application of washings 

to fruit wounds, which are then challenged with the fungal spores; 

following a few days of incubation, microorganisms are isolated from the 

non-infected wounds. Under these conditions, the predominant 

microorganisms on the culture medium were of yeast species. Tests for 

antagonism were performed with pure cultures under in vivo conditions. 

Lorito et al. (1993) attempted to isolate selective biocontrol agents by 

utilizing media with purified polymers, such as chitin, in order to detect 

chitinase activity exhibited by the antagonistic microorganisms. Isolation 

based on chitinase activity of the antagonist offers some level of selection 

for properties potentially important to fungal control, but effective 

microorganisms which do not produce chitinases could be missed by the 

screening process. Some level of selection could also be achieved by using 

media made of pasteurized or autoclaved homogenates of the pathogens 

(Valois et al., 1996). This method may select only the microorganisms 

that can live on specific cellular content. A simple selection method for 

isolating bacterial antagonists to B, cinerea, which is both a pre- and a 

postharvest pathogen of strawberries, was recently developed by Moline 

et al. (1999); it is based on the ability of selected antagonistic bacteria to 




grow on insoluble material consisting primarily of cell-wall material 

obtained following the extraction of freeze-dried mycelium. 

In order to act as effective biocontrol agents and to qualify for 

development for commercial use on harvested crops, the antagonists 

should meet certain criteria. Moreover, determining the features needed 

for an antagonist to serve as a successful biocontrol agent is the first step 

before selecting potential antagonists. Wilson and Wisniewski (1989) 

listed the desirable characteristics of a potential antagonist: (1) 

genetically stable; (2) effective at low concentrations; (3) simple nutrient 

requirements; (4) capable of surviving adverse environmental conditions; 

(5) effective against a wide range of pathogens and on various fruits and 

vegetables; (6) resistant to pesticides; (7) a non-producer of metabolites 

deleterious to human health; and (8) non-pathogenic to the host. Various 

criteria connected with commercial development aspects, such as being 

easy to dispense and compatible with commercial handling practices, 

have also been mentioned. These desired features have guided many 

scientists in their biocontrol studies. 

In selecting an antagonist suitable for postharvest application, we 

need to look for those that are well adapted to survival and growth in 

wounds or on the produce surface under storage conditions, and that 

have an "adaptive advantage" over specific pathogens (Wilson and 

Wisniewski, 1989). For example, Rhizopus stolonifer is more sensitive to 

low temperatures than many microorganisms; therefore, an antagonist 

well adapted to low temperatures might prove advantageous against this 

pathogen. Another example is the advantage of a Candida oleophila 

strain in reducing the level of Penicillium expansum infection in 

nectarines under storage conditions: the effectiveness of the antagonistic 

yeast was not reduced by controlled atmosphere (CA) storage or by 

application of a commercial fungicide, therefore, it can be applied under 

CA conditions which are important for maintaining fruit quality, and in 

combination with dicloran which will, at the same time, prevent infection 

by other pathogens, such as Rhizopus (Lurie et al., 1995). 

For satisfactory control of postharvest pathogens of pome cultivars, 

the antagonist should be able to function under cold-storage conditions. 

Thus, the capability of several epiphytic bacteria isolated from apple 

leaves to control postharvest fungi on cold-stored apples, or that of a new 

strain of the yeast, Candida sake to control P. expansum, Botrytis 

cinerea, and R, stolonifer under various cold-storage conditions, is of 

importance when considering commercial application (Vinas et al., 1996; 

Sobiczewski et al., 1996). Since low temperatures and, in some cases. 




controlled atmospheres are applied for prolonged storage for other crops 

as well, Arul (1994) pointed out the importance of testing the 

effectiveness of antagonists under the intended storage conditions rather 

than at ambient temperatures or in air. 

The need for biocontrol agents with resistance to the chemicals 

commonly used in fruit and vegetable production has been emphasized 

by Spotts and Sanderson (1994). This feature is important since 

biocontrol agents applied postharvest would contact residues of 

fungicides which had been applied within a few days before harvest or 

those applied postharvest to control decay. 

While selecting antagonistic isolates, Sobiczewski et al. (1996) 

emphasized microorganisms that are capable of inhibiting several 

postharvest pathogens of the host. By screening of the antagonistic 

effects of 107 isolates of epiphytic bacteria originating from apple leaves, 

they found that only six of them satisfied this need and were capable of 

inhibiting both B. cinerea and P. expansum, the two main postharvest 

apple pathogens. 

In a recent study, Schena et al. (1999) found that isolates of the 

common yeast-like fungus, Aureobasidium pullulans at high 

concentrations (10^ and 10'^ cells ml-i), were able to control Penicillium 

digitatum on grapefruit, B, cinerea, R. stolonifer and Aspergillus niger on 

table grapes, and B, cinerea and K stolonifer on cherry tomatoes. Less 

decay control was exhibited at lower concentrations. The ability of 

A. pullulans isolates markedly to suppress postharvest decay caused by a 

range of pathogens on various fruits, and its ability to survive and 

increase its population under a variety of field conditions and during cold 

storage, suggested that this widespread and well adapted saprophytic 

fungus be considered as a biocontrol agent against postharvest pathogens 

(Leibinger et al., 1997; Schena et al., 1999). It is interesting to note that 

some of the A pullulans isolates significantly reduced decay caused by 

four different postharvest pathogens that could not be controlled with a 

single chemical fungicide. Furthermore, the resistance of A. pullulans to 

some commonly used fungicides (Lima et al., 1997) suggested the 

possible integrated use of this antagonist with chemical fungicides, to 

control postharvest rots. 

However, A. pullulans is characterized by extreme morphological and 

cultural variability and genetic instability (Bulat and Mironenko, 1992), 

whereas genetic stability has been mentioned as an important feature 

required for a successful biocontrol agent by Wilson and Wisniewski 

(1989). Schena et al. (1999) have recently confirmed the high genetic 




variability of A pullulans, by obtaining 41 isolates from the surfaces of 

fruits and vegetables cultivated in Italy. They found that only a few 

isolates had the same genetic patterns and showed similar biocontrol 

activities. 

Research in the past decade has mainly focused on microorganisms 

that are antagonistic to 'wound pathogens'. Since infection of wounds by 

fungal spores is very rapid, rapid colonization and growth at the wound 

site is a key characteristic of a successful antagonist (Droby et al., 1996). 

The features required for effective biocontrol agents should, therefore, 

include: the ability of the antagonist to colonize wounds; a rapid rate of 

growth in surface wounds; effective utilization of the nutrients present in 

the wound; and the capability to survive and develop at the infection 

sites better than the pathogen and to do so under a wider range of 

temperature, pH and osmotic conditions (Droby et al., 1996). 

An important point in the selection of biological control antagonists is 

the necessity to avoid strains or isolates that may injure either the 

plants they are intended to protect or other plants of economic 

importance. Hence, screening tests to prevent the use of harmful 

microorganisms should be a part of any effort to develop biocontrol 

antagonists (Smilanick et al., 1996). Pseudomonas syringae strains, for 

instance, can occupy wounds on the peel of citrus fruit and reduce the 

incidence of postharvest rots initiated by wound pathogens. However, 

some P. syringae pv. syringae strains are pathogens of citrus and other 

plants and, therefore, strains should not be approved for biological 

control until their risk of virulence to many hosts has been determined 

(Smilanick et al., 1996). 

B. INTRODUCTION OF ANTAGONISTS FOR DISEASE 

CONTROL 

To identify antagonists as promising agents, a screening system 

should simulate natural inoculation, and the inoculum should be applied 

in the proper infection courts at the proper time (Smilanick, 1994). The 

selection of the inoculation method used for in vivo screening is, 

therefore, critical to a successful strategy. A biocontrol agent has 

generally been introduced to the wound site prior to the arrival of the 

pathogen or shortly thereafter (Smilanick, 1994). Applying the 

antagonists after inoculation of the pathogen involves eradicative action. 

The synthetic fungicide imazalil, for instance, controlled green mold 



Biological Control Til 

decay in lemons effectively if applied 24 h after inoculation, while the 

antagonistic bacterium Pseudomonas cepacia was effective only if applied 

within 12 h after inoculation (Smilanick and Denis-Arrue, 1992). The 

antagonistic yeast Debaryomyces hansenii was effective against wound 

pathogens in citrus fruits if applied 3 h after the fruit was inoculated 

with the pathogen, but was ineffective 7 h after inoculation (Chalutz and 

Wilson, 1990). 

The time of application of the antagonist was found to be an important 

factor in the control of Lasiodiplodia theobromae in banana fruits by 

Trichoderma viride and other Trichoderma spp.: the highest reduction in 

the infection rate was achieved when the antagonistic fungus was 

applied 4 h prior to inoculation with the pathogen. There was less 

disease reduction following simultaneous application, and the least 

disease inhibition occurred when the Trichoderma was applied 4 h after 

inoculation (Mortuza and Ilag, 1999). These results demonstrated that 

the antagonistic Trichoderma species were not effective against 

infections already established in the fruits and indicated that the nature 

of the biocontrol activity is protective. 

Another factor affecting the effectiveness of the antagonist is the 

presence of moisture in the wound. The biocontrol of Botrytis cinerea on 

apples was more effective when the antagonistic yeast Candida oleophila 

was applied to fresh wounds rather than to 1-day-old wounds (Mercier 

and Wilson, 1995). Introducing C. oleophila to wounds in apple fruit prior 

to inoculation with B. cinerea significantly reduced the percentage of 

gray mold of the fruit after 14 days at 18°C. However, as the fruit surface 

dries, moisture rapidly becomes a limiting factor for yeast growth, 

therefore, the application of the antagonist should follow the occurrence 

of wounding as closely as possible (Mercier and Wilson, 1995). It is likely 

that the poorer establishment of C. oleophila on dry wounds prevented it 

from multiplsdng to population levels that would be inhibitory to 

B. cinerea, although a dry environment could also interfere with the 

mode of antagonistic action. 

Zehavi et al. (2000) inoculated berries with B, cinerea, Aspergillus 

niger and Rhizopus stolonifer in order to study the activity of epiphytic 

microorganisms isolated from table and wine grapes, against these 

pathogens. They concluded that wound inoculation is ideal for initial 

screening of antagonists but that this situation differs from that in the 

field. On the other hand, dipping grape bunches in an antagonist 

suspension and then sprasdng them with the pathogen spore suspension 

provides a closer simulation of a field biological control system, where the 




fungi are sparse, most berries are not wounded and the antagonist, 

applied as a spray, covers the fruit. 

Reductions in the percentage of decay, or in its rate of development in 

storage have been reported, following both field sprays with the 

antagonistic organism suspension and a postharvest application. 

Preharvest application 

Since the infection of fruits by postharvest pathogens often occurs in 

the field prior to harvest (see the chapter on Postharvest Disease 

Initiation - Pathogen Penetration into the Host), it is no wonder that 

preharvest application of the antagonist may sometimes be advantageous 

in controlling postharvest diseases. For this approach, successful 

biocontrol strains should be able to tolerate not only low nutrient 

availability but also UV-B radiation and climatic changes (Schena et al., 

1999). 

Field sprays of strawberry flowers with antagonistic non-pathogenic 

Trichoderma isolates resulted in a decreased incidence of gray rot (B. 

cinerea) during storage (Tronsmo and Dennis, 1977). Similarly, 

application of epiphytic isolates of the yeast-like fungus Aureobasidium 

pullulans, one of the most widespread saprophytes in the phylosphere, to 

strawberries grown under plastic tunnels, markedly reduced storage 

decay by both S. cinerea and R, stolonifer (Lima et al., 1997). The 

antagonists were more effective when applied at the flowering stage than 

at fruit maturity. Under these conditions, A. pullulans showed 

significantly higher activity against Botrytis rot than the fungicide 

vinclozolin. 

An isolate of A. pullulans was found to reduce gray mold in table 

grapes significantly when applied several times in the field, and its effect 

was not significantly different from that of the chemical control 

(iprodione) (Schena et al., 1999). It is worth noting that populations of 

this antagonist rapidly increased on grape berries under field conditions, 

and that they increased further during cold storage (0°C). Moreover, this 

antagonist was able to survive and increase its population when 

transferred to a new environment characterized by high temperature and 

low relative humidity. Considering its high biocontrol efficacy and its 

ability to survive and control gray mold in diverse environmental 

conditions, this cosmopolitan yeast-like fungus was considered as a 

potential biocontrol agent, especially where the use of chemical 

protection is restricted (Schena et al., 1999). 




Application of the two epiphytic fungal antagonists, A, pullulans and 

Epicoccum purpurescens, obtained from healthy peach blossoms, to 

cherry blossoms, reduced the number of quiescent infections of Monilinia 

fructicola in green cherries (Wittig et al., 1997). Selected strains of A. 

pullulans, Rhodotorula glutinis and Bacillus subtilis, isolated from leaf 

and fruit surfaces, were found to suppress postharvest decay 

development by P. expansum, B, cinerea and Pezicula malicorticis, when 

applied in combined mixtures to apple trees in the field (Leibinger et al., 

1997). A combination of two strains of A pullulans and one strain of JR. 

glutinis suppressed rotting of apples to the same extent as the commonly 

used fungicide euparen. It was further noted that applications before 

harvest are also of great interest because European regulations 

governing integrated pest management do not allow postharvest 

treatments of apples (Leibinger et al., 1997). Application of cell 

suspensions of the yeasts R, glutinis and two Cryptococcus species to 

pear fruits in the field, 3 weeks prior to harvest, was found to maintain 

high population levels through harvest. The three yeast species provided 

significant postharvest control of blue mold (P. expansum), gray mold (JB. 

cinerea) and side rots (Cladosporium herbarum, Alternaria alternata and 

Phialophora malorum). The most consistent decay control, however, was 

provided by Cryptococcus infirmo-miniatus, although the decay incidence 

and type of decay observed varied among years and among pear cultivars 

(Benbow and Sugar, 1999). 

A reduction in pathogen development following postharvest 

application of the antagonist has been recorded for pineapple fruits on 

which field sprays with non-pathogenic strains of Penicillium 

funiculosum suppressed decay caused by pathogenic strains of this 

fungus during storage (Lim and Rohrbach, 1980). 

A significant reduction in the incidence of green mold rot {Penicillium 

digitatum) in stored grapefruits was also achieved by spraying the fruits, 

prior to picking, with yeast cells of Pichia guilliermondii (Droby et al., 

1992). Such a treatment seems to reduce the potential inoculum level of 

the pathogenic spores which are naturally located on the fruit prior to 

harvesting. 

Postharvest application 

Postharvest application of the antagonist is often done by postharvest 

sprays or by bringing wounds on the fruit peel in contact with it. 

Spraying with suspensions of Trichoderma harzianum, T. viride, 



230 


Gliocladium roseum and Paecilomyces variotii, resulted in a partial 

control of Botrytis in strawberry fruits and of Alternaria in lemon fruits 

(Pratella and Mari, 1993). In the case of Alternaria rot in lemons, 

biological control by the fungus Paecilomyces was more effective than the 

conventional treatment with iprodion; and in the case of Fusarium rot in 

potato tubers, control by T, harzianum was more effective than the 

conventional benomyl treatment (Figs. 37, 38). However, some 

Trichoderma strains have been found to be pathogenic to harvested 

fruits, which may limit its possible use to only a few strains. Moreover, 

spraying strawberries with this antagonistic fungus has no effect on the 

quiescent infections of Botrytis, which comprise an important proportion 

of total storage infection. 

A significant reduction in storage decay was achieved by bringing 

several yeast species in direct contact with wounds in the peel of 

harvested fruit. This procedure resulted in the suppression of the main 

wound pathogens in citrus fruit, including P. digitatum, P, italicum and 

E 

100 

c5 40 

CO 

3 

80 + 

60 + 

20 + 

0 

~l 

4- 

n Control 

• Trichoderma 

m Benomyl 0.05% 

The treatment 

Fig. 37. The antifungal activity of Trichoderma harzianum compared with 

conventional fungicide, benomyl, on potato tubers. Statistical differences (at p= 

0.05) are indicated by different letters through the application of the Duncan's 

Multiple Range Test. (Reproduced from Pratella and Mari, 1993 with 





E 

'v- 

CO 

CO 

60 

50 

40 

^ 30 + 

o 

o 

c 20 + 

0 

*o 

c 

-S 10 -f 

0 

• Control 

• Paedlomyces 

li Iprodione 0.05% 

The treatment 

Fig. 38. The antifungal activity of Paedlomyces variotii compared with a 

conventional fungicide, iprodione, on lemon fruit. Statistical differences (at p= 

0.05) are indicated by different letters through the application of the Duncan's 

Multiple Range Test. (Reproduced from Pratella and Mari, 1993 with 


Geotrichum candidum (Chalutz and Wilson, 1990); of B. cinerea in apples 

(GuUino et al., 1992; Mercier and Wilson, 1995; Roberts, 1990; 

Wisniewski et al., 1988); of B. cinerea, P. expansum and Phialophora 

malorum in pears (Chand-Goyal and Spotts, 1996a; Sugar and Spotts, 

1999); of P. expansum in apples and nectarines (Wilson et al., 1993; Lurie 

et al., 1995), and B. cinerea, R. stolonifer and A alternata in tomatoes 

(Chalutz et al., 1991). However, not all the pathogens react similarly to a 

given antagonist. For instance, under natural infection conditions, 

dipping grapes in a cell suspension of yeasts of the genera Kloeckera and 

Candida, was effective in controlling Rhizopus decay but had no effect on 

Aspergillus decay caused by A. niger in storage (McLaughlin et al., 1992). 

Experiments focused on B, cinerea indicated that the efficacy of the 

antagonist was affected both by the concentration of yeast cells in the 

wound and by the number of pathogen spores used for inoculation. When 

wound inoculation was done with a high concentration of Botrytis spores 



232 


(10^ spores ml-i), a reduced percentage of infection could be achieved only 

by the highest yeast concentration (10^ cells ml"i). On the other hand, 

when wounds were inoculated with low concentrations of spores, decay 

suppression could be achieved by each of the antagonist concentrations 

tested, including the lowest concentration (Chalutz et al., 1991) (Fig. 39). 

A similar trend was shown for Trichoderma species antagonistic to 

various fungal pathogens: the best activity was generally observed at 

high concentrations of the antagonist and the lower inoculum levels of 

the pathogen (Elad et al., 1982; Mortuza and Ilag, 1999). 

The marked antagonistic effects attributed to various yeast species 

raised the question of the antagonistic capacity of industrial yeasts 

commonly used in food processing. Screening of industrial yeasts for 

control of P. digitatum in lemons showed that four out of 150 isolates 

completely controlled the green mold development. These included one 

isolate of Saccharomyces cerevisiae and three Kluyveromyces isolates. 

None of these isolates was found to produce antibiotics (Cheah and Tran, 

1995). 

Pichia guilliermondii (cells/ml)



In fact, postharvest decay control in pear fruits by the yeasts, 

Rhodotorula and Cryptococcus has been demonstrated following both 

field sprays (Benbow and Sugar, 1999) and postharvest application 

(Chand-Goyal and Spotts, 1996a). Similarly, the efficiency of yeasts of 

the genera Pichia and Hanseniaspora in controlling Rhizopus and 

Botrytis decay in grapes has been expressed following both sprays in the 

vineyard prior to harvest, and dipping of harvested fruit in the 

antagonist yeast cell suspension (Ben Arie et al., 1991). 

C. MODE OF ACTION OF THE ANTAGONIST 

Elucidation of the mechanisms by which antagonists inhibit 

postharvest pathogens is important for the development of a more 

reliable procedure for the effective application of known antagonists and 

for providing a rationale for the selection of more effective antagonists 

(Wilson and Wisniewski, 1989). Several modes of action have been 

suggested to explain the biocontrol activity of antagonistic microorganisms 

(Droby et al., 1992; Droby and Chalutz, 1994; Wilson et al., 1994): 

(a) the secretion of antibiotic compounds; 

(b) competition for nutrients at the wound site; 

(c) direct effect of the antagonist or enzymes secreted 

by it, on the pathogen; 

(d) induction of host defense mechanisms. 

(a) The secretion of antibiotic compounds by the antagonist. 

Examples of microorganisms which inhibit the pathogen via the 

production of antibiotics are the bacteria. Bacillus subtilis and 

Pseudomonas cepacia. The early study of Gutter and Littauer (1953 ) 

found B, subtilis able to inhibit the development in culture of the main 

pathogens of harvested citrus fruits. More than 30 years later, it was 

found that this antagonist was effective against fungal development in 

citrus fruit (Singh and Deverall, 1984) and against Monilinia fructicola 

in peaches and cherries (Pusey and Wilson, 1984; Utkhede and Sholberg, 

1986). Analytical tests of the antibiotic substance secreted by an active 

isolate of B. subtilis indicated the presence of iturins, which are cyclic 

peptides made of seven a acids and one p acid. They are characterized by 

a wide antifungal spectrum including fungi pathogenic to man and 

plants (Pusey, 1991; Gueldner et al., 1988). It is interesting to note that 




the inhibition of Monilinia development within the fruit has been 

achieved by the use of both cell-free filtrates of the bacterium and of the 

living cells separated from the liquid medium on which they grew, 

although autoclaved cells offered no protection (Pusey, 1991). These facts 

indicated that living cells of the Bacillus may act on the fruit either by 

producing antifungal metabolites or by antagonizing the pathogen in 

another way (such as competition for nutrients). 

The bacterium, P. cepacia inhibits Botrytis cinerea and Penicillium 

expansum growth in apples via the antibiotic compound, pyrrolnitrin 

(Janisiewicz and Roitman, 1988). Trials performed with strawberries 

have shown that dipping the fruit in p5n:rolnitrin alone delayed gray 

mold development by several days, but had no effect on the extent of 

decay (Takeda et al., 1990) (Fig. 40). 

Experiments with Penicillium digitatum-inoculaied lemon fruit 

(Smilanick and Denis-Arrue, 1992) showed that P. cepacia was the most 

effective of several Pseudomonas species in controlling green mold decay. 

The bacterium developed rapidly in the wound area, did not cause any 

2 3 4 

Days in storage 

Fig. 40. Effect of pyrrolnitrin on rot development on Tribute' strawberry fruit 

stored at room temperature. Fruit were dipped in water (O) or 250 mg 

pyrrolnitrin/liter (•) and stored at 18°C. Bars indicate LSD, p=0.05. (Reproduced 

from Takeda et al., 1990 with permission of the American Society for 

Horticultural Science). 




visible injury and prevented green mold development when it was applied 

within 12 h of inoculation. Laboratory tests confirmed that fimgal inhibition 

in vitro was caused by the presence of psrrrolnitrin in the culture medium. 

However, experiments also showed that when the fruit was inoculated 

with pyrrolnitrin-resistant P. digitatum mutants, inhibition of decay 

development was still recorded, even though the spores of the resistant 

isolate were capable of germination in the presence of the antibiotic 

substance. This result suggests that the antibiotic substances produced by 

the Pseudomonas are not the only means by which the bacterium 

functions, and that the presence of Pseudomonas cells in the wound also 

involved a similar antifungal effect. Furthermore, another species of 

Pseudomonas - P. fluorescence - which had no inhibiting effect on the 

Penicillium growth in culture, was capable of significantly reducing the 

growth rate of the green mold in the harvested fruit. 

Various antibiotic compounds which control human diseases have 

been tested against plant diseases: chlortetracycline, cycloheximide, 

fungicidin, griseofulvin, mycostatin and streptomycin, were found to be 

effective in controlling serious postharvest diseases. These include 

bacterial soft rot caused by Erwinia carotovora, gray rot caused by 

B, cinerea and brown rot caused by M fructicola (Goodman, 1959). 

However, none of these compounds is used commercially to control 

postharvest diseases since, in addition to their high cost, their 

application to fresh fruits and vegetables would endanger their medicinal 

effectiveness when needed by humans (Pusey, 1991). The possibility of 

rapid development of pathogen resistance towards antibiotic substances 

may be another obstacle in the practical use of antibiotic-producing 

microorganisms for decay control. 

As the list of microorganisms effective in suppressing pathogen 

development without being capable of producing antibiotic compounds 

grows, much research has been focused on understanding and defining 

their mode of action in the fruit. 

(b) Competition for nutrients between the antagonist and the 

pathogen. A fresh wound is a good source of nutrients for invading 

microorganisms. A delicate balance apparently exists at the wound site 

between the propagules of the antagonist and the pathogen, which affects 

the interaction between them and will determine whether or not the 

wound becomes the site of infection. As was previously mentioned, to 

compete successfully with the pathogen at the wound site, the antagonist 

should be better adapted than the pathogen to various environmental and 




nutritional conditions: it should grow more rapidly than the pathogen, 

while using low concentrations of nutrients, and should be able to survive 

under conditions that are unfavorable to the pathogen (Droby et al., 1991). 

One can assume that the antagonist, which grows rapidly in the 

wound by utilizing nutrients located there, will deprive the pathogen of 

available nutrients and inhibit spore germination and germ-tube 

elongation, which are the stages prior to pathogen establishment in the 

tissues. Thus, it was found that the rapid development of the 

antagonistic yeast, Pichia guilliermondii on grapefruit peel at a broad 

range of temperatures and at various levels of relative humidity, enables 

it to populate the wound within 24 h, while P. digitatum spores are still 

at their initial stages of germination (Droby et al., 1992). Similarly, the 

development of the yeast cells in wounds of tomato peel preceded the 

development of Rhizopus stolonifer, B. cinerea or Alternaria alternata, 

which had also been introduced to the same wound (Chalutz et al., 1991). 

Several findings support the hypothesis that the main mechanism by 

which P. guilliermondii inhibits B, cinerea and other postharvest 

pathogens, is competition for nutrients at the wound site (Chalutz et al., 

1991): (1) the addition of nutrients to the wound during inoculation 

markedly reduced the antagonistic effect of the yeast, and the degree of 

reduction depended on the concentration of nutrients added; (2) a 

marked reduction in the growth rate of the pathogen occurred only under 

conditions of limited nutrition, when the antagonist cells were cultured 

with the pathogen on a poor synthetic medium, and there was no growth 

inhibition on a rich medium (PDA); (3) the rapid rate of growth of the 

yeast at the wound site during the critical first 24 h of incubation. The 

non-specific nature of the antagonistic yeast and its ability to inhibit 

several wound pathogens of diverse hosts, such as citrus, apple, grape 

and tomato, support the hypothesis that competition for nutrients is the 

major mode of action of Pichia in decay suppression. 

A similar mode of action was suggested also for non-pathogenic yeasts 

of the genus Cryptococcus which have been isolated from a natural 

epiphytic population on leaves and fruits of apples and pears (Roberts, 

1991). These yeasts, which effectively prevented or reduced postharvest 

diseases of apples, pears and cherries, were able rapidly to colonize, to 

prosper and to survive in wounds for long periods under a broad range of 

temperatures (0-20°C). Other features of these yeasts, which could be 

important for the storage of apples, are their ability to survive in 

C02-enriched controlled atmospheres and their resistance to various 

fungicides, such as benomyl, sodium or^/iophenylphenate and rovral. 




However, the antagonistic capacity of Cryptococcus spp. depended on their 

concentration as well as on the storage temperature (Roberts, 1991). 

Biocontrol of the gray mold on apples by two isolates of another yeast, 

Metschnikowia pulcherrima, was found to be reduced or totally 

suppressed by the addition of several nitrates. This finding supports the 

hypothesis that competition for nutrients plays a role in the biocontrol 

capability of the yeast against S. cinerea. Under culture conditions, 

however, both isolates of the antagonist inhibited spore germination and 

mycelial growth of the pathogen, regardless of restrictive nutrient 

conditions (Piano et al., 1997). 

Non-pathogenic species of Erwinia, such as Erwinia cypripedii, 

showed an antagonistic activity against various isolates of Erwinia 

carotovora, the causal agent of soft rot in many vegetables (Moline, 

1991). Dipping carrot disks in a cell suspension of the antagonistic 

Erwinia, followed by inoculating them with active pathogenic isolates of 

E. carotovora, prevented the tissue maceration typically caused by the 

pathogen; whereas dipping the disks in a cell suspension previously 

heated, to inactivate the antagonistic cells, canceled the antagonistic 

effects. The antagonistic Erwinia also significantly reduced the incidence 

of soft rot caused by E. carotovora, in tomatoes and peppers, after it was 

introduced to a fresh wound in the fruit (Moline, 1991). Since the 

antagonistic strain of Erwinia did not exhibit any antibiotic activity, its 

inhibitory effect on disease development was hypothesized to be the 

result of competition for substrate in the wound site, between the 

pathogenic and the antagonistic Erwinia spp. This hypothesis is 

supported by the finding that various bacteria serve as biocontrol agents 

in the soil, through their ability to bind iron, thus limiting the pathogen 

growth in the micro-environment (Neilands, 1981; Leong, 1986). 

(c) Direct effect of the antagonist or its enzymes, on the 

pathogen. Several electron microscope observations (Wisniewski et al., 

1988, 1991) showed that the antagonist yeast cells may injure the 

pathogen directly. Co-culturing P. guilliermondii with the pathogenic 

fungi, J5. cinerea or P. expansum demonstrated the ability of the yeast to 

attach fungal hyphae closely (Photo 7). In many instances, individual 

yeast cells appeared to be embedded within depressions in the hyphal 

wall and in some areas extensive pitting and holes were observed in the 

fungal hyphae (Photo 8). In contrast, co-culturing B, cinerea with a 

non-antagonistic yeast elicited only a loose attachment to the fungus, 

with no pitting in the hyphae. 



238 


Photo 7. Coculture of Botrytis cinerea and the antagonistic yeast Pichia 

guilliermondii: tenacious attachment of the yeast cells to fungal hyphae. (From 

Wisniewski, Biles and Droby, 1991). 

Photo 8. Coculture of Penicillium expansum and the antagonistic yeast Pichia 

guilliermondii: formation of pitting and holes in the fungal hyphae. (From 

Wisniewski, Biles and Droby, 1991). 




Apart from the effects of direct contact of the antagonistic yeast with 

the pathogen hyphae, filtrates of the antagonist cells can produce higher 

levels of gluconase than filtrates of the non-antagonistic yeast, and the 

gluconase activity is the cause of cell wall degradation at the sites of 

attachment. These findings raised the suggestion that the firm 

attachment to the fungus, in conjunction with the enhanced activity of 

cell-wall degrading enzymes, may have an important role in the 

biological activity of P, guilliermondii, and that its efficacy is not 

dependent only on its competition with the pathogen for nutrients 

(Wisniewski et al., 1991). However, there is still the possibility that the 

firm attachment enhances the utilization of nutrients by the antagonist, 

at the site of invasion, and thereby blocks access to the available 

nutrients by the pathogen (Wisniewski et al., 1988). 

Various studies with species of Trichoderma, which are known as 

effective biocontrol agents of several important plant pathogenic fungi, 

highlighted the role of lytic enzymes, including glucanases, chitinases 

and proteinases, in the capability of the antagonist to attack the 

pathogen and thus to reduce disease incidence (Chet et al., 1993; 

Goldman et al., 1994, Lorito et al., 1993; Mortuza and Hag, 1999). 

Trichoderma harzianum isolates were found to produce chitinases and 

glucanases when grown on live mycelium of Sclerotium rolfsii and 

Rhizoctonia solani in soil (Elad et al., 1982). The chitinolytic system of 

T. harzianum is made up of six distinct chitinolytic enzymes: two 

P-l,4-A^-acetylglucosaminidases (exoenzymes) and four endochitinases 

(Haran et al., 1996). Both the levels and the expression patterns of these 

enzymes are specifically affected by the pathogenic fungus attacked by 

the Trichoderma. The parasitic interaction with R. solani involved the 

expression of both the endochitinase activities and the exotype activity. 

During the mycoparasitic interaction with S, rolfsii, however, only the 

exotype activities of two P-l,iV-acetylglucosaminidases were detected. It 

was suggested that the differential expression of T. harzianum chitinases 

might influence the overall antagonistic ability of the fungus against a 

specific pathogen. 

When 15 Trichoderma isolates were tested for their antagonistic 

ability against Lasiodiplodia theobromae, the greatest inhibition in dual 

culture was exhibited by T. harzianum, followed by T viride (Mortuza 

and Hag, 1999). For each species, however, inhibition increased with 

increasing density of spores in the inoculum. All the Trichoderma 

species reduced spore germination and inhibited germ-tube elongation of 

L. theobromae. Microscopic investigation demonstrated direct parasitism 




and coiling of T. harzianum and T, viride around the hyphae of 

L. theobromae. In addition to the direct parasitism, and deformation of 

some cells of the pathogenic fungus, granulation of the cytoplasm and 

disintegration of the pathogen hyphal walls of L. theobromae also 

occurred without intimate contact between the hyphae. It was, therefore, 

suggested that the two Trichoderma species could produce antifungal 

metabolites that contributed to their antagonistic activity. In fact, 

culture filtrates of the antagonists were found to be more suppressive of 

the conidia of the pathogen than spore suspensions, suggesting the 

involvement of certain metabolites in the antagonistic effect of 

Trichoderma species on L. theobromae. Both antibiosis and direct 

parasitism have also been suggested to be the modes of action of 

T. harzianum in reducing the incidence of postharvest stem-end rot 

(Botryodiplodia theobromae), anthracnose (Colletotrichum gloeosporioides) 

and brown spot (Gliocephalotrichum microchlamydosporum) on 

rambutan fruits (Sivakumar et al., 2000). 

An isolate of T. harzianum, which is used commercially prior to 

harvest, to control B, cinerea on grapes and greenhouse crops, was found 

to retard spore germination and germ-tube elongation of the pathogen. 

This retardation was associated with a limiting of disease severity, 

probably because of reduced penetration of B, cinerea germ tubes into the 

host. However, the efficient antagonistic isolate was also found to reduce 

the activity of hydrolytic enzymes of the pathogen (Kapat et al. 1998). 

Levels of cutin esterase, pectin methyl esterase, exopolygalacturonase, 

endopolygalacturonase and pectate lyase were all reduced when B, cinerea 

was grown with the active isolate of T. harzianum, either in a liquid 

culture or on the surface of bean leaves. The antagonist had no effect on 

the carboxymethyl cellulase activity of the pathogen. When different 

isolates of T, harzianum were used, a correlation was found between the 

enzyme inhibition capacity of the antagonist and its disease suppression 

efficacy, supporting the hypothesis that the inhibition of the activity of 

pathogenic enzymes of JB. cinerea is a mechanism by which the biocontrol 

agent affects disease (Kapat et al., 1998). 

(d) Induction of the host defense mechanism. One of the 

explanations offered for the mode of action of the antagonistic yeast, 

P. guilliermondii lay in its ability to stimulate wound healing or to induce 

other defense mechanisms in the host (Droby et al., 1992). Experiments 

with Pic/iia-inoculated citrus fruit showed that the yeast stimulated 

ethylene production and raised the levels of the enzyme phenylalanine 




ammonia lyase (PAL). The involvement of these processes in the 

enhanced defense mechanisms in citrus fruit has previously been 

described by G.E. Brown and Barmore (1977) in tangerines inoculated 

with C. gloeosporioides. 

Candida famata (isolate F35) was found to be one of the most active 

yeasts tested against P, digitatum in citrus fruits: it was able to reduce 

decay by 90-100% in artificially wounded fruits (Arras, 1996). Scanning 

electron microscope observations revealed a rapid colonization of the 

fungal mycelium at the wound sites, with numerous yeast cells strongly 

attached to the hyphae, exhibiting lytic activity and rapid alterations. 

Furthermore, when the yeast was inoculated into artificial wounds, 

either alone or with the pathogen, it stimulated the fruit to produce the 

phytoalexins, scoparone and scopoletin at the wound site. The 

concentrations of the phytoalexins depended significantly on the time lag 

between inoculation with the antagonist yeast and inoculation with the 

pathogen. Four days after inoculating the fruit with the yeast alone, 

scoparone concentration reached 124|ig gi fresh weight, 12 times higher 

than that in the non-inoculated wound. The concentration reached only 

47|xg gi when the antagonist was inoculated together with the pathogen, 

and only 37|Lig g^ when the pathogen alone was introduced into the 

wounds. Since scoparone inhibits spore germination of P. digitatum at 

46 ng g-i and germ tube elongation at 29^g g^ (Kim et al., 1991), it was 

concluded that the phytoalexin had already reached fungitoxic 

concentrations within a few days of the application of the antagonistic 

yeast (Arras, 1996). 

Increased resistance to infection can result not only from enhanced 

production of phytoalexins but also from preformed inhibitory 

compounds. It was thus found that a non-pathogenic mutant of 

Colletotrichum magna was capable of enhancing the preformed 

antifungal diene compound in avocado fruit peel and could, therefore, 

protect the fruit against infection by C. gloeosporioides (Prusky et al., 

1994). 

Recently, Ippolito et al. (2000) related the capability of the 

antagonistic yeast-like fungus, Aureohasidium pullulans to control decay 

in apple fruits inoculated with B, cinerea and P. expansum to its ability 

to enhance the activities of the enzymes, p-l,3-gluconase, chitinase and 

peroxidase in the treated fruits, in addition to its capacity to overcome 

the pathogen in competition for nutrients and space. These enzymes are 

all considered to be involved in host defense mechanisms. Chitinase and 

P-l,3-gluconase are capable of hydrolyzing fungal cells and so inhibiting 




fungal growth (Schlumbaum et al., 1986), while peroxidases are involved 

in the formation of structural barriers against pathogen invasion (El 

Ghaouth et al., 1998). A. pullulans could thus induce further decay 

control through a multiple-component mode of action against the 

pathogen (Ippolito et al., 2000). 

D. ANTAGONIST MIXTURES TO IMPROVE DISEASE 

BIOCONTROL 

The development of biological control has made much progress during 

the last decade. Today, with several biocontrol treatments approved for 

commercial application and others undergoing the approval process, 

research is being focused on improving both the antagonist efficacy and 

the control system. One of the approaches to improving biocontrol agents 

is by selecting combinations of antagonists that will act better than each 

of the components separately. 

A combination of the bacterium Pseudomonas syringae and the pink 

yeast Sporobolomyces roseus proved to have a marked advantage over 

each of the antagonists in controlling Penicillium expansum in apples, 

both in reducing the incidence of wound infections and in limiting rot 

diameter (Janisiewicz and Bors, 1995). After application of the mixtures, 

populations of S. roseus in the wounds were found to be consistently 

lower than those after individual applications, whereas populations of 

P. syringae were not affected by the presence of the other antagonist. 

When 35 nitrogen sources were tested for utilization by the antagonists, 

both S. roseus and P. syringae were found to utilize 14 sources, whereas 

P. syringae utilized an additional nine compounds. On the other hand, 

more carbon sources were utilized by S. roseus than by P. syringae. It 

was concluded that the populations of the antagonists in apple wounds 

form a stable community, dominated by P. syringae. This domination was 

attributed to the ability of P. syringae to use nitrogen sources, which 

form the limiting factor in the carbon-rich apple wounds. This is an 

example of the possibility of exploiting the differing nutritional features 

of the antagonists to achieve improved biological control at the wound 

site. 

In a further study, Janisiewicz (1996) reported on the development of 

antagonist mixtures that were superior to individual antagonists in 

controlling blue mold (P. expansum) of apples; the development was 

based on ecological knowledge of the distributions of the antagonists in 




time and space, and of their nutritional *niche' differentiation. The 

microorganisms in these mixtures, which were isolated from exposed 

apple tissue before harvest, comprised mainly yeast populations. 

Following screening for their ability to control P. expansum on apple 

fruits, they were grouped into various nutritional clusters. Preference 

was given to antagonists colonizing the same fruits, followed by those 

colonizing different fruits but isolated at the same time. Nutritional 

differences between two antagonists in a mixture allowed populations of 

both antagonists to flourish in the same wound. The results indicated 

that combinations of antagonists that occupy different nutritional niches 

and coexist in the infection area are more effective in biological control 

treatments than the individual antagonists. 

The advantage of antagonistic pairs over a single antagonist was 

described by Schisler et al. (1997) in the control of Fusarium dry rot 

(Gibberella pulicaris, anamorph Fusarium sambucinum) in stored potato 

tubers. The search for biological means to control this disease was 

stimulated by the development, over the years, of resistance to 

thiabendazole, the conventional chemical treatment. When the pathogen 

was challenged with pairs of antagonistic bacterial strains, it was found 

that successful pairs reduced Fusarium dry rot by an average of 70% 

versus controls, a level of control comparable with that obtained with 100 

times the inoculum dose of a single antagonist strain. The successful 

coexistence of these pairs was partly attributed to the fact that 

compatible strains possessed differing carbon substrate utilization 

profiles. Similarly to the demonstration by Janisiewicz (1996) that 

combining antagonists on the basis of *niche' differentiation enhanced the 

possibility of improved control of P. expansum on apples, diverse niches 

are also likely to be found in potato wounds. These could result in the 

exposure of G. pulicaris to higher total bacterial populations and wider 

ranges of nutrient competition when challenged by co-existing mixtures 

of microbial strains than when challenged by a single antagonist 

(Schisler et al., 1997). It does seem that determination of the substrate 

utilization profiles of strains of antagonist pairs, as well as evaluation of 

the colonization characteristics of successful and unsuccessful pairs of 

potato dry rot antagonists, would provide further clues as to the nature 

of the disease control success of some antagonistic pairs. However, the 

success of antagonistic pairs also may be attributable to the individual 

strains of the pair possessing complementary modes of action (Schisler et 

al., 1997). 




E. COMBINED TREATMENTS TO IMPROVE DISEASE 

BIOCONTROL 

Various additives have been shown to increase the effectiveness of 

some antagonistic microorganisms in controlhng postharvest decay. The 

addition of CaCb to antagonistic yeast suspensions was found to enhance 

their biocontrol activity and reduce the populations of yeast cells 

required to give effective control (McLaughlin et al., 1990; Droby et al., 

1997). A combination of the yeast antagonist, Pichia guilliermondii (10'^ 

cells ml-i) with CaCh (136 mM) in dip application, significantly decreased 

the incidence of green mold caused by Penicillium digitatum in 

grapefruit wounds (Droby et al., 1997). Both spore germination and germ 

tube elongation of P. digitatum decreased with increasing CaCl2 

concentration. In addition, increased CaCh concentration also resulted in 

the inhibition of pectolytic activity of a crude enzyme preparation of 

P. digitatum. Hence, the effects of calcium in reducing infection may be 

due to its effects on the host tissue, via enhanced cell-wall resistance to 

enzymatic degradation, or to direct effects on the pathogen by interfering 

with spore germination and inhibiting fungal pectolytic activity. 

However, the yeast also strongly maintained calcium homeostasis, and 

this ability probably allowed it to grow in a microenvironment which is 

inhibitory to the pathogen. Enhancement of biocontrol of the blue mold 

{Penicillium expansum) on apples has been achieved by adding 

nitrogenous compounds to suspensions of the bacterial antagonist 

Pseudomonas syringae applied to the fruits (Janisiewicz et al., 1992). 

Other compounds capable of enhancing the activity of some antagonistic 

microorganisms are sugar analogs. The addition of 2-deoxy-D-glucose to 

the antagonists, P. syringae and Sporobolomyces roseus was found to 

enhance their antagonistic effect against P. expansum (Janisiewicz, 1994). 

This combination enabled the concentration of the antagonist required for 

the biocontrol of blue mold on apple fruits to be reduced tenfold. El 

Ghaouth et al. (1995) found that out of the various sugar analogs tested as 

potential fungicides against apple and peach pathogens, only 

2-deoxy-D-glucose was effective in controlhng decay in inoculated fruits. 

Recently this compound was found to be compatible with the antagonistic 

yeast, Candida saitoana (El Gaouth et al., 2000b). Although the growth of 

C. saitoana in vitro was reduced by 2-deoxy-D-glucose, the yeast grew 

normally in the presence of this compound when applied to wounds. 

A combination of the antagonistic yeast with a low dose of 0.2% of the 

sugar analog, applied to apple wounds prior to inoculation with B, cinerea 




and P. expansum, or to lemon and orange wounds prior to inoculation with 

P. digitatum, significantly enhanced the biocontrol activity of C. saitoana 

against decay. The same combination was also effective against 

established infections when applied within 24 h after inoculation. In other 

words, the combination of the antagonistic yeast with the sugar analog 

had both protective and curative effects against the postharvest 

pathogens. Furthermore, its effectiveness on citrus fruits was found to be 

similar to that of the common fungicide, imazalil. It was also emphasized 

that the curative activity gained by the combined treatment represented 

an improvement over the currently available microbial biocontrol products 

that confer only a protective effect (El Ghaouth et al., 2000b). 

Another combination aimed at improving the biocontrol of fruit decay 

by C. saitoana is that of the antagonistic yeast with glycolchitosan, a 

combination known as "a bioactive coating" (El Ghaouth et al., 2000a). 

The use of a bioactive coating became possible because the presence of 

glycolchitosan in apple wounds and on the fruit surface does not affect 

the natural increase of the yeast population. The combination of the 

antagonistic yeast with 0.2% glycolchitosan was more effective in 

controlling the natural infection caused by B. cinerea and P. expansum in 

various apple cultivars than either the yeast or the glycolchitosan alone. 

The bioactive coating was either similar or superior to thiabendazole in 

suppressing decay; it was also more effective than the antagonistic yeast 

alone in controlling natural infection of oranges and lemons (mainly by 

P. digitatum) and the control level was equivalent to that achieved with 

2000 g ml-i imazalil (Fig. 41). 

Genetic manipulation of biocontrol fungi is another approach to 

improving the antagonist ability to control disease. To enhance their 

antagonistic potential against pathogenic fungi, Lalithakumari et al. 

(1996) selected two parent isolates of Trichoderma species for genetic 

manipulation: Trichoderma harzianum, which is an efficient biocontrol 

agent against plant pathogens, and Trichoderma longibrachiatum, which 

is tolerant to copper sulfate and carbendazim. The second isolate was 

selected as a co-parent because development of fungicide tolerance 

potential in a biocontrol agent is believed to be of great importance for 

integrated disease management. It was found that protoplast fusion of the 

two isolates, taken from young mycelia following cell-wall digestion, 

resulted in fusants that exhibited an enhanced antagonistic effect against 

several pathogens, along with tolerance to copper and carbendazim. 



246 

25 4 

2 

I 20 

£ 15 

I 10 + 

0. 

5 

0 


Q Control 0 biocoating 

A 

iinazaiii 

Hamlin oranges Pinapple oranges Valencia oranges 

Fig. 41. Effect of the combination of Candida saitoana with 0.2% glycolchitosan 

on decay of orange cvs. Hamhn, Pineapple and Valencia after 21, 28 and 21 

days at IS^'C, respectively. Bars within each cultivar with the same letter are 

not significantly different according to Duncan's Multiple Range Test (p=0.05). 

(Reproduced from El Ghaouth et al., 2000a with permission of the American 


F. INTEGRATION INTO POSTHARVEST STRATEGIES 

Several studies have highlighted the advantages of postharvest 

application of biological control agents over field or soil application. The 

major advantages are: (1) the convenience of bringing the antagonist in 

contact with the commodity, compared to its addition to the soil; (2) the 

possibiHty of acting under controlled conditions, created and maintained 

during storage and transportation (Wilson and Wisniewski, 1989); (3) the 

possibility of applying the biocontrol agents during the commercial 

process which the fruit and vegetables undergo in the packinghouse. For 

instance, it is possible to introduce Bacillus subtilis into the wax applied 

to peaches in the packing house to protect them from the brown rot 

caused by Monilinia fructicola (Pusey et al., 1988). The antagonistic 

yeast Pichia guilliermondii can be introduced into the wax mixture 

applied to citrus fruit, or applied as a separate step, in the packinghouse. 




Compatibility between a microbial antagonist and a synthetic 

fungicide offers the option of using the antagonist in combination with 

reduced levels of the fungicide. Applying the yeast antagonist, Pichia 

guilliermondii to citrus fruit in combination with substantially reduced 

concentrations of thiabendazole (TBZ) reduced Penicillium digitatum 

decay to a level similar to that achieved by the currently recommended 

concentration of TBZ applied alone (Droby et al., 1993b). Thus, by 

adapting an integrated pest management system, we may expect not 

only to gain effective pest control but we can also maintain very low 

levels of chemical residues (Hofstein et al., 1994). The biological agent 

must, however, have low sensitivity to any of the supplemented chemical 

fungicides. 

The intensive studies on biocontrol of postharvest diseases have led to 

the registration of two biological products for commercial postharvest 

applications to citrus fruits: Aspire, which is Candida oleophila, and 

BioSave'^ 1000, which is Pseudomonas syringae (Brown, G.E. and 

Chambers, 1996; Brown, G.E. et al., 2000). In evaluating the efficacy of 

biological products for the control of citrus fruit pathogens. Brown and 

Chambers (1996) found that significant control of P. digitatum was 

obtained by each of the biological products but that the level of control, as 

well as consistency, were usually less pronounced than those obtained 

with standard rates of the fungicides, thiabendazole or imazalil. More 

recently Brown, G.E. et al. (2000) found that the major factor affecting 

the efficacy of the biological control is how quickly and how well the yeast 

colonizes injuries to the citrus fruit surface, including minor injuries 

involving only oil vesicles. This indication followed the finding that the 

peel oil was toxic to the Candida cells but not to spores of P. digitatum. 

However, combining Aspire with each of the chemicals used improved the 

results; sometimes combinations with a low rate of fungicide were 

sufficient to achieve effects similar to those obtained by the chemicals at 

standard rates. 

GRAS (generally recognized as safe) compounds or natural products of 

plant origin, which have been suggested as alternatives to synthetic and 

conventional fungicides (see the Chapter on Chemical Control - Generally 

Recognized as Safe Compounds and Natural Chemical Compounds), 

could also be used in combination with biocontrol agents, complementing 

their activity. Pathogens treated with such antifungal substances might 

be weakened and become more vulnerable to the antagonist activity 

(Pusey, 1994). 




The integration of postharvest biocontrol into modern production, 

storage and handUng systems must begin before harvest. Several 

preharvest factors that affect fruit quahty may have profound effects on 

the efficacy of postharvest biological control agents (Roberts, 1994). 

Preharvest calcium sprays during the growing season of apples and 

pears, as well as calcium application by immersion infiltration at 

postharvest, can increase fruit firmness, decrease the incidence of certain 

disorders and enhance resistance to postharvest infection (see the 

chapter on Means for Maintaining Host Resistance - Calcium 

Application). As was previously mentioned, calcium amendments and 

postharvest application of some antagonistic yeasts can be additive in 

reducing fruit decay and can significantly increase disease control 

compared with either treatment alone (McLaughlin et al., 1990; Droby et 

al., 1997). The advantages in increased firmness, enhanced resistance to 

postharvest decay and enhanced biocontrol efficacy under some 

circumstances reflect the multiple benefits of integrating postharvest 

biological control with cultural and production practices (Roberts, 1994). 

Fruit maturity at harvest and at the application of antagonists is 

another factor affecting postharvest biological control. Late-picked, 

over-mature fruits are more susceptible to decay than are fruits picked at 

optimal storage maturity (Sommer, 1982). Working with apples and 

pears, and with different species of the antagonistic yeast Cryptococcus, 

Roberts (1990, 1994) found that fruit maturity markedly affected 

biocontrol efficacy: while excellent control was achieved on freshly 

harvested fruit, treatments of ripened fruit gave much lower levels of 

control. On the assumption that the infection process can be initiated at 

harvest, it would be advantageous to treat fruit with biocontrol agents as 

quickly as possible after harvest and to cool the fruit as rapidly as 

possible, to retard pathogen development. In fact, studies with 

Mi/cor-inoculated pears and antagonistic Cryptococcus species 

demonstrated maximal biocontrol effect when the yeasts were applied to 

the fruit soon after harvest (Roberts, 1990). The principle is to retard 

pathogen development while allowing the antagonistic microorganisms to 

colonize wound sites. 

Postharvest factors, too, may have a major impact on the effectiveness 

of biological control. Temperature management, which is a critical factor 

in the maintenance of fruit quality and in pathogen development (see the 

chapter on Factors Affecting Disease Development), may also enhance 




biological control of storage decay. Janisiewicz et al. (1991) demonstrated 

that as the storage temperature of apples and pears decreased, there was 

a reduction in the concentration of pyrrolnitrin (a metabolite of 

Pseudomonas cepacia and other Pseudomonas spp.) needed to protect the 

fruit from gray mold (Botrytis cinerea) and blue mold {Penicillium 

expansum). 

An integrated strategy to control postharvest decay in pome and stone 

fruits has been advanced in recent years (Sugar et al., 1994; Spotts et al., 

1998; Willett et al., 1992); it comprises several pre- and postharvest 

components (Sugar et al., 1994): 

(1) Alteration of fruit nutrient status. This was found to influence the 

susceptibility of pome fruits to decay, and includes calcium applications, 

either as sprays during the growing season (Sugar et al., 1991) or by 

pressure infiltration into the fruit (Conway and Sams, 1983), which 

increase fruit calcium content and reduce the severity of decay. Lower 

fruit nitrogen content has also been associated with reduced disease 

severity. Management of pear trees for low fruit nitrogen (influenced by 

timing of fertilizer application) combined with calcium chloride sprays 

was found to reduce decay severity more than low nitrogen alone (Sugar 

et al., 1992). 

(2) Maturity at harvest. The severity of several postharvest diseases of 

pears was found to increase as the fruit approached maturity or as 

harvest was delayed, within the range of harvested maturity (Spotts, 

1985). 

(3) Controlled atmosphere storage. Atmospheres with reduced O2 and 

elevated CO2 can reduce the severity of postharvest fungal decay in pome 

fruits by inhibiting fruit senescence (Chen et al., 1981), thereby 

maintaining host resistance to infection (Barkai-Golan, 1990). 

(4) Application of low level of fungicide. All the components of the 

integrated program were found to be compatible with thiabendazole, 

which is used for postharvest decay control in pears (Roberts, 1991). This 

is relevant when the combined treatment is aimed at controlling the blue 

mold (P. expansum) in harvested pears. Thiabendazole is not effective 

against Phialophora malorum, the causal agent of side rot of pears 

(Willett et al., 1992). 

Several yeast species, such as Cryptococcus laurentii, are capable of 

colonizing wounds of pear fruit under conditions of low temperature 

(0°C), ambient or reduced O2, and ambient or elevated CO2 (Roberts, 

1991) and can, therefore, integrate into postharvest strategies. The 




integration of early harvest, low fruit nitrogen, high fruit calcium, yeast 

or yeast + one-tenth of the label rate of thiabendazole, along with a 

controlled atmosphere (2% O2, 0.6% CO2), was found to reduce blue mold 

severity in inoculated fruit by 95, 61 and 98% in three successive years, 

after 2-3 months at 0°C. Side rot caused by P. malorum was completely 

controlled when the combined treatments included the antagonistic 

Cryptococcus, early harvest and high fruit calcium. 

Control of brown rot of sweet cherry was achieved by postharvest 

application of a combination of the antagonistic yeast C. laurentii or 

C. infirmo-miniatus, with low doses of iprodione to the fruit (Chand-Goyal 

and Spotts, 1996b). However, following the ban on the application of 

iprodione less than 7 days before harvest, Spotts et al. (1998) showed that 

a combination of a single preharvest treatment with iprodione, in 

conjunction with postharvest biological control by C. infirmo-miniatus, 

gave significantly better control of the brown rot in stored sweet cherries 

than iprodione alone, although the yeast by itself had no suppressive effect 

on decay development. The effectiveness of such a combined treatment 

resulted from the fact that M fructicola was sensitive to the residues of 

iprodione, whereas the antagonistic yeast was resistant to them. 

In contrast to its lack of effect on M fructicola, the yeast alone did 

suppress the development of P. expansum, which infected cherry fruits 

naturally. Combinations of preharvest iprodione and postharvest 

Cryptococcus applications resulted in the prevention of blue mold 

infection in storage (Spotts et al., 1998). 

In addition, brown rot in sweet cherries, which was reduced by 

modified atmosphere packaging alone, could be further reduced as a 

result of synergism between the antagonistic yeast and the modified 

atmosphere. The O2 and CO2 percentages within the sealed package, 

after 42 days of storage at 0.5°C, were 11.4 and 5.1%, respectively; 

reduction of decay under these conditions was attributed to the 

accumulation of CO2 in the atmosphere. Furthermore, when CO2 

dissolves in water, carbonic acid is produced and the pH is lowered. 

Such conditions may favor yeast growth. Thus, the Cryptococcus-MA 

synergism may result from a combination of suppression of M fructicola 

and stimulation of C. infirmo-miniatus (Spotts et al., 1998). When these 

two postharvest treatments - biological control and modified 

atmosphere packaging — were combined with preharvest iprodione 

spray, the incidence of the brown rot was reduced from 41.5% in the 

control to only 0.4%. 




A combined strategic approach was elaborated by Eckert (1991), who 

proposed to control wound-infecting pathogens by a series of treatments: 

(1) disinfection of the fruit surface and environment; (2) eradication or 

suppression of fungal spore germination at wound sites by a combination 

of fungicides; and (3) reduction of wound susceptibility to infection by the 

addition of biocontrol antagonists which act as protective agents. Since 

the biocontrol efficiency of the antagonist is enhanced by reduction of the 

spore concentration of the pathogen in the wound (see Fig. 39), a 

sanitation program that reduces pathogen contamination in water 

systems or on the equipment in the packing house may determine the 

level of success of a biological control program. 



CHAPTER 12 

NOVEL APPROACHES FOR ENHANCING HOST 

RESISTANCE 

Synthetic fungicides are currently the primary means used for 

controlhng postharvest diseases. The resistance developed by postharvest 

pathogens to some fungicides, and the withdrawal of a number of key 

fungicides in response to health concerns over pesticide contamination, 

have stimulated the search for alternative technologies for postharvest 

disease control. Among the various possible alternative means of 

control, much attention has been drawn to the wide range of natural 

substances with antimicrobial activity. Plant- and animal-derived 

fungicides may offer safe alternatives to synthetic fungicides (Wilson et 

al., 1994), and the potential use of these antimicrobial agents is 

discussed in the chapter. Chemical Control - Natural Chemical 

Compounds. Other alternatives to chemical treatments are the various 

physical means of suppressing postharvest decay development. These 

are discussed in the chapters. Means for Maintaining Host Resistance, 

and Physical Means. The use of antagonistic microorganisms, often 

isolated from plant surfaces, as biocontrol agents is another alternative 

and quite promising direction. The chapter on Biological Control is 

dedicated to this subject. 

However, many of these treatments may also act as elicitors of 

enhanced resistance responses in harvested fruits and vegetables (Biles, 

1991; Wilson et al., 1994). Fruits and vegetables may defend themselves 

against pathogen attack or further colonization of the tissue by inducing 

defense mechanisms in response to initial infection or to various 

chemical, physical or biological elicitors. These induced defenses are 

described as active mechanisms, which require host metabolism to 

function (Keen, 1992). Disease occurs when a potential pathogen not only 

circumvents the passive defenses, such as the structural barriers or the 

preformed antimicrobial compounds of the host, but also avoids 

elicitation of active defense responses in the tissues (Jackson and Taylor, 

1996; Hutcheson, 1998). New approaches for controlling postharvest 

diseases have been based on activation of the natural defense responses 

induced by the host itself, by modulating them with suitable elicitors, as 



Novel Approaches for Enhancing Host Resistance 253 

well as on utilization of the resistance genes of a host via genetic crop 

modification or genetic engineering techniques. 

A. INDUCED RESISTANCE 

Inducible resistance in harvested tissues has recently joined the 

general concept that resistance in plants can be enhanced by modulating 

their natural defense mechanisms (Kuc, 1995). Activation of defense 

responses in harvested crops has been demonstrated in various 

host-pathogen interactions via application of physical, chemical or 

biological elicitors. This chapter will focus on the resistance-eliciting 

properties of these treatments and display their potential defense 

reactions. 

1. PHYSICAL ELICITORS 

Modified atmospheres, heat treatments, gamma radiation and 

ultraviolet (UV) light have been the main physical elicitors suggested as 

possible resistance inducers. 

Modified Atmospheres 

While conditions which stimulate ripening, such as ethylene 

application to various fruits, enhance decay incidence, conditions which 

inhibit ripening, such as low storage temperatures and controlled or 

modified atmospheres, contribute to decay suppression or retardation 

(see the chapter on Factors Affecting Disease Development). It has thus 

been found that storing avocado fruits in sealed plastic bags, along with 

the production of a C02-enriched atmosphere within the bag, delays fruit 

ripening. When ripening is delayed, the normal decomposition of a 

preformed antifungal diene compound in the fruit peel is retarded and, 

under these conditions, the appearance of anthracnose symptoms 

(Colletotrichum gloeosporioides) is delayed (Prusky et al., 1991). In a 

recent study, Ardi et al. (1998) showed that exposure of freshly harvested 

avocado fruits to CO2 resulted in increased concentrations of the 

antioxidant epicatechin and of the antifungal diene compound in the 

peel. The normal decrease in the antifungal activity in the peel during 

ripening has been attributed to the activity of lipoxygenase, which 

oxidizes the antifungal compound to a non-active compound, while the 

activity of lipoxygenase is regulated by the epicatechin naturally present 




in the fruit peel during ripening (Prusky et al., 1985b). Thus, by 

producing a high-C02 atmosphere, the increased concentrations of 

epicatechin during ripening may lead to increased antifungal activity in 

the host and, therefore, to maintaining the quiescent state of the fungus 

and reducing decay development. 

A high-C02 atmosphere may also induce higher levels of resorcinols in 

mango fruits, resulting in the retardation of Alternaria alternata 

infection (Prusky and Keen, 1995). Similarly, application of 

subatmospheric pressure to mango fruits retards ripening and, in 

parallel, retards the decrease in the level of the preformed antifungal 

resorcinol compounds in the peel. This decrease occurs naturally 

during ripening. Under these conditions the renewed development of 

A. alternata, which is found in a quiescent state in the fruit, is delayed 

(Droby et al., 1986). 

Heat Treatments 

Heat treatment can affect many physiological processes of the 

harvested fruit and vegetable. Depending on fruit species, cultivar, and 

physiological age, on the one hand, and on the temperature and length of 

exposure to heating, on the other hand, this treatment may lead to 

retardation of the ripening process. In many cases, along with the 

retardation of fruit ripening and the maintenance of firmness during 

storage, heating may also induce enhanced resistance of the host to 

pathogens. 

Various mechanisms by which heat treatment may induce host 

resistance have been suggested. Heat may function as a stress factor, 

inciting the accumulation of phytoalexins in the host tissue. A range of 

coumarin-derived antifungal compounds was detected in the peels of 

citrus fruits, following heat treatment (Ben Yehoshua et al., 1992; Kim et 

al., 1991). Kim et al. (1991) reported on the enhanced accumulation of 

the antifungal coumarin scoparone in heat-treated Penicilliuminoculated 

lemon fruits, and a correlation was drawn between the level of 

the antifungal activity in the host tissues and the development of disease 

resistance. Heat treatments may also result in the inducement of specific 

proteins of various molecular weights, known as 'heat shock proteins', in 

the plant tissue (Freeman et al., 1989; Vierling, 1991). A correlation was 

drawn between the accumulation of *heat shock proteins' and the 

enhancement of thermotolerance of the fruit following heat treatments. 

The inducement of such proteins in fruits and their consequent enhanced 

thermotolerance may improve the efficacy of heat treatments in reducing 




postharvest decay, through the possibiHty of using higher and more 

effective temperatures. 

Gamma Radiation 

Ionizing radiation can suppress decay development by directly 

affecting fungal spores and hyphae. It may also suppress decay 

indirectly, by dela5dng the ripening and senescence processes of 

harvested fruits and vegetables and contributing to the maintenance of 

their natural resistance to pathogen invasion or development. However, 

gamma radiation may also function as a stress factor, capable of inducing 

phytoalexin formation. Induction of the phytoalexins, scopoletin and 

scoparone was reported in citrus fruit following gamma irradiation at 1-4 

kGy (Riov, 1971; Riov et al., 1971), and their antifungal activity has been 

demonstrated (Dubery and Schabort, 1987). However, the part that the 

induced phytoalexins play in the complex of radiation effects on the 

pathogen or the defense responses of the host to effective radiation doses 

has not yet been evaluated. 

Low-dose Ultraviolet (UV) Light 

UV illumination is another physical treatment that may induce 

resistance against pathogen infection (Wilson et al., 1994). UV-C 

(wavelength below 280 nm) light at low doses was found to induce 

resistance in a wide array of commodities, such as onions (Lu et al., 

1987), carrots and peppers (Mercier et al., 1993), tomatoes (Liu et al., 

1993), sweet potatoes (Stevens et al., 1990), peaches (Lu et al., 1991), 

various citrus fruits (Chalutz et al., 1992; Droby et al., 1993a; Porat et 

al., 1999; Rodov et al., 1992), and table grapes (Nigro et al., 1998). 

The enhanced resistance that follows UV treatment may be mediated 

by the activation of various defense responses in the host tissue. The first 

such response is the accumulation of phytoalexins to inhibitory levels. In 

carrot slices, the UV-induced resistance of the tissue to Botrytis cinerea 

and Sclerotinia sclerotiorum infection coincided with the induction of the 

phytoalexin, 6-methoxymellein (Mercier et al., 1993). The induced 

resistance was expressed only after 1 week of storage, when the 

concentration of 6-methoxymellein in the tissue had reached inhibitory 

levels. The content of this compound in UV-treated slices that were held 

at 1 and 4'^C remained elevated for up to 35 days after treatment. 

Another phytoalexin, scoparone (6,7-dimethoxy-coumarin), has been 

linked to UV-induced resistance in citrus fruits (Rodov et al., 1992). Its 

accumulation in kumquat fruits reached its peak (530 \ig gO 11 days 




after illumination (254 nm), but its concentration declined rapidly, 

returning to trace levels typical of non-illuminated fruit, 1 month after 

treatment. The accumulation of the phytoalexin was correlated with 

increased antifungal activity in the flavedo, and resulted in enhanced 

resistance of the fruit to infection by Penicillium digitatum. Such a 

defense against infection was achieved when illumination was applied 

before inoculation with P. digitatum, without direct contact of the 

pathogen with the light. Moreover, illumination of previously inoculated 

fruit failed to prevent decay development. The experiments with 

kumquats clarified that the fruit/pathogen interaction depends on the 

relative rates of fungal growth and of resistance development by the host. 

Inoculation of the fruit before UV treatment gives an advantage to the 

pathogen, while illumination applied 2 days before inoculation was 

sufficient to improve fruit resistance. Host resistance is thus dependent 

on the existence of a lag period between illumination and inoculation, 

during which the phytoalexin can be accumulated and suppress 

subsequent pathogen development (Rodov et al., 1992). 

The enhancement by UV treatment of citrus fruit resistance to 

P. digitatum infection has also been explained in part by the induction of 

the key enzymes in the secondary metabolite pathway. It was thus found 

that in citrus fruits, the onset of UV-induced resistance coincided with 

the induction of phenylalanine ammonia lyase (PAL) (a key enzyme in 

the phenylpropanoid pathway) and peroxidase activities (Chalutz et al., 

1992; Droby et al., 1993a). PAL activity in the peel of grapefruits 

increased within 24 hours after UV treatment and remained elevated for 

72 hours, while peroxidase activity reached its maximum 72 hours after 

treatment. Both enzymes are considered to play a role in the induced 

resistance of plants against pathogens. 

An additional explanation for the induced resistance of citrus fruits 

following UV treatment has recently been offered by Porat et al. (1999), 

who found that exposure of grapefruits to UV light (254 nm), followed by 

inoculation with P. digitatum, resulted in the accumulation of chitinase 

protein in the peel tissue. Whereas UV treatment or wounding alone had 

almost no effect on the levels of p-l,3-endoglucanase protein, the 

combination of the two treatments induced the accumulation of both 

chitinase and glucanase proteins. Since the two pathogenesis-related 

proteins are hydrolysers of fungal cell wall polymers and may inhibit 

fungal development, it was suggested that the resistance of citrus fruit to 

fungal development induced by UV treatment had been mediated by the 




induced accumulation of chitinase and glucanase protein (Porat et al., 

1999). 

Although the potential of UV light as an inducer of defense reactions 

has been proven for various fruits and vegetables, this treatment by itself 

is unlikely to provide disease control comparable with that provided by 

synthetic fungicides. El Ghaouth (1994) suggests that in order to extend 

UV effects during a significant storage period, UV treatment should be 

combined with other alternatives such as a biological control by means of 

antagonistic microorganisms. 

Studying the effects of UV-C illumination on the natural microbial 

epiphytic population on the surface of grape berries, Nigro et al. (1998) 

found a significant increase in the population of antagonistic yeasts and 

bacteria on berries treated with 0.25-0.5 kJ m-2 of UV-C energy. These 

results indicate that the UV-C light doses capable of inducing disease 

resistance have no negative impact on the epiphytic population and may 

even elevate the populations of some of the efficient antagonistic yeasts 

and bacteria which may serve as natural biological control agents on the 

fruit (see the chapter on Biological Control). 

2. CHEMICAL ELICITORS 

Among the chemical elicitors, we include those already well known, as 

well as newly introduced compounds that have been found to be 

associated with natural defense processes. 

Ethylene as Inducer of Hypersensitive Response 

Some disease-resistant plants restrict the spread of fungal, bacterial 

or viral pathogens to a small area around the point of initial penetration, 

where a necrotic lesion appears. This phenomenon, referred to as the 

hypersensitive reaction, may lead to acquired resistance to subsequent 

pathogen attack after the initial inoculation with the lesion-forming 

pathogens. The ability of ethylene to induce a hypersensitive response 

had already been demonstrated in the 1970s by G.E. Brown (1975, 1978): 

treatment of harvested Robinson tangerines with ethylene was found to 

cause a loss of chlorophyll and an increase in the content of carotenoids 

in the fruit. In parallel, the treatment stimulated the formation of 

infection hyphae by the appressoria of Colletotrichum gloeosporioides 

present on the fruit peel, and enhanced disease development. However, 

when the appressoria are removed from the peel before ethylene 

application, the ethylene-treated fruit, which becomes orange in color, 

develops resistance to invasion by subsequently applied fungal spores. In 




this case, the penetration of epidermal cells by the hyphae of 

Colletotrichum results in the formation of several layers of necrotic cells 

around the hyphae. The necrotic cells of the host were found to be filled 

with phenolic compounds which restricted fungal development to the site 

of penetration and prevented further spread of the disease (Brown, G.E., 

1978). 

Chitosan and Defense Reactions 

Chitosan, a deacetylated derivative of chitin, has demonstrated 

fungicidal activity against many fungi, including soil-borne and 

postharvest pathogens. Because of its polymeric nature, chitosan can also 

form gas-permeable films and create an internal modified atmosphere. It 

has, therefore, the potential for use as an edible antifungal coating 

material, regulating gas and moisture exchange by fruits and vegetables 

(Wilson et al., 1994). For discussion of these features of chitosan, see the 

chapter. Chemical Control -- Natural Chemical Compounds. However, in 

addition to its direct antifungal capability and its polymeric nature, 

chitosan is also a potential elicitor of phytoalexins (Kendra et al., 1989) 

and of various low-molecular-weight, pathogenesis-related (PR) proteins, 

such as chitinases, chitosanases and P-l,3-glucanases. 

By inducing these PR proteins, chitosan application may be able to 

promote defense responses in the tissues. These enzymes hydrolyze the 

main components of fungal cell walls (Boiler, 1993; Bowles, 1990) and are 

considered to play a major role in constitutive and inducible resistance of 

plants against invading pathogens (El Ghaouth, 1994). 

The inducement of such antifungal hydrolases by chitosan has been 

recorded in strawberries, bell peppers and tomato fruits, in which they 

have remained elevated for up to 14 days after treatment. In 

chitosan-treated bell peppers the production of lytic enzymes was 

followed by a substantial reduction of chitin content of the cell walls of 

invading fungal hyphae, as exhibited by a reduction of chitin labeling in 

the walls (El Ghaouth and Arul, 1992). Studies with bell peppers showed 

that chitosan treatment caused severe cytological damage to hyphae of 

invading Botrytis cinerea and interfered with the capacity of the fungus 

to secrete polygalacturonases. These activities were expressed in the 

reduced maceration of the host cell-wall components, pectin and 

cellulose, by the pathogen (El Ghaouth et al., 1997). The induction of lytic 

enzymes in harvested tissue by prestorage treatment with chitosan 

could, therefore, lead to the restriction of fungal colonization. The 




persistence of defense enzymes in the tissue following elicitation by 

chitosan may also contribute to the delay in the reactivation of quiescent 

infections, which naturally takes place when tissue resistance declines. 

Because of these features, chitosan has been considered a promising 

means for enhancing disease resistance and has been offered as a possible 

alternative to synthetic fungicides (El Ghaouth, 1994). 

Salicylic Acid and Defense Reactions 

Plant infection by fungi, bacteria and viruses may lead to a 

hypersensitive response which prevents pathogen spread, by the 

development of necrotic lesions at the site of infection. In addition, the 

localized infection may induce enhanced resistance against further 

infection by a variety of pathogens. This phenomenon is known as 

systemic acquired resistance (Raskin, 1992; Buchel and Linthorst, 1999). 

Along with the development of a hypersensitive reaction and systemic 

acquired resistance to subsequent pathogen attack, the systemic 

synthesis of low-molecular-weight pathogenesis-related (PR) proteins has 

been recorded (Carr and Klessing, 1989). Among the various PR proteins 

discovered, we find the enzymes, P-l,3-glucanases and chitinases, which 

are associated with defense responses. 

Although PR proteins are induced in pathological situations, they can 

also be induced by the application of certain chemicals that partly mimic 

the effects of pathogen infection, as well as by wounding or other stresses 

(Buchel and Linthorst, 1999). Salicylic acid is a natural compound capable 

of inducing such proteins even in the absence of pathogenic organisms. 

Salicylic acid, which belongs to the diverse group of plant phenolics, 

has ubiquitous distribution in plants. Many plant scientists have used 

salicylic acid and aspirin (acetylsalicylic acid) interchangeably in their 

experiments. Aspirin undergoes spontaneous hydrolysis to salicylic acid, 

although it has not been identified as a natural plant product. 

Increasing evidence suggests that endogenous salicylic acid plays an 

important role in the activation in plant tissues, of defense responses 

against pathogen attack (Kessman et al., 1994; Buchel and Linthorst, 

1999). It is no wonder that recent development in the area of new 

antifungal compounds has focused on this compound and its functional 

analogs. Exogenic application of salicylic acid or aspirin has been found 

to induce PR proteins in leaves of some crops (Renault et al., 1996); no 

information, however, is available on the ability of such treatments to 

induce PR proteins in fruits, where they might be induced in reaction to 

wounding or other stresses. On the other hand, studies with pear cell 




suspension culture indicated that salicylic acid may act as an 

inhibitor of ethylene biosynthesis, by blocking the conversion of 

1-aminocyclopropane-l-carboxylic acid (ACC) to ethylene (Leslie and 

Romani, 1986) and, therefore, may function as an inhibitor of fruit 

ripening. Studies with tomato fruits showed that salicylic acid may 

inhibit the increased expression of ACC gene, which is activated by fruit 

wounding (Li et al., 1992). By functioning as an inhibitor of ethylene 

production, salicylic acid would be able to enhance disease resistance 

against pathogens that do not attack non-ripening tissues. 

Antioxidants 

Antioxidants are another class of compounds that can reduce 

postharvest decay by modulating the natural fruit resistance. The 

antifungal diene, which is responsible for the inhibition of Colletotrichum 

gloeosporioides development in the young unripe avocado fruit, is 

apparently oxidized by lipoxygenase during fruit ripening, allowing the 

fungus to resume colonization of the fruit tissues. The activity of this 

enzyme in the peel of ripening fruit is regulated by the natural antioxidant 

epicatechin (Prusky and Keen, 1993). Laboratory studies showed that 

infiltration or dip treatment of avocado fruit with several antioxidants, 

such as a-tocopherol, butylated hydroxytoluene, butylated hydroxyanisole 

and tert-hutyl hydroquinone, inhibited lipoxygenase activity, retarded the 

decrease of the antifungal diene and, consequently, inhibited the 

development of anthracnose (Prusky et al., 1985b). Following these findings 

Prusky et al. (1995) showed that a dip or spray of avocado fruit (cvs. Hass 

and Fuerte) with a commercial formulation of the antioxidant, butylated 

hydroxyanisole (1200 iiig a.i. ml^) or the antioxidant combined with the 

fungicide prochloraz (250 |xg a.i. ml-i) consistently reduced the incidence of 

postharvest anthracnose caused by C. gloeosporioides in small and 

semi-commercial experiments. The effect of the antioxidant plus prochloraz 

lasted longer than that of the antioxidant alone, although prochloraz alone, 

even at higher concentrations, did not always reduce decay incidence. It was 

concluded that the antioxidant, which is a common food additive, prevented 

the conversion of quiescent infections into active ones, a process associated 

with fruit ripening and reduced resistance to infection. 

3. BIOLOGICAL ELICITORS 

The ability of various antagonistic microorganisms to act as 

alternatives to systemic fungicides and control postharvest diseases may 

be connected with their ability to produce and secrete antibiotic 




substances, or their ability to injure the pathogen directly. In many 

cases, these microorganisms, which grow rapidly and colonize wounds, 

have been hypothesized to act by competing with postharvest wound 

pathogens, for space and nutrients (see the chapter on Biological 

Control). However, in several cases, antagonistic yeasts can also act as 

inducers of resistance in the host tissue. 

The antagonistic yeast, Pichia guilliermondii, which is effective in 

controlling a wide variety of postharvest diseases in citrus fruits, apples 

and peaches, has been shown to induce enhanced levels of phenylalanine 

ammonia lyase (PAL) in citrus fruit peel (Wisniewski and Wilson, 1992), 

indicating the induction of a defensive response. P. guilliermondii, as 

well as an isolate of the yeast Candida famata, which is similarly 

effective against Penicillium digitatum in citrus fruits (Arras, 1996), 

contributes to enhanced host resistance by inducing the formation of the 

phytoalexin scoparone or of scoparone plus scopoletin, respectively, in the 

fruit peel (Rodov et al., 1992; Arras, 1996). 

Studies with unripe avocado fruits indicated that their resistance to 

anthracnose, caused by Colletotrichum gloeosporioides, was related to the 

presence of an antifungal diene compound in the peel (Prusky and Keen, 

1993). It has been suggested that fruit resistance to infection could be 

modulated, not only by delaying the normal decline of the antifungal 

diene by retarding fruit ripening, but also by increasing its synthesis. 

Challenge inoculation with the avocado pathogen, C. gloeosporioides 

(Prusky et al., 1990), or with a non-pathogenic mutant of Colletotrichum 

magna (Prusky et al., 1994) led to increased levels of the antifungal 

diene. These results suggest that the non-pathogenic Colletotrichum 

mutant is capable of enhancing the natural defense mechanism of 

avocado fruits, leading to the prolongation of the quiescent period of the 

pathogen and thus to disease inhibition. 

Wild and Wilson (1996) have recently detected a host defense reaction 

in apples, which reduced decay development in fruits that had been 

challenged by Penicillium expansum, the typical blue mold fungus of 

apples. The application of the protein synthesis inhibitor, cycloheximide 

prevented the reaction and resulted in more than 700% increase in 

decay. Following cycloheximide application, inoculation of apples with 

the citrus pathogen, P. digitatum resulted in green mold rot development, 

although the fungus does not normally attack apple fruits. Once the 

citrus Penicillium became established in the apple, it progressed at a 

higher rate than that of the apple Penicillium, It was also found that if 




rot caused by P. digitatum progressed through an apple and came into 

contact with a damaged region, it developed around the damage site 

leaving a halo of uninfected tissue. This area was suggested to be the 

location in which the host defense reaction occurred (Wild and Wilson, 

1996). 

In a recent study, Ippolito et al. (2000) found that the antagonistic 

yeast-like fungus, Aureobasidium pullulans multiplied rapidly in apple 

wounds, and reduced decay incidence in apples inoculated with Botrytis 

cinerea and P. expansum. However, they also found that the antagonist 

was capable of inducing the activities of P-l,3-glucanase, chitinase and 

peroxidase in the treated wounds on the fruits. This induction was 

exhibited in a transient increase in the enzymatic activity which started 

24 h after the application of the antagonist, and reached a maximum 48 

or 96 h after the treatment. The three enzymes are considered to be 

potentially important in host resistance mechanisms: chitinase and 

P-l,3-glucanase are hydrolases, capable of hydrolyzing fungal cells 

(Schlumbaum et al., 1986; Wilson et al., 1994), while peroxidase is 

involved in lignin formation and the production of structural barriers 

against pathogens (El Ghaouth et al., 1998; Chittoor et al., 1999). It was 

thus suggested that the induced activity of the glucano-hydrolases and 

peroxidase, together with the capacity to out-compete the pathogen for 

nutrients and space may be the basis of the biocontrol activity of 

A pullulans (Ippolito et al., 2000). 

B. GENETIC MODIFICATION OF PLANTS 

1. DISEASE-RESISTANT TRANSGENIC PLANTS 

Genetic transformation of plants for desired traits - such as improved 

yield, increased size or enhanced disease resistance - is not a new 

concept. Classical breeding has always provided new plant varieties 

which have desirable characteristics. However, the process can be too 

slow and inexact. Often pathogens can mutate too quickly for breeding of 

disease resistance varieties by classical methods. Progress in the 

development of bioengineering techniques provided the opportunity to 

modify plants, and enabled new transgenic plants with greater disease 

resistance to be developed more rapidly than via classical methods 

(Mount and Berman, 1994). A transgenic plant contains, within its 

genome, a foreign DNA that has been introduced artificially via genetic 




engineering. The creation of such plants involves the introduction, from 

unrelated plant species, of genes for disease resistance (Boiler, 1993). 

This technology not only allows for a wider genetic diversity but it may 

enable us to add multiple, diverse resistance genes to one plant variety. 

Furthermore, creating plant cultivars with inheritable genes for disease 

resistance is the most environmentally sound strategy for disease 

management. 

Desirable target genes for creating useful transgenic plants are 

usually isolated from plant viruses, bacteria, fungi or other plants, 

depending on the traits desired. Widely used genes are those known to be 

inhibitory to fungal growth, and they are often induced following fungal 

invasion (Boiler, 1993; Broekaert et al., 1995). These genes were reported 

to provide a quantitative improvement in resistance when introduced 

into a transgenic host in greenhouse studies (Jongedijk et al., 1995). 

However, the selection of beneficial target genes for pathogen resistance 

requires a thorough understanding of the pathogen, the host and the 

host-pathogen interactions. This knowledge can come from a multitude of 

genetic and physiological experiments. 

2. SOURCES OF GENES FOR BIOENGINEERING PLANTS 

Mount and Berman (1994) pointed out several natural compounds 

known to have antimicrobial activity against pathogenic fungi or 

bacteria, which could be possible sources for genes valuable for 

bioengineering of plants. The natural antibiotic compounds present an 

example of such sources. Transgenic plants with genes for antibiotic 

production may prevent initial infection of plant tissue by postharvest 

pathogens. Sources for novel antibiosis genes may come from plants 

themselves or from the many microorganisms being studied for use as 

biological control agents. If the antibiosis is due to a compound with 

isolatable genes, the genes might be suitable for bioengineering of plants. 

However, the widespread use of antibiotics is not recommended, mainly 

because of the possibility that the pathogen could rapidly develop 

resistance to the antibiotic compounds. 

Chitinases and P-l,3-glucanases are other antifungal compounds 

which are effective against fungal cell-wall polymers and are believed to 

be involved in plant defense mechanisms against fungal infection. Once a 

hydrolase gene that is effective against the pathogen has been identified, 

a desired transgenic plant can be created through molecular 

manipulation. The insertion of a chitinase gene into tobacco and canola 

plants was shown to result in enhanced resistance to Rhizoctonia solani 




but not to Cercospora nicotianae, suggesting that other factors besides 

chitinase, may be involved in disease resistance (BrogUe et al., 1991). A 

later study describes enhanced resistance to fungal attack in transgenic 

plants by co-expression of chitinase and glucanase genes (Zhu et al., 

1994). Other microbial enzymes normally induced in plants, such as 

peroxidase and other pathogen-defense-related compounds, might also be 

exploited for bioengineering (Bowles, 1990; Mohan and Kolattukudy, 

1990). 

The knowledge and understanding of postharvest disease etiology and 

physiology may be of much help in selecting new genes and developing 

new transgenic plants, resistant to postharvest diseases. For example, 

the knowledge that wounding leads to fungal penetration into the host 

and triggers postharvest disease development (Barkai-Golan and 

Kopeliovitch, 1981; Brown, G.E. and Barmore, 1983; Eckert and 

Ratnayake, 1994) may lead to genetic alteration of the plant to make it 

more physically resistant to wounding or to enhance the production of 

wound-healing compounds to shorten the time that the wounded tissue is 

vulnerable to pathogen attack (Mount and Berman, 1994). Similarly, 

understanding the role of the pathogen cell-wall-degrading enzymes in 

plant tissue deterioration may lead to the creation of new transgenic 

plants by which the pathogen polygalacturonase (PG) is suppressed. 

Following the characterization of PG-inhibiting proteins from pears by 

Stotz et al. (1993), it was suggested that the pear promoter might affect a 

fruit-specific expression of the gene, resulting in the inhibition o£ Botrytis 

cinerea. It was further suggested that the characterization of the protein 

inhibitor from pears should lead to the expression of PG-inhibiting 

proteins in transgenic plants and possibly to the inhibition of decay 

development. In fact, Powell et al. (1994) reported that transgenic tomato 

fruits expressing the gene of fungal PG-inhibiting glycoproteins of pears, 

were more resistant to B. cinerea than the control fruits. 

Using nor (non-ripening) or rin (ripening inhibitor) tomato mutants 

that block many aspects of the ripening process of the fruit, including 

softening of the tissues and color development, Barkai-Golan et al. (1986) 

and Giovannoni et al. (1989) found that the tomato's own PG plays an 

important part in the total PG activity in the host during pathogenesis, 

and probably takes a major role in the degradation of cell wall pectic 

substances. Following these findings, it was further suggested that the 

suppression of fruit PG might be useful in increasing fruit shelf life 

(Giovannoni et al., 1989). Bioengineering this enzyme has been 

accomplished with tomatoes (Kramer et al., 1990; Smith et al., 1988) and 




the resulting produce had a longer shelf life with no effect on fruit 

texture or color development. 

C. MANIPULATION OF ETHYLENE BIOSYNTHESIS AND 

GENETIC RESISTANCE IN TOMATOES 

The importance of ethylene ('the ripening hormone') in accelerating 

ripening and senescence in harvested fruits, and the considerable 

shortening of shelf life through its effects (see the chapter, Factors 

Affecting Disease Development - the Effects of Ethylene) led to the 

search for ways to suppress its influence in a reversible manner. Among 

the solutions offered to achieve this goal were the prevention of ethylene 

production by the plant tissue or the construction of a mutant plant 

whose fruits would not ripen until treated with ethylene (Theologis, 

1992). 

Elucidation of the pathway for ethylene synthesis in higher plants by 

Yang and Hoffman (1984) has been a major contribution, not only to the 

understanding of the biochemistry of this process, but also to the 

possibility of manipulating it to suppress or prevent ethylene production. 

An efficient way to prevent ethylene synthesis in tomato fruits was the 

inhibition of 1-aminocyclopropane-l-carboxylic acid synthase (ACC 

synthase), a key enzyme in the biosynthesis of ethylene, by an 

ACC-synthase antisense transgene. This function led to an almost 

complete inhibition of the ethylene precursor, ACC synthase, and to the 

production of mutant tomato plants with non-ripening fruits (Oeller et 

al., 1991). Using tomato plants in which synthesis of an ethylene-forming 

enzyme had been inhibited by an antisense gene, Picton et al. (1993) 

showed that the degree of inhibition of ripening was dependent upon the 

stage of development at which the fruits were detached from the plant: 

the effects were much more pronounced when fruits were detached from 

the vine before the onset of color change. Application of exogenous 

ethylene to such fruits only partially restored fruit ripening: it failed to 

increase lycopene accumulation to the level in the normal ripening fruits, 

and the ethylene-treated fruits demonstrated persistent resistance to 

over-ripening and shriveling. 

Another approach to the manipulation of ethylene biosynthesis was 

reported by Klee et al. (1991), who introduced a bacterial gene encoding 

an ACC-metabolizing enzyme into tomato plants. As a result of this 

procedure, inhibition of the accumulation of ACC and, consequently, a 




reduction in ethylene production and a marked inhibition of the ripening 

of the tomato fruit, were recorded. 

Inhibition of fruit ripening by these approaches has not only had a 

direct effect of prolonging the physiological life of the harvested fruit, but 

also an indirect effect on decay development, which is strongly related to 

the stage of ripening. The relationship between the stage of maturity and 

the resistance of harvested fruit to postharvest diseases is probably 

associated with the declining ability of the mature tissue to produce 

defensive compounds and structural barriers. Furthermore, the pathogen 

is capable of producing and activating pectolytic enzymes only during 

fruit ripening or in the mature fruit, where it decomposes the pectic 

compounds of the fruit cell walls, thus causing tissue degradation. 

Studies at the genetic level were also dedicated to direct enhancement 

of fruit disease resistance (Martin et al., 1993). This was achieved in a 

plant-pathogen system in which a single resistance gene in the plant 

responded specifically to a single avirulence gene in the pathogen, i.e., in 

a 'gene-for-gene' system. Such interactions have been described for 

various plant-pathogen pairs (Keen and Buzzel, 1991). Susceptibility to 

disease results in plant-pathogen systems in which either of two genes - 

the plant resistance gene or the pathogen avirulence gene - is absent 

from the interacting organisms. 

Martin et al. (1993) used the relationship between tomato and the 

bacterium Pseudomonas syringae pv. tomato, the causative agent of 

bacterial speck, to isolate a plant resistance gene by map cloning. Since 

the avirulence gene from the bacterium was found to induce resistance 

specificity in tomato cultivars containing the plant resistance gene, it 

was concluded that the interaction between the tomato and the 

bacterium involves a 'gene-for-gene' system, and when tomatoes 

susceptible to Pseudomonas are transformed with the plant resistance 

gene they become resistant to the pathogen. 

The tomato plant offers many advantages for the cloning of a 

resistance gene on the basis of their position on the genetic linkage map; 

since the tomato has been the subject of more than 50 years of plant 

breeding, many loci have been identified that confer resistance to various 

fungi, bacteria and viruses (Martin et al., 1993). 



POSTHARVEST DISEASE SUMMARY 

FOUR FRUIT GROUPS 

The present chapter presents the major postharvest diseases of four 

specific groups of fruits: subtropical and tropical fruits; pome and stone 

fruits; soft fruits and berries and solanaceous fruit vegetables. Aspects 

addressed include the life cycles and modes of infection of the pathogens, 

factors affecting disease development, and approaches to disease 

prevention and suppression. 

These descriptions should give us not only a general view on the 

various aspects of the postharvest pathology of different fruits, but may 

also enable us to compare among the diseases elicited by a given 

pathogen on a variety of hosts and among those elicited by different 

pathogens on a certain host. This chapter highlights two important 

features common to many postharvest diseases: (a) that wound 

pathogens play a dominant part in postharvest pathology, sometimes as 

major pathogens responsible for serious losses, and sometimes as minor 

pathogens, frequently associated with senescent or weakened tissue; 

(b) the importance of quiescent infections, characteristic of many 

postharvest pathogens at some stage between their arrival at the host or 

their initial penetration into the tissues, and the development of an 

active disease. 

Since understanding the nature of the pathogens and their modes of 

infection forms the basis for the development and subsequent application 

of suitable control methods, notes on control measures appropriate to 

each group of comodities accompany the disease descriptions. 

Furthermore, since many postharvest studies in the last decade have 

focused on the development of new control measures, especially new 

alternatives to synthetic fungicidal compounds, these new options are 

included in the notes. However, we should always keep in mind that 

detailed chemical, physical and biological means for disease control, as 

well as means for enhancing or eliciting host resistance, are addressed in 

separate chapters, for which the present chapter is intended as a 

supplement. Comprehensive descriptions of postharvest diseases of fruits 

and vegetables have recently been given by Snowdon (1990, 1992) and by 

Beattie et al. (1989; 1995). 




I. SUBTROPICAL AND TROPICAL FRUITS 

This group of fruits comprises unrelated species, most of which are 

chmacteric (pineapple being an exception). All the fruits of tropical and 

subtropical origin are characterized by their perishability: the harvested 

fruits have lost most of the intrinsic resistance characteristic of young 

and immature fruits, are rich in nutrients, have high moisture contents 

and are susceptible to attack by several pathogenic fungi (Eckert, 1990). 

The storage life of tropical fruits is relatively short - generally a few 

weeks under optimal conditions. The use of refrigeration to extend their 

storage life is limited since these fruits are sensitive to chilling injury 

and cannot be stored below their critical chilling range (Wang, 1990). 

Sensitivity to cold storage varies with the fruit species or cultivar, and 

also with the state of maturity. Because of their sensitivity to chilling 

injury, bananas should be stored at 13-16°C, lemons, grapefruits or 

mangoes at 10-15°C and papayas at 7-10°C. Under these conditions, 

pathogens such as Colletotrichum gloeosporioides or C. musae will 

continue to develop on mango or banana fruits, respectively, as will 

Penicillium digitatum, P. italicum and Geotrichum candidum on various 

sensitive cultivars of citrus fruits. In fact, postharvest decay losses of 

tropical and subtropical fruits have been estimated to be up to 50%, 

especially in developing countries where postharvest handling and 

storage are not optimal (Eckert, 1990). However, the demand for tropical 

fruits in the markets of non-producing countries continues to increase, as 

indicated by the expansion of trade among widely separated countries, 

although to reach some markets the fruits must undergo journeys long in 

terms of distance and time (Burden, 1997). 

CITRUS FRUITS 

1. Wound Pathogens 

Penicillium digitatum Sacc. (the green mold fungus) and 

P. italicum. Wehmer (the blue mold fungus) are the main wound 

pathogens of citrus fruits, causing the most common and the most 

devastating postharvest diseases. They occur in all citrus growing 

countries, worldwide and may attack the fruits in packinghouses, in 

transit, in storage and in the market. The green mold, which is specific to 

citrus fruit, is more prevalent than the blue rot, and may account for 

most of the postharvest decay of citrus fruit in storage. The two fungi 



Postharvest Disease Summary 269 

may appear together in the same lot or even on the same fruit. Early 

studies by Fawcett and Berger (1927) have shown that the green mold 

fungus grows more rapidly at moderate temperatures and, therefore, it 

predominates during short-term transit and storage. Penicillium 

italicum may sometimes develop in storage as a hyper- parasite over the 

green mold decay. 

Conidia of the Penicillia are present during the season in the 

atmosphere of citrus growing areas, particularly in citrus packinghouses 

and their surroundings, on the packinghouse equipment or on the hands 

of selectors and packers (Barkai-Golan, 1966). Disease is 

characteristically initiated through wounds and mechanical injuries 

sustained during harvesting, packing and handling (Kavanagh and 

Wood, 1967). Wounds may also result from piercing by the 

Mediterranean fruit fly and other fruit-piercing insects (Roth, 1967). 

Fruits are particularly susceptible to infection during wet or humid 

weather, and the postharvest temperature is another factor determining 

the green and blue mold fungi development. The optimal temperature 

range for both fungi is 20-27°C, within which the fruits may rot within a 

few days. Although fungal growth is reduced at lower temperatures, a 

very slow rate has still been recorded at 4.5-10°C, allowing the fungi to 

progress under these conditions when storage is extended or in overseas 

shipments. At 0-l°C the growth of the two Penicillia is arrested, but 

these temperatures result in chilling injury, expressed in pitting and 

internal physiological injury (Smoot et al., 1983). 

At an early stage both fungi cause a soft rot of the peel. Following 

this stage, a white mycelium develops from the center of the affected 

soft area; it later starts sporulating from the center of the colony, which 

is the older part of the infected area. The sporulating part becomes olive 

green in the case of P. digitatum and blue in the case of P. italicum. The 

rot at this stage is characterized by three circles. The colored 

sporulating center (1) is surrounded by (2) a band of white mycelium 

which has not yet sporulated or has only just begun to form 

conidiophores, or conidiophores plus sterigmata (which will later bear 

the spores). This white band is surrounded in turn by (3) a definite band 

of water-soaked peel, which has not yet developed the white fungal 

mycelium. 

Softening of the peel tissue is associated with the activity of pectolytic 

enzymes produced by the pathogen during pathogenesis. Under dry 

conditions the decayed fruit shrinks and becomes 'mummified'. The blue 

mold spreads directly from the decayed fruit into uninjured, healthy 




fruits, causing 'nesting'. The green mold typically infects adjacent fruits 

that have been injured (Barmore and Brown, 1982). The dusting of sound 

fruit with Penicillium spores from decayed fruit, termed soilage, is often 

of greater economic importance in retail cartons than is the decayed fruit 

(Stange and Eckert, 1994). 

Penicillium digitatum is capable of producing ethylene during its 

development (Biale, 1940; Ilag and Curtis, 1968). The production of 

ethylene, which is considered to be the ripening hormone, increases fruit 

respiration (Achilea et al., 1985a; Biale and Shepherd, 1941), hastens 

peel coloring and accelerates button senescence (hence leading to the 

initiation of stem-end rotting). By producing and releasing ethylene, 

P. digitatum reduces the storage life of healthy fruits in the same 

container or sometimes even in the same room. 

Geotrichum candidum Link ex Pers., the sour rot fungus, has been 

reported from most citrus growing areas. The disease is usually of less 

importance than the green mold and blue mold decays or the stem-end 

rots. It is, however, particularly important after prolonged wet seasons 

when heavy losses have been recorded. Furthermore, the importance of 

sour rot may generally be underestimated because initial infections are 

easily overgrown by other molds (Smoot et al., 1983). 

Sour rot is primarily a disease in storage and in transit, although 

infection may also occur on the tree. The spores (oidia), which are 

thin-walled cells derived from the fragmentation of fungal hyphae, are 

soil inhabitants being splashed during irrigation or rain to low hanging 

fruits or contaminating the fruit by soil contact. Infection may initiate 

preharvest at injuries caused by insects or via mechanical wounds 

sustained during harvesting and packinghouse handling (Laville, 1974). 

It is found most often on lemons, limes and grapefruits, which are often 

stored for extended periods, but they may infect other citrus fruits as 

well. Ripe fruit is more susceptible to the pathogen than green and 

immature fruit (Baudoin and Eckert, 1985). 

Early symptoms are similar to those of the green and blue mold 

decay, being a water-soaked spot that enlarges along with or following 

the growth of the fungus within the fruit tissues. At a later stage, a thin 

water-soaked, off-white mycelium is developed on the affected area. 

When a total decomposition of the fruit tissues occurs, a sporecontaining 

juice leaks from the rotten fruit and can infect healthy 

adjacent fruits. 




2. Stem-End Pathogens 

The principal stem-end rot pathogens of citrus fruits are: 

(i) Diplodia natalensis P.E., frequently referred to as 

Botryodiplodia theobromae Pat. [(perfect state: Physalospora 

rhodina Berk. & Curt.) Cooke]; 

(ii) Phomopsis citri Fawcett (perfect state: Diaporthe citri Wolf); 

(Hi) Dothiorella gregaria Sacc. (perfect state: Botryosphaeria 

ribis Grossenb. & Duggar). 

(Hi) Alternaria citri Ell. & Pierce, which was renamed A alternata 

(Fr.) Keissler pv. citri (Solel, 1991). 

Stem-end rots caused by D, natalensis and P. citri are the major 

postharvest diseases in citrus fruits grown in humid subtropical areas 

with high rainfall during the growing season. These fungi, as well as 

D. gregaria, may assume the sexual state, namely perithecia, which give 

rise to ascospores. However, this form is not commonly found and the 

asexual spores, borne within pycnidia, are the only spores important for 

infection. The pycnidia are produced in dead wood and in living tissues of 

stems and leaves. 

Infection may be initiated at any stage of fruit development, when 

wind and splashes of rain carry the pathogen spores to the surface of 

immature fruits on the tree. However, immature fruits are resistant to 

invasion, and the fungi remain quiescent in floral remnants under the 

sepals of the fruit and do not become active until the buttons become 

senescent and begin to separate from the fruit (Brown, G.E. and Wilson, 

1968). Diplodia usually progresses rapidly down the central axis, 

frequently reaching the stylar-end; it grows along the tissues that divide 

the fruit into segments of the pulp, taking on the appearance of fingers 

that connect the rotted ends of the fruit (Smoot et al., 1983). Similar 

symptoms can arise from Dothiorella development. Phomopsis decay is 

characterized by some shriveling of the decayed tissue, and this helps to 

separate the decayed from the sound tissue. The fungus similarly 

progresses down the core but does not reach the stylar end until most of 

the rind has decayed. Infection by the three fungal species may, however, 

originate at injuries on the side or at the stylar end of the fruit. 

Stem-end rots caused by Diplodia, Phomopsis and Dothiorella may 

occur in the same lots or even on the same fruit, and it is often difficult or 

impossible to distinguish among them. When Diplodia and Phomopsis 

develop on the same fruit, an important factor in determining which will 

predominate is the temperature (Smoot et al., 1983): the optimum 




temperatures for growth are 30°C for Diplodia and 23°C for Phomopsis 

and Dothiorella. In general Phomopsis occurs throughout the harvest 

season, whereas Diplodia is more common during the ethylenedegreening 

season and in the late spring, when there are warmer 

weather and frequent rains. Under humid conditions a superficial, fine 

white mycelium may be developed on the Phomopsis-iniected fruits. 

Surface growth is seldom seen on Diplodia-intected areas. 

Alternaria citri is widely distributed in citrus-growing areas. Being 

dispersed by wind and air currents in the grove, the conidia can be found 

at the stem end, as well as on or underneath the 'button' of citrus fruits 

of any age. In Navel oranges, fungal conidia may be located at the stylar 

end (Singh and Khanna, 1966). However, infection may occur only if the 

fruit has been injured or physiologically weakened by unfavorable 

growing conditions. After harvest, infection may originate in peel injuries 

or more frequently through the stem end, resulting in stem-end rotting. 

Freshly harvested lemons, however, are resistant to Alternaria infection 

while immature. Infection through the button occurs only at the 

senescent stage, when the fungus is capable of progressing into the fruit, 

affecting the central core and to some extent the inner tissues of the rind. 

In this case, internal rot is frequently developed before the appearance of 

external disease symptoms (Brown, G.E. and McCornack, 1972). 

In oranges, grapefruits and some hybrids, the internal tissue becomes 

black; hence the common name for Alternaria infection - the black rot 

(Singh and Khanna, 1966). In Navel oranges, infection usually originates 

at the stylar end. The ability of the fungus to penetrate the fruit through 

senescent buttons may explain the prolonged prevalence of A, citri. 

Similarly, citrus fruits that have been degreened by ethylene to 

accelerate color development are liable to be infected by A, citri. Ethylene 

treatment, by hastening senescence and thereby the death of the green 

button, predisposes the fruit to invasion by stem-end fungi. In contrast, 

the application of growth-regulating substances such as 2,4-D retards 

senescence of the fruit button and delays disease initiation (Eckert and 

Eaks, 1989). 

3. Phytophthora spp. 

Phytophthora citrophthora (Smith & Smith) Leon., Phytophthora 

hibernalis Carne, Phytophthora nicotianae (van Breda de Haan) 

var. parasitica (Dastur) Waterh. and other Phytophthora species are 

the causal organisms of brown rot of citrus fruits. 

Brown rot is a major fruit disease in all citrus growing areas and is 




particularly prevalent after late-season rains, which play an important 

part in disease development. The fungi are common in soil and survive in 

their sexual state as thick-walled oospores. Fungi in the asexual state - 

sporangia and zoospores - are formed under moist conditions (Hough et 

al., 1980) and the mobile, swimming zoospores are splashed by rain or 

irrigation water onto low-hanging fruits in the tree. Rains and moisture 

can also carry the zoospores to higher fruits. Infection occurs only when 

the fruit remains wet for a relatively long period, during which the 

fungus may penetrate directly into the non-injured peel (Feld et al., 

1979). If the fruit dries before the zoospores can germinate and penetrate 

into the peel, the infection process is arrested. Fruits infected on the tree 

will later drop onto the ground and, in fact, the appearance of infected 

fruits on the ground around the tree may indicate the presence of an 

active infection on the tree. At harvest, fruits which had unnoticed 

incipient infections on the tree are carried along with healthy fruits into 

the packinghouse. In such a case, the disease will develop during storage 

or transit. 

The decayed area on citrus fruit is firm and leathery, characterized by 

brownish peel discoloration. Under humid conditions, a white delicate 

mycelium usually develops on the infected area. Another 

characterization of the brown rot is its distinctive aromatic or fermented 

odor (Smoot et al., 1983). During storage, the disease can spread by 

contact with other fruits (Klotz and DeWolfe, 1961). At a later stage, 

secondary soft rot bacteria may invade fruits infected by brown rot and 

turn the firm decay into a soft one. 

4. Colletotrichum gloeosporioides (Penz.) Sacc. [Perfect state: 

Glomerella cingulata (stonem.) Spauld. & v. Schrenk] 

Colletotrichum gloeosporioides, the causal fungus of anthracnose, is 

not included in the list of the major pathogens in citrus fruits. However, 

anthracnose may be a serious disease of tangerines, tangerine hybrids 

and other mandarin fruits harvested early in the fall, when long periods 

of ethylene exposure are required for promoting fruit color (Brown, G.E., 

1975); it is usually only of minor importance on oranges, grapefruits and 

lemons. 

The fungus exhibits both the sexual state (perithecia with ascospores) 

and the asexual state (acervuli with conidia). Acervuli with conidia are 

formed in dead branches in the grove. Conidia from this source are 

spread by winds and rains and may infect citrus fruit on the tree only 

when it has been weakened by drought, frost damage or unfavorable 




cultural conditions. The infection may remain latent in the form of 

appressoria, in which case disease is manifested only during transport 

and storage (Fisher, 1970; Brown, G.E., 1975). Anthracnose may invade 

the fruits after harvest, in the packinghouse, in storage or in the market; 

it gains entrance to the fruit via mechanical injuries or dead buttons. 

Mature green fruits are sensitive to infection after prolonged exposure to 

ethylene during the degreening, since this treatment renders the intact 

peel easily penetrated by the pathogen. Affected fruits are characterized 

by the development of sunken lesions that, under humid conditions, form 

abundant salmon-pink masses of spores that later turn into brown-black 

masses. 

The incidence of anthracnose can be reduced by washing, which 

removes the quiescent appressoria (Smoot and Melvin, 1971; Brown, 

G.E., 1975), before degreening with ethylene. This, however, cannot be 

done commercially because washing retards the degreening process. 

Several factors affect the severity of anthracnose: (1) orange-colored 

fruits are more resistant to the disease than pale or green-colored ones 

(Brown, G.E. and Barmore, 1976); (2) degreening with ethylene at 

concentrations higher than those required for optimal chlorophyll 

degradation (5 |LI1 per liter of air) favors anthracnose development; 

(3) fungicide application prior to degreening reduces anthracnose 

development (Brown, G.E. and Barmore, 1976). Resistance to 

anthracnose was induced in mature green tangerines treated with 

ethylene after being washed to remove dormant appressoria (Brown, G.E. 

and Barmore, 1977). Application of ethephon (a chemical which produces 

ethylene on degradation) to Robinson tangerines 5-7 days before harvest 

significantly reduced the incidence of anthracnose (Barmore and Brown, 

G.E., 1978); control was attributed to the accumulation in the interior of 

the fruit of ethylene which induced physiological changes leading to the 

development of resistance. 

5. Trichoderma viride Pers. ex S.F. Gray 

This fungus, which is responsible for Trichoderma rot in storage, is 

frequently considered to be a pathogen of minor importance. It can, 

however, cause serious loss in some citrus-growing areas. 

The fungus is known as a soil inhabitant and is capable of 

decomposing cellulose. Germinating conidia enter through mechanical 

injuries on the fruit, at the stem end or the stylar end (Gutter, 1961). 

This fungus is often overgrown by other organisms, particularly by 

Penicillium digitatum with which T. viride is often confused because of 




the development of green or green-yellow masses of conidia on the 

infected area under humid conditions (Gutter, 1961). In the presence of 

P. digitatum, Trichoderma growth is stimulated, probably by the 

production and emanation of ethylene by P. digitatum; this enhances 

fruit senescence and consequently fruit susceptibility (Brown, G.E. and 

Lee, 1993). T. viride can sometimes be spread into healthy uninjured 

fruits during storage, causing 'nesting'. 

Control Measures 

The possibility of penetration of the wound pathogens into the fruit via 

injuries in the peel shows why the control of insects in the grove and 

careful handling throughout the harvesting process, to prevent skin 

breaks and bruises, are of primary importance in preventing the 

development of infections. The severity of postharvest wound infections 

is also directly related to the damage caused to the crop by rough 

handling after harvest (Sommer, 1982). Thus, careful handling should 

always be a continuous concern; the fact that preharvest fungicide sprays 

in the grove have not always been effective in reducing postharvest decay 

is probably because most injuries occur during harvesting or 

packinghouse handling. 

Sanitation to minimize the presence of infective inoculum is also of 

great importance for disease prevention. It includes the removal of fallen 

or decayed fruit from the grove and from the packinghouse and its 

surroundings, sanitation of packinghouses by fumigation (with 

formaldehyde or other fumigants), sanitation of field boxes with 

disinfecting solutions or steam, and unloading and cleaning fruit arriving 

in the packinghouse in a separate area, to reduce contamination in the 

processing, packing and storage areas (Smoot et al., 1983). 

In addition to sanitation procedures, strategies for control of 

postharvest diseases incited by wound pathogens include: (a) inactivation 

of germinating spores in fresh wounds; (b) protection of the peel from 

infection at a later time by depositing a fungitoxic residue in wounds or 

on the surface of the fruit; and (c) inhibition of Penicillium sporulation on 

the surface of decaying fruits and of subsequent contact spread in storage 

(Eckert, 1990). Non-selective, water-soluble salts of fungicides, such as 

sodium or^/io-phenylphenate or sodium carbonate, applied generally 

prior to 1960, were fairly effective against wound pathogens if applied to 

the fruit soon after harvest (before the pathogen penetrated the tissue). 

The introduction of the systemic benzimidazole compounds offers highly 

effective fungicides for controlling wound infection by Penicillium spp. 




At high dosage rates they have also provided a protective barrier on the 

fruit surface, that inhibits Penicillium sporulation on decasdng fruits 

(Eckert and Ogawa, 1985). Imazahl, a systemic fungicide characterized 

by a different mode of action from that of the benzimidazole compounds, 

effectively controlled P, digitatum and P. italicum on citrus fruits, and its 

control extended to isolates resistant to the benzimidazole compounds. 

Both groups of fungicides, however, are ineffective against sour rot 

caused by Geotrichum candidum (Eckert and Ogawa, 1985). Following 

the evolution of Penicillium strains resistant to imazalil, as to other 

previously effective fungicides, new strategies and new chemicals are 

constantly being evaluated and developed. (See the chapter on Chemical 

Means - Postharvest Chemical Treatment). 

Eckert and Eaks (1989) emphasized that since only P. digitatum 

biotypes resistant to the fungicides can sporulate on the treated fruit, 

fungicide application creates an ideal situation for the buildup of 

resistant types of P. digitatum, which would lead to poor decay control. 

Curing citrus fruits (holding them at temperatures and humidities 

conducive to wound healing) has long been reported as a non-chemical 

method for controlling decay by wound pathogens (Hopkins and Loucks, 

1948; Brown, G.E., 1973). Healing of wounds and control of postharvest 

decay has also been achieved by combining individual wrapping of citrus 

fruit in plastic film, which leads to the formation of a water-saturated 

atmosphere within the wrap, with curing at 36°C for 3 days 

(Ben-Yehsoshua et al., 1987). Stange and Eckert (1994) showed that 

dipping lemons in a surfactant solution prior to curing gave better 

control of the green mold than curing alone. 

Much research has been conducted on the development of biological 

control means as alternatives to chemical treatments, for controlling 

postharvest decay (Wilson and Wisniewski, 1989; Brown, G.E. and 

Chambers, 1996). Chalutz and Wilson (1990) and Droby et al. (1992) 

found that yeast species of the genera Debaryomyces and Pichia are 

capable of inhibiting wound pathogens because of their rapid 

development in wounds in citrus fruit peel. Arras (1996) found that 

Candida famata was one of the most active yeasts against P. digitatum 

in wounded fruits, while Smilanick and Denis-Arrue (1992) and 

Smilanick et al., (1996) demonstrated that strains of the bacterium 

Pseudomonas syringae could reduce the incidence of postharvest rot by 

occupying the wounds. Isolates of the common yeast-like fungus, 

Aureobasidium pullulans were reported by Schena et al. (1999) to control 

P. digitatum at high concentrations on grapefruit. 




Recently, two biological products have been registered for commercial 

postharvest applications to citrus fruits - Aspire, which is Candida 

oleophila, and BioSave"^^ 1000, which is P. syringae (Brown, G.E. and 

Chambers, 1996). Evaluating the efficacy of these biological products 

against citrus fruit pathogens, G.E. Brown and Chambers (1996) found 

that although significant control of P. digitatum was obtained with each, 

the level of control and its consistency were usually less pronounced than 

those obtained with standard rates of the usual chemicals (thiabendazole 

or imazalil). Combining each of these chemicals with Aspire improved the 

results and sometimes combinations of Aspire, with a low rate of 

fungicide, were sufficient to achieve effects similar to those obtained by 

the chemicals at standard rates. The biological products were not 

effective against stem-end rots caused by Diplodia natalensis or 

Phomopsis citri (Brown, G.E. and Chambers, 1996). 

Several approaches have been used to control stem-end rots of citrus 

fruits arising from latent infections. Since infections at the stem end are 

related to the ability of the pathogens to invade the fruit through the 

senescent button tissues, these fruits have been controlled for over four 

decades with 2,4-D in order to retard senescence of the button, which 

usually harbors a quiescent infection of one of the stem-end pathogens. 

Maintaining the button green and young delays fungal penetration into 

the fruit (Eckert and Eaks, 1989). 

Debuttoning of fruit at harvest was found to remove the source that is 

responsible for initial infection. Similarly, pruning out dead limbs and 

twigs to reduce the level of inoculum has also been found effective, but it 

is not economically practical (Smoot et al., 1983). The control of stem-end 

rots of citrus fruits was made possible by the introduction of the systemic 

fungicides (thiabendazole, benomyl, imazalil and prochloraz). Apparently, 

these systemic compounds penetrate the tissues and provide action 

against the pathogen at sites not accessible to the traditional 

water-soluble fungicides (Brown, G.E., 1981; Eckert, 1990). 

Since brown rot is initiated in the grove, its control should start with 

preharvest disease control. Fungicidal sprays of fixed copper compounds 

applied to the lower fruits of the tree before the onset of the rainy season 

were found beneficial in suppressing disease development (Timmer and 

Fucik, 1975). Spraying the ground beneath the trees was found effective 

in inhibiting spore formation on the soil surface (Solel, 1983). Preventive 

measures also include the removal of fallen fruit from the grove, routine 

disinfection of the picking boxes and ensuring that they are not left on 

the soil during rainy or foggy periods (Smoot et al., 1983). 




The benzimidazole compounds and imazalil applied to control the 

wound pathogens of citrus fruit are not effective against Phytophthora, 

On the other hand, metalaxyl is uniquely effective in eradicating 

incipient infections of Phytophthora and has no effect on the development 

of other postharvest diseases (Eckert and Ogawa, 1985). This compound 

delayed the development of brown rot on citrus fruit (Cohen, 1981), 

suppressed the growth of Phytophthora on the fruit and prevented 

contact spread of the disease on inoculated grapefruit at 11°C. Later 

studies (Ferrin and Kabashima, 1991) described the development of 

metalaxyl-resistant isolates of P. parasitica as indicated by the increased 

metalaxyl concentration needed for the suppression of mycelium growth 

and of sporangia and chlamidospore formation. Fosetyl aluminum 

applied after harvest was found to provide both protective and curative 

action against P. parasitica infection (GauUiard and Pelossier, 1983). 

Hot water dip treatments for 3 min at 46-49°C are effective in 

eradicating Phytophthora infections when applied shortly after infection 

(within a few days), while the fungus is still confined to the external 

layers of the fruit peel where heat will penetrate rapidly (Klotz and 

DeWolfe, 1961). 

Disease control is dependent on careful handling and shipping of 

strong, top-quality fruits and the avoidance of prolonged storage. 

Harvesting of fruit at prime maturity can avoid the need for prolonged 

degreening of the immature fruit with ethylene, which generally 

increases fruit susceptibility to anthracnose. Preharvest ethephon 

sprays, which promote fruit ripening with relatively small color changes 

from green to yellow or orange, may allow the crop to be picked at prime 

maturity and may also lead to triggering the resistance mechanisms of 

the fruit (Barmore and Brown, G.E., 1978). 

Suppression of citrus stem-end rots greatly depends on proper 

refrigeration during storage and transportation, and temperatures 

between 4 and 7°C are required to control these rots in transit. 

However, for various citrus fruits, such as lemons, limes, grapefruits, 

pomelos and certain hybrids, which are sensitive to low temperatures 

and develop chilling injuries, higher temperatures should be 

maintained. Since the minimum growth temperature of Colletotrichum 

gloeosporioides is 9°C (Sommer, 1985), temperatures below 10°C would 

delay the appearance of anthracnose symptoms. Storage temperatures 

below 10°C also result in a very slow growth of the wound pathogen 

G. candidurriy which has a high optimal growth temperature of 25-30''C 




(Laville, 1974), and suppress growth of the fungus Trichoderma, whose 

minimum growth temperature is about 5°C (Gutter, 1963). 

BANANA 

The most important diseases of bananas are: crown rot, caused by a 

complex of pathogens; anthracnose, incited by Colletotrichum musae; and 

fruit rot, caused by Botryodiplodia theobromae. 

1. Crown Rot Fungi 

Several fungi may be involved in banana crown rot, which is the 

principle cause of postharvest disease losses of bananas in international 

trade. They include C. musae, (Berk. & Curt.) v. Arx, Fusarium spp. 

[mainly F. pallidoroseum (Cooke) Sacc], Verticillium theobromae (Turc.) 

Mason & Hughes, B, theobromae Pat., Acremonium spp., Cephalosporium 

spp. and Ceratocystis paradoxa (Dade) Moreau. 

Crown rot pathogens commonly grow saprophytically and sporulate on 

senescent flower parts and leaf debris in the plantation. The spores are 

spread to the developing hands by rain splash and wind (Meredith, 

1971). Harvested bananas carry the spores of pathogenic fungi into the 

tank of water, in which they are floated to permit latex to flow from the 

cut tissue of the crown (the portion of the node that has been severed 

from the stem). The tank (or the 'dehanding' tank) is believed to be the 

major site of crown rot inoculation, and the newly exposed tissue is 

vulnerable to infection (Eckert, 1990; Shillingford, 1977). 

2. Colletotrichum musae (Berk. & Curt.) v. Arx 

This fungus is one of the components of the crown rot complex, and the 

principle cause of banana anthracnose: one of the major diseases of 

bananas in all producing countries. The fungus exists on debris in its 

asexual state, as conidia located in a special acervulus. Spores are 

liberated by rain splash or irrigation water and dispersed by air currents 

onto young green fruits. Following germination under moist conditions, 

the conidia produce appressoria, which remain quiescent on the fruit 

skin. When the fruit ripens and its susceptibility to invasion increases, 

the fungus becomes active and infects the fruit. The presence of an 

antifungal compound in the unripe banana fruit has been related to the 

latency of C. musae in this fruit (Mulvena et al., 1969). 




The infection of uninjured, immature fruit before harvest results in 

the formation of numerous, small dark circular spots which enlarge and 

tend to coalesce (Simmonds, 1963). However, Colletotrichum may cause a 

"non-latent" anthracnose, which is initiated at abrasions and scars 

sustained during harvesting and handling (Slabaugh and Grove, 1982). 

In this case, large lesions are produced (Shillingford and Sinclair, 1978). 

Both types of lesion eventually carry salmon-pink spore masses. 

3. Botryodiplodia theobromae Pat. 

In addition to being a component of the crown rot complex pathogens, 

B. theobromae is also responsible for banana finger rot. The abundance of 

conidia produced on decaying vegetation in banana plantations form the 

source of primary infection. Conidia are disseminated by air currents and 

rain to dsdng flower parts and are later capable of infecting the fruit 

when it ripens. Rotting usually begins at the tip of a finger but may occur 

at any wound on the fruit surface. The rot progresses rapidly at high 

temperatures, its optimum being about 30^C. Under these conditions, 

within a few days the rot becomes soft and dark, and typical black 

pycnidia appear, which give rise to conidia (Williamson and Tandon, 

1966). Thus, in hot and humid weather, the harvested fruits can be 

expected to develop finger rot during transport and ripening. The disease 

may be serious in fruits held at a high temperature for more than 14 

days in transit (Strover, 1972). 


Control of postharvest diseases of banana fruits depends on several 

steps. The fruit should be harvested at the correct stage of maturity and 

handled carefully to avoid injury. Fallen leaves and flower bracts should 

be removed in the plantation and the packing station to reduce the 

numbers of infecting spores. The washing water in the tank, in which the 

banana 'hands' float to permit latex to flow from the cut crown, should be 

changed frequently to minimize the inoculum level. Along with reducing 

the inoculum level, wounds should be protected against fungal infection 

(Shillingford and Sinclair, 1978; Slabaugh and Grove, 1982). Application 

of a fungicide soon after dehanding is important, to protect wounds and 

to prevent infection of the cut crown surface (Eckert and Ogawa, 1985). 

Postharvest treatment with a systemic fungicide was found to be more 

effective in controlling decay than preharvest sprays (Ram and Vir, 

1983). Cooling is important to suppress decay development, and it should 

be initiated as soon as possible after cutting the fruit. 




Studying the potential for biocontrol of Lasiodiplodia theobromae in 

banana fruits, Mortuza and Ilag (1999) found that Trichoderma viride is 

capable of reducing rotting by up to 65% in inoculated banana fruits. The 

time of application of the antagonist and the inoculum levels of both the 

pathogen and the antagonist are important factors in the biocontrol 

activity. The best activity was exhibited at high concentrations of the 

antagonist and low concentrations of the pathogen, and when the 

antagonist was applied 4 hours prior to the pathogen. 

MANGO 

The most serious postharvest diseases of mango are anthracnose, 

caused by Colletotrichum gloeosporioides; and stem-end rot, caused by a 

complex of fungi, of which Botryodiplodia theobromae is the most 

common. 

1. Colletotrichum gloeosporioides (Penz.) Sacc. [perfect state: 

Glomerella cingulata (Stonem.) Spauld. & v. Schrenk]. 

This pathogen causes anthracnose, which is responsible for severe 

losses of fruit in all mango-growing countries. It is the same fungus that 

causes anthracnose in avocado, citrus and papaya fruits. 

C. gloeosporioides may exhibit both the asexual state (acervuli with 

conidia) and the sexual state (perithecia and ascospores). The asexual 

conidia are produced in wet seasons on dead twigs and leaves, from 

which they are washed down to the fruit. Perithecia have also been 

found on dead twigs and leaves but they probably do not play an 

important role in anthracnose infection (Fitzell and Peak, 1984). 

Disease is initiated by conidia in unripe mango fruits in the plantation, 

the fruit being susceptible to infection from blossoming and during its 

developmental stages on the tree. Germinating conidia give rise to 

appressoria, but at this stage or after the production of fine 

subcuticular hyphae, the infection remains quiescent. Fungal growth is 

renewed only when the fruit ripens (Daquioag and Quimio, 1979). The 

resistance of unripe mango fruits has been attributed to the presence of 

preformed antifungal compounds in the peel, which act on the 

subcutaneous hyphae from germinating appressoria (Prusky and Keen, 

1993). A fruit with quiescent infection, without any visible symptoms, is 

liable to be stored and will start to rot later, during storage or 

marketing. 





Stem-end rot is a very serious postharvest disease of mango, 

responsible for heavy losses during transit and storage. The fungi 

involved in stem-end rots are mainly: Botryodiplodia theobromae Pat. 

[perfect state: Physalospora rhodina (Berk. & Curt.) Cooke], Phomopsis 

spp. and Dothiorella spp. In their asexual state - pycnidia with conidia - 

they persist in the orchard on dead wood. The sexual state (perithecia 

and ascospores) is also occasionally exhibited (Alvarez and Lopez, 1971). 

The conidia are washed down by rainwater, thus contaminating the fruit. 

Stem-end rots arise by invasion of the cut stem of the fruit at harvest or 

shortly after harvest (Pathak and Srivastava, 1969), but may originate at 

injuries in other locations on the fruit. The rot develops rapidly at room 

temperature, causing a soft-watery decay, which involves the whole fruit 

within several days, and is characterized by the development of minute 

black pycnidia over the lesions. 

3. Alternaria alternata (Fr.) Keissler 

This fungus usually develops from quiescent infections on the surface 

of mango fruits, where it creates typical black spots. Germinating conidia 

can penetrate the immature fruit via lenticels but infection remains 

quiescent until the onset of ripening after harvest (Prusky et al., 1983). 

The resistance of young mango fruits to fungal development has been 

related to the presence in the peel of fungitoxic concentrations of 

antifungal resorcinol compounds (Droby et al., 1986). 


The major strategies for control of postharvest diseases of mangoes 

include regular sprays in the plantation, to reduce quiescent infections of 

C. gloeosporioides, Anthracnose can be controlled by immersing 

mature-green fruits in hot water at 55°C for 5 min after harvest, but this 

treatment does not satisfactorily control Diplodia stem-end rot (Spalding 

and Reeder, 1978). The heat treatment may slightly injure the fruit and 

accelerate the change in peel color from green to yellow. The addition of 

various fungicides to the hot water enabled the concentration of the 

fungicide and the level of the heat treatment to be reduced compared 

with those required when either was applied alone, resulting in improved 

control of anthracnose and stem-end rot (Spalding and Reeder, 1986b) 

and, in turn, a greater amount of fruit acceptable for marketing. 

The efficiency of quarantine heat treatments (developed for fruit fly 

disinfestation in mangoes) as a means for postharvest disease control, 




was evaluated by Coates and Johnson (1993). High-humidity (> 95% RH) 

hot air treatment alone (to a core temperature of 46.5°C for 10 min) 

reduced the incidence of anthracnose in mangoes stored for 14 days at 

13°C prior to ripening at 22°C. A treatment consisting of high-humidity 

hot air combined with either a heated fungicide (benomyl) or an 

unheated superior fungicide (prochloraz) gave complete control of 

anthracnose under these storage conditions. The hot air treatment alone 

gave no control of stem-end rot caused by Dothiorella and Lasiodiplodia 

in mangoes stored at 13°C prior to ripening. A supplementary hot 

fungicide treatment was required for acceptable control of this disease in 

cool-stored mangoes. A low level of gamma radiation (1000 Gy), which is 

aimed at eradication of fruit fly infestations of mango fruits, improved 

the efficacy of hot water against both anthracnose and stem-end rots 

(Spalding and Reeder, 1986b). Effective control of postharvest diseases of 

mangoes during short-term storage at 20°C was provided by hot 

fungicide treatment followed by low-dose irradiation. Satisfactory disease 

control during long-term controlled atmosphere storage (5% O2 and 

1.5-2% CO2 at 13°C) was achieved when mangoes were treated with 

heated and unheated fungicides combined with low-dose irradiation 

(Johnson et al., 1990). Irradiation in excess of 600 Gy caused surface 

damage in Kensington Pride mangoes, while cultivars Kent and Zill show 

more tolerance to irradiation. 

A postharvest unheated fungicide application may substantially reduce 

the incidence of Alternaria black spot on mangoes (Prusky et al., 1983). 

Moreno and Paningbatan (1995) reported on the biocontrol of Diplodia 

stem-end rot of mango, by the antagonistic fungus, Trichoderma viride. 

The antagonistic activity was enhanced at higher concentrations of the 

antagonist and at lower concentrations of the pathogen. 

PAPAYA 

Similarly to other tropical and subtropical fruits, the major postharvest 

diseases of papaya are anthracnose, caused by Colletotrichum 

gloeosporioides, and stem-end rots, caused by Botryodiplodia theobromae 

or Phomopsis spp. Other fungi may be associated with stem-end rots of 

papayas, such as Phoma caricae-papayae and Phytophthora palmivora. 

Various surface rots, caused by species of Rhizopus, Fusarium, 

Alternaria, Stemphylium and other pathogens, may infect papaya fruits 

and reduce their postharvest life. 





Glomerella cingulata (Stonem.) Spauld. & v. Schrenk] 

Anthracnose, caused by this pathogen, is generally initiated by the 

asexual state of C. gloeosporioides (acervuli with conidia), although the 

sexual state (perithecia and ascospores) has been recorded. Conidia are 

produced on dying leaves or petioles and are spread by rain and air 

currents onto the fruit. Under moist conditions, the conidia germinate 

within a few hours to form appressoria and infection hyphae, which 

penetrate the cuticle and form quiescent infections in the immature fruit 

(Dickman and Alvarez, 1983). Renewed growth and characteristic disease 

symptoms, including fungal colonization and the development of an 

abundance of salmon-pink conidia, appear when the fruit ripens after 

harvest. 

C. gloeosporioides is also responsible for multiple small lesions of the 

'chocolate spot' type (Dickman and Alvarez, 1983), which similarly arise 

from quiescent infections but only occasionally develop into anthracnose 

lesions. 


Botryodiplodia theobromae Pat. and Phomopsis spp. are characteristic 

stem-end pathogens of tropical and subtropical fruits. 

Infection takes place by conidia (produced within pycnidia), which are 

liberated and dispersed during wet conditions in the orchard. 

Germinating spores penetrate via newly cut stem ends or through 

crevices between the peduncle and the fruit flesh (Chau and Alvarez, 

1979), but infection may also be initiated at any injury incurred by the 

fruit shortly before harvest, and which has not yet healed. 

Phoma caricae-papayae (Tarr) Punith. (perfect state: Mycosphaerella 

caricae H.& P. Sydow) can infect uninjured developing fruit during wet 

weather (Simmonds, 1965). Infection may be initiated anywhere on the 

fruit surface, by asexual spores (conidia) or by sexual spores (ascospores 

produced within perithecia) (Chau and Alvarez, 1979). Infection may, 

however, occur via the cut stem during harvest resulting in the formation 

of stem-end rot (Srivastava and Tandon, 1971). 

3. Phytophthora palmivora (Butler) Butler 

Phytophthora infection is initiated during the rainy season by the 

asexual state of the fungus (sporangia with zoospores), although sexual 

spores (oospores) have also been recorded. Following dispersal of the 

lemon-shaped sporangia, the zoospores, which are liberated during wet 




weather, are capable of infecting uninjured papaya fruits. Infected fruits 

fall to the ground and serve as a source of new infections in developing 

fruits (Srivastava and Tandon, 1971). Invasion may, however, occur at 

the cut stem and result in stem-end rot development (Alvarez and 

Nishijima, 1987). 

4. Rhizopus stolonifer (Ehrenb. Ex Fr.) Lind 

Rhizopus rot, caused by Rhizopus stolonifer, is a destructive rot of 

papaya. Spores of the fungus are ubiquitous and infection originates only 

via injuries inflicted during harvesting and handling. Severe problems 

occur particularly during extended rainy periods (Nishijima et al., 1990). 

The fungus infects mature fruits, causing a soft watery rot. Under 

warm humid conditions, a loose white mycelium bearing black sporangia 

(giving rise to sporangiospores) is being developed. Infected fruits, which 

rapidly collapse, cause contact infection of neighboring fruits during 

shipment and storage (Quimio et al., 1975). Nishijima et al. (1990) found 

that the optimum temperature for disease development was about 25°C 

and that no disease developed at 5 or 35°C; they also showed that the 

fungus is consistently capable of infecting the fruit through lesions 

caused by Colletotrichum gloeosporioides, Phomopsis sp. or Cercospora sp. 


Postharvest diseases of papaya are controlled by frequent fungicide 

sprays in the plantation followed by postharvest hot water and fungicide 

treatments (Eckert, 1990). The frequent fungicide sprays in the 

plantation control quiescent infections of Colletotrichum and incipient 

infections of Phytophthora, and reduce the inoculum levels of Phoma 

(Mycosphaerella), Botryodiplodia and other pathogens that invade the 

fruit through wounds (Alvarez et al., 1977; Bolkan et al., 1976). The 

traditional treatment of hot water at 48°C for 20 min was used in Hawaii 

for many years to control anthracnose, stem-end rots and incipient 

Phytophthora infections (Akamine, 1976; Aragaki et al., 1981). However, 

this treatment delayed color development and caused a slight heat 

injury, accompanied by an increase in Stemphylium infection (Glazener 

and Couey, 1984). A combination of shorter, hotter water sprays (54°C 

for 3 min) followed by a fungicide treatment, was recommended for 

papayas destined for long distance shipment (Couey and Alvarez, 1984). 

In order to satisfy the quarantine requirements for eradication of fruit 

flies in papaya shipments to fly-free zones, the double hot water dip (30 

min at 42°C followed by 20 min at 49''C) was adopted. This treatment, 




coupled with regular field spraying programs, also provides control of 

postharvest diseases of papaya (Alvarez and Nishijima, 1987). However, 

in the light of the sensitivity of various papaya cultivars to heat injury, a 

single hot dip (49°C for 15 min) was suggested as optimal for disease 

control with minimum effects on fruit quality (Nishijima, 1995). 

A hot water dip at 49°C for 20 min is effective in reducing early 

Rhizopus infections, mycelium being more sensitive to heating than 

fungal spores. Decay was further reduced and almost eliminated when a 

hot water treatment was coupled with fungicide field sprays. The main 

control measures against Rhizopus soft rot are, however, sanitation 

procedures. These include the removal of decaying fruit from areas 

around the packinghouse, and regular disinfestation of floors, packing 

lines and other packinghouse equipment and of bins used for transferring 

fruit from the field to the packer (Nishijima et al., 1990). Holding the 

fruit at 7-10°C will retard decay development during storage. 

AVOCADO 

The most significant postharvest diseases of avocado fruits are: 

anthracnose, caused by Colletotrichum gloeosporioides; fruit rot, caused 

by Dothiorella gregaria; and stem-end rots, caused by Botryodiplodia 

theobromae, Dothiorella gregaria, Alternaria spp. and Phomopsis spp. 

(Darvis, 1982; Muirhead et al., 1982). Other, less significant postharvest 

pathogens include Fusarium spp., Pestalotiopsis versicolor, Rhizopus 

stolonifer, Pseudocercospora purpurea, Trichothecium roseum, Penicillium 

spp., and the bacteria Erwinia carotovora and Pseudomonas syringae, 

1. Colletotrichum gloeosporioides (Penz.) Sacc. (Perfect state: 

Glomerella cingulata (Stonem.) Spauld. & v. Schrenk) 

This fungus, which is responsible for anthracnose, may exhibit both 

the asexual state (acervuli and conidia) and the sexual state (perithecia 

and ascospores), but the asexual conidia play the main role in fungal 

infection. Initial infection of avocado peel takes place in the plantation 

during the growing season but, since avocados do not ripen on the tree, 

the infection remains quiescent in the immature fruits (Binyamini and 

Schiffmann-Nadel, 1972). Quiescence is exhibited by appressoria that are 

formed by germinating conidia and are located on the fruit surface, or by 

thin infection hyphae that penetrate under the cuticle or the external 

layers of the epidermis (Muirhead, 1981b; Prusky et al., 1990). Renewed 




fungal growth and complete penetration of the fruit peel occur only after 

the onset of ripening (Prusky and Keen, 1993). Infected avocados may 

thus be harvested without any visible symptoms and the disease 

develops later during storage or marketing. Typical symptoms of 

anthracnose, which appear only when the fruit begins to soften, include 

circular dark spots and, later, sunken lesions which, under humid 

conditions, give rise to masses of salmon-pink conidia. 

2. Dothiorella gregaria Sacc. (perfect state: Botryosphaeria ribis 

Grossenb. & Duggar) 

Dothiorella may exhibit both the asexual state (pycnidia with conidia) 

and the sexual state (perithecia and ascospores) on dead twigs and 

leaves. Under wet conditions spores may infect the fruits on the tree, via 

their stomata and lenticels, and the infection then remains quiescent. 

Progressive lesions generally develop when the fruit begins to soften 

after harvest (Labuschagne and Rowell, 1983). The same fungus, alone or 

in association with other fungi, is also responsible for stem-end rot 

(Muirhead et al., 1982). 

3. Stem-End Fungi 

The stem-end fungi are found on dead branches and bark of avocado 

trees. Botryodiplodia theobromae Pat. [perfect state: Physalospora 

rhodina (Berk. & Curt.) Cooke], is typically a wound pathogen and 

avocado infections frequently take place during harvest. The decay 

originates at the stem end and proceeds, rather uniformly, towards the 

blossom end. It may infect the pedicel during fruit development but, 

since avocados do not ripen on the tree, the infection remains quiescent 

until after harvest, when the fruit ripens. Dothiorella gregaria and 

Phomopsis spp., however, are capable of causing latent infections in 

developing fruits (Peterson, 1978). Generally, stem-end rots caused by 

Alternaria alternata are less common. However, after prolonged 

commercial use of the systemic fungicide, thiabendazole to treat 

avocados, Alternaria alternata, which is insensitive to this fungicide, has 

become a major cause of stem-end rot in stored fruits (Zauberman et al., 

1975). 


Anthracnose development may be reduced by preventing injuries to 

the fruit, in the orchard or during harvesting and handling (Smoot et al., 

1983). The disease is particularly important during prolonged storage. 




since the fruit may mature and ripen during this period. Both orchard 

spraying and postharvest fungicidal dips have been used to control 

postharvest avocado diseases (Darvis, 1982; Muirhead et al., 1982). 

A reduction of the incidence of postharvest anthracnose of avocado was 

recorded after dipping or spraying with an antioxidant compound, which 

led to the maintenance of host resistance. A combination of the 

antioxidant with fungicide (prochloraz) application reduced decay 

consistently and for a longer period, while fungicidal treatments alone 

did not always result in disease reduction (Prusky et al., 1995). 

Looking for a non-chemical method to control postharvest diseases of 

avocados in South Africa, Korsten et al. (1995) evaluated the inhibitory 

capacity of bacteria isolated from avocado leaf and fruit surfaces, against 

fruit pathogens. Bacillus subtilis, when incorporated into the commercial 

wax and applied to the fruit in the packinghouse, was found to be the 

most effective antagonist against the Dothiorella/Colletotrichum fruit rot 

complex and against stem-end rot. 

Under commercial conditions, decay development can be delayed by 

storing the fruit at 5-6*^0 for short-term shipping or storage. However, 

several cultivars are sensitive to such low temperatures and should be 

stored at higher temperatures. A controlled atmosphere of 2% O2 and 

10% CO2 at 7°C can extend the storage period of avocados (Spalding and 

Reeder, 1975). When the fruit must be held for longer periods, a 

combination of low temperatures and fungicide treatments is needed 

(Muirhead et al., 1982). 

PINEAPPLE 

The main postharvest diseases of pineapple are black rot, caused by 

Thielaviopsis paradoxa, and fruitlet core rot, caused by several wound 

pathogens. 

1. Thielaviopsis paradoxa (de Seynes) Hohnel [perfect state: 

Ceratocystis paradoxa (Dade) Moreau] 

This fungus is the cause of black rot, which is the most significant 

postharvest disease of pineapple (Rohrbach and Phillips, 1990), and 

which is sometimes referred to as soft rot or stem-end rot, depending on 

the mode of infection. The fungus survives in plant debris in the soil in 

the form of thick-walled chlamydospores. Infective asexual conidia are 

splashed onto the fruit by rain. The sexual state (perithecia with 




ascospores) has also been recorded occasionally (Liu and Rodriguez, 

1973). Infection may originate in the field via growth cracks or via insect 

punctures, but fungi generally penetrate into the fruit through the cut 

stem end when the fruit is removed from the stalk or through injuries 

inflicted during harvesting and handling (Rohrbach and Phillips, 1990). 

The disease develops rapidly at tropical temperatures. The fungal 

optimal growth temperature is approximately 26°C and it becomes 

inactive below 10°C. Moist conditions at harvest are important for decay 

development. High percentages of decay in a shipment usually indicate 

that the fruit was harvested during or shortly after a prolonged rainy 

period (Smoot et al., 1983). 

2. Fruitlet Core Pathogens 

Several fungi, such as species of Cladosporium, Penicillium and 

Trichoderma, can invade wounds during harvest but frequently develop 

as surface molds (Eckert, 1990). Some fungi, including species of 

Fusarium and Penicillium, and bacteria, including species of Erwinia, 

Pseudomonas and Acetobacter, either singly or in combination, may cause 

"fruitlet core rot" (Rohrbach and Taniguchi, 1984). While Penicillium 

funiculosum enters the developing floret through the unopened bud, 

Fusarium moniliforme var. subglutinans and the pathogenic bacteria 

infect the developing fruit via the open flower (Rohrbach and Phillips, 

1990). Fungal spores or bacterial cells are deposited in the floral cavities 

by water splash or by insects that both carry the pathogens and damage 

the tissue prior to infection (Bolkan et al., 1979; Mourichon, 1983). 

Infection results in brown soft rot of the axis of individual fruitlets and 

can rarely be detected from the outside of the fruit. A longitudinal section 

reveals the extension of the affected area towards the heart of the fruit 

(Snowdon, 1990). The decay may develop in ripe or mature fruit, in the 

field or during transit and marketing. 


Since fungal infection is associated with the presence of wounds, 

careful handling is needed at all stages to prevent injury, and packing 

has to be designed to prevent the sharp 'crown' leaves from piercing 

adjacent fruits (Snowdon, 1990). Damaged and wet fruits and those with 

an excessive number of growth cracks should be excluded from fresh fruit 

shipments (Smoot et al., 1983). Measures to reduce fruitlet core rot 

incited by the various pathogens start in the field, with the control of 

insects that spread the disease (Rohrbach and Phillips, 1990). 




For several decades black rot was controlled by disinfection of the cut 

area of the fruit with sodium or^/io-phenylphenate, sodium salicylanilide 

or benzoic acid within 2 h after cutting (Eckert and Ogawa, 1985). 

Several studies indicated that better control of black rot could be 

achieved by postharvest fungicide applications (Cho et al., 1977; Eckert, 

1990; Hernandez Hernandez, 1990), which should be applied within 6 to 

12 h after harvest. Thiabendazole and benomyl may also be added to a 

wax formulation applied to harvested fruits to control internal browning 

and water loss, although they are less effective when mixed with wax 

than when applied in water (Cho et al., 1977). 

Refrigeration at 7-8°C arrests fungal development, even in the 

presence of deep wounds, open for fungal penetration (Hernandez 

Hernandez, 1990). This temperature is suitable for ripe fruit during 

storage or transportation, but it may cause mature green fruits to fail to 

ripen normally and to develop good flavor, therefore higher storage 

temperatures (10-13°C) are required for such fruits. 

PERSIMMON 

Several fungi are involved in postharvest decay of persimmon fruits. 

The most important are species of Alternaria, Botrytis, Colletotrichum, 

Phoma, Cladosporium, Penicillium, Mucor and Rhizopus. 

Alternaria alternata (Fr.) Keissler 

This pathogen causes black spot disease or Alternaria rot. Direct 

penetration of developing fruits can occur during the entire growth period of 

the fruit. The infection remains quiescent and regains activity only after 

harvest, when the fruits ripen (Prusky et al., 1981). Symptoms generally 

appear after 10 weeks of storage at -1°C and, at a later stage, the disease 

may involve the entire fruit surface. In addition, fungal penetration 

frequently occurs via small cracks formed on the fruit shoulder beneath the 

calyx as the fruit approaches harvest maturity. In the high humidity 

environment of the rainy season, preharvest disease symptoms appear 

beneath the cal}^ of riper fruits in the orchard (Prusky et al., 1981; Perez et 

al., 1995). When the season is dry, no decay is recorded at harvest. 


Protective fungicide treatments in the orchard are ineffective in reducing 

decay development beneath the calyx (Perez et al., 1995). A preharvest 




spray with gibberelic acid (GA3), applied in the orchard 10-14 days before 

harvest, delays fruit softening and extends the storage life of the fruit 

(Ben-Arie et al., 1986). Following three GA3 sprays (20 |Lig ml-i) during the 

development of the fruit in the orchard, the cal5^ was found to remain erect 

until harvest and a smaller area was covered with the black spot disease 

after 3 months of storage at 0°C. GA3, at concentrations of up to 200 |Lig ml-i, 

had no direct effect on fimgal growth in vitro or on inoculated fruit, and the 

effect of GAs on disease development was attributed to the enhancement 

of fruit resistance to infection (Perez et al., 1995). 

GUAVA 

The fact that the guava fruit peel is easily broken leads to postharvest 

infections by many wound pathogens (Eckert, 1990). These include: 

Colletotrichum gloeosporioides, Botryodiplodia theobromae, Ceratocystis 

paradoxa and species of Phomopsis, Phoma, Fusarium, Pestalotia, 

Penicillium, Aspergillus, Rhizopus, Mucor and Erwinia (Adisa, 1985b; 

Arya et al., 1981; Brown, B.I. et al., 1984; Singh and Bhargava, 1977; 

Ramaswamy et al., 1984; Wills et al., 1982). 


Glomerella cingulata (Stonem.) Spauld & v. Schrenk] 

Anthracnose, caused by this pathogen, is the most significant disease 

of guavas (Adisa, 1985). The fungus, which characteristically also infects 

other tropical and subtropical fruits, attacks most cultivars of guava, 

although several cultivars have been found to offer some resistance 

(Tandon and Singh, 1970). 

2. Botryodiplodia theobromae Pat. 

Botryodiplodia rot, caused by this pathogen, is another disease 

common to various tropical and subtropical fruits. It may invade guavas 

in the orchard, causing a dry stem-end rot of developing fruits. Decay of 

ripe fruits results from infection through the stem end or via wounds and 

causes a soft breakdown of the tissue (Adisa, 1985; Srivastava and 

Tandon, 1969). 

3. Phomôpsis spp. 

Infection by these pathogens may also be initiated either in the 

orchard or after harvest (Srivastava and Tandon, 1969); it usually occurs 




at the stem end, or at the distal end in the region of the persistent calyx. 

Affected areas of the skin turn soft and wrinkled along with the 

appearance of numerous pycnidia with the asexual spores. 


Sanitation activities in the orchard, careful harvesting and handling 

and preventive preharvest fungicide sprays may reduce postharvest 

diseases of guava incited by wound pathogens (Srivastava and Tandon, 

1969; Ito et al., 1979; Ramaswamy et al., 1984). Good control has been 

achieved by postharvest fungicide dips of guava inoculated with the 

wound pathogens Pestalotia, Phoma, Gloeosporium and Aspergillus 

(Singh and Bhargava, 1977; Arya et al., 1981). Wills et al. (1982) found 

that under natural infection conditions, a heated fungicide was more 

effective than some single fungicides at ambient temperatures. 

Refrigeration at 5°C enables prolongation of the postharvest life of 

guavas by up to 3 weeks while lower temperatures may cause chilling 

injury (Brown, B.I. and Wills, 1983). 

LITCHI 

Litchis may be infected by many postharvest pathogens during storage 

and transport (Prasad and Bilgrami, 1973; Scott et al., 1982). Pathogens 

characteristic of tropical and subtropical fruits, such as Botryodiplodia 

theobromae and Colletotrichum gloeosporioides, do not spare this one. 

Wound pathogens, such as Rhizopus spp. or Geotrichum candidum, for 

which wounding is a prerequisite for infection, may penetrate the fruit 

shell via cracks resulting from sun scorch or via punctures produced by 

fruit-piercing insects (Scott et al., 1982). Such injuries not only provide 

an avenue for fungal penetration into the fruit but may also allow the 

exudation of juices that stimulate mold development (Prasad and 

Bilgrami, 1973). However, the main postharvest problem with litchis is 

associated with physiological changes exhibited in darkening and loss of 

the natural red color of the fruit. This phenomenon involves the formation 

of polyphenols in response to desiccation and injury (Akamine, 1976). 


Disease control should begin in the orchard, with preventive measures 

against fruit insects, including 'bagging' of developing fruits (Prasad and 

Belgrami, 1973). In order to reduce postharvest decay, the fruits can also 




be treated with a wax formulation containing fungicide (Prasad and 

Bilgrami, 1973) or dipped in a fungicide solution, which may be heated or 

unheated (Scott et al., 1982; Brown, B.I. et al., 1984). The physiological 

changes in the natural fruit color can be controlled by packing the fruit 

in a plastic film that maintains a high relative humidity around it and 

reduces moisture loss (Akamine, 1976), or by altering the pH of the fruit 

pericarp (Fuchs et al., 1993). 

POME FRUITS 

II. POME AND STONE FRUITS 

Eckert and Ogawa (1988) divided the major postharvest pathogens of 

pome fruits into two groups: (a) those that cause quiescent infections of 

lenticels, including Gloeosporium album, G, perennans and Nectria 

galligena; and (b) those that preferably enter through wounds after 

harvest, including Penicillium expansum, Botrytis cinerea, Monilinia 

spp., Mucor spp., Rhizopus spp., Alternaria alternata, Stemphylium 

botryosum and Cladosporium herbarum. The rots in the lenticels are 

initiated in the orchard in the late summer, in areas with late summer 

rainfall and are a major problem in apples grown in the United Kingdom 

and Northern Europe (Edney, 1983). In drier apple production areas, the 

main problems are caused by wound pathogens that invade the fruit 

after harvest through injuries sustained during harvesting and handling 

and via puncture wounds, bruised lenticels, etc. In fact, the wound 

pathogens are of major importance in all apple and pear production 

areas. Other important pathogens of pome fruits are species of 

Phytophthora that may become a serious problem during rainy seasons 

for fruit from orchards with heavy soils (Edney, 1978), and 

Colletotrichum gloeosporioides (Syn. Gloeosporium fructigenum), the 

bitter rot fungus, which is capable of direct penetration of the intact skin 

(Brook, 1977), while harvested fruits are infected via injuries. 

Botryosphaeria spp., the black and white rot fungi, are of importance in 

several areas of the USA (Snowdon, 1990). 

Pathogens of minor importance that may occasionally be found on 

harvested apples and pears include Stemphylium botryosum, 

Cladosporium herbarum, Trichothecium roseum, and species of 




Phomopsis, Nigrospora, Fusarium, Epicoccum, Aspergillus, Trichoderma 

and others (Snowdon, 1990). 

A. Penicillium expansum Link 

The blue mold rot, caused by P. expansum, is the commonest and one 

of the most destructive rots of harvested apples and pears. Other 

Penicillium species, such as P. cyclopium, P. crustosum and P. verrucosum, 

may occasionally cause the blue mold rot (Barkai-Golan, 1974; Hall and 

Scott, 1989a). 

Germinating conidia invade the fruit mainly through wounds or 

bruises incited during harvesting and handling. Penetration can also 

take place via the lenticels, under favorable conditions (Baker and Heald, 

1934), or at the site of infection by other pathogens, such as species of 

Gloeosporium, Phytophthora and Mucor (Snowdon, 1990). The 

susceptibility of lenticels to penetration is enhanced in over-mature fruit, 

during prolonged storage, or by bruising or puncturing (Hall and Scott, 

1989a). The fungus produces pale brown to brown soft-watery spots that 

enlarge rapidly under shelf life conditions. Under humid conditions, 

conidia-bearing conidiophores, grouped to form coremia, are formed on 

the surface of the lesion. As the conidia mature, they become blue-green 

and form masses which give the decay its typical color. Since decay 

development is favored by high humidity, the blue mold is more of a 

problem on fruits stored or shipped in plastic film liners (Hall and Scott, 

1989a). Decay can progress, albeit slowly, during cold storage; rapid 

development begins when the fruits are transferred to warmer 

conditions. The fungus can spread during the long months of storage, by 

contact between infected and sound fruit, forming 'nests' of decay. 

Several strains of P. expansum can produce the mycotoxin patulin 

while they develop in apples and pears. The mycotoxin may be highly 

toxic to animal tissue and may also display carcinogenic and mutagenic 

properties (Stott and BuUerman, 1975). The ability to produce patulin 

and the amounts of patulin produced depend on fungal strain, fruit 

cultivar, storage temperature and storage atmosphere (Lovett et al., 

1975; Paster et al., 1995). The amounts of patulin produced by different 

strains of the fungus in Golden Delicious apples were found to range 

from 2 to 100 |ag g^ (Sommer et al., 1974). However, patulin production 

by a given strain has also been found to differ at different temperatures 

(Paster et al., 1995). Stud5dng patulin production in apples stored under 

controlled atmosphere of 1% CO2, 3% O2 at 1°C, Lovett et al. (1975) 

reported that only one of the two Penicillium strains tested produced 




mycotoxin, and the amounts produced were considerably reduced, as 

compared with production in air. Sommer et al. (1974) found that two 

strains of P. expansum produced patuHn in Golden Delicious apples 

stored in a controlled atmosphere containing 7.5% CO2 and 2% O2 at 

23°C, although at markedly lower levels than in air. Working with other 

P. expansum strains, Paster et al. (1995) found that patulin production 

was inhibited in Starking apples stored in an atmosphere of 3% CO2, 2% 

O2 at 25°C, although fungal growth was still 70% of that of the control. 

Under 3% CO2, 10% O2, patulin production was only slightly reduced. 

B. Botrytis cinerea Pers. [perfect state: Botryotinia fuckeliana (de 

Bary) Whetzel] 

The gray mold rot caused by this fungus can cause heavy losses of 

pome fruits, particularly pears. The fungus survives as sclerotia in the 

soil and on plant debris, and under favorable conditions asexual spores 

(conidia) are formed. Under wet conditions the conidia may infect dying 

blossoms (Combrink et al., 1983) and remain quiescent in the flower 

parts (DeKock and Holz, 1991). Fruit infection occurs later, after renewal 

of the pathogen growth during storage and marketing. B. cinerea can 

infect the fruit through wounds incurred during harvesting and 

handling, or directly via skin breaks (Spotts and Peters, 1982a). In fact, 

the resistance of various apple cultivars to infection by B, cinerea and 

other wound pathogens has recently been related to the force required to 

break the fruit skin (Spotts et al., 1999). 

Lesions are dry and firm at first, but become soft as the rot advances. 

Under humid conditions, abundant gray-brown conidia are produced. 

Sclerotia may also be formed on well advanced lesions. The fungus 

spreads very readily during storage by contact between infected and 

sound fruit, forming 'nests* of decaying fruit. This type of infection 

markedly increases the infection rate in storage (Edney, 1978). In 

addition, J5. cinerea may penetrate apple fruits via the sinus between the 

calyx and the core cavity, in apple cultivars characterized by open 

sinuses, resulting in core rot (Spotts et al., 1988). 

C. Monilinia spp. 

Monilinia rot may be incited by three species of Monilinia: Monilinia 

fructigena (Aderh. & Ruhl.) Honey, which is widespread in Europe, Asia 

and South America, but is uncommon in North America; M fructicola 

(Wint.) Honey, which occurs in North and South America, South Africa, 

Japan, Australia and New Zealand; and Monilinia laxa (Aderh. & Ruhl.) 




Honey, which occurs occasionally in Europe, Asia and some regions of 

North America (Snowdon, 1990). 

The three species are wound pathogens capable of invading apple and 

pear fruits before and after harvest (Edney, 1983). Fruits infected early 

in the growing season develop rot on the tree, exhibit firm lesions and 

eventually turn into shriveled 'mummies', which may remain attached 

to the tree or fall to the ground. Infections initiated in the field continue 

to develop in storage, where they spread by contact between infected 

and sound fruits (Cole and Wood, 1961). The fungus survives the winter 

and, when environmental conditions become favorable, produces 

asexual spores (conidia). The conidia are dispersed throughout the 

orchard and infect the fruits via wounds. The fungi may exhibit the 

sexual state (apothecia with ascospores) and, on liberation, the 

ascospores may infect the flower parts, thus spreading the disease into 

developing fruits. 

D. Gloeosporium album Osterw. [perfect state: Pezicula alba 

Guthrie] and Gloeosporium perennans Zeller & Childs [perfect 

state: Pezicula mâlicorticis (Jackson) Nannf.] 

These two lenticel pathogens cause Gloeosporium rot. Infection is 

spread by the asexual spores (conidia produced in acervuli), which are 

dispersed by rain and contaminate the fruit during the growing season 

(Edney, 1983). Following germination on the fruit surface, the fungus 

enters the developing fruits via lenticels (Bompeix, 1978) and remains 

quiescent until after harvest (Noble and Drysdale, 1983). The sexual 

state (apothecia with ascospores) has been recorded but is not regarded 

as an important source of inoculum. G. perennans produces circular rots, 

frequently with a yellow center, that have led to the common names 

'target spot' or 'bull's eye' rot. However, identification of the species is 

mainly by microscopic examination. 

E. Cylindrocarpon mali (AUesch.) WoUenw. [perfect state: Nectria 

galligena Bresad.] 

This lenticel pathogen is responsible for Cylindrocarpon or Nectria 

fruit rot. The fungus exhibits both the sexual (perithecia with 

ascospores) and the asexual (conidia produced in sporodochia) states, 

and both forms may be found in cankers on the tree. Conidia are 

dispersed by rain splash while ascospores are frequently carried by air 

currents to greater distances (Swinburne, 1983). Infection may occur 

through the calyx-end or the stem-end and result in the fruit decaying 




on the tree. However, the fungus can penetrate via the lenticels and 

remain quiescent until after harvest. The quiescent stage has been 

related to the synthesis of the phytoalexin, benzoic acid in the tissues; 

after harvest, with the loss of the phytotoxic activity of the phytoalexin, 

the pathogen resumes active growth and decay is developed 

(Swinburne, 1983). The resistance of apple tissues to decay may, 

however, change because of the development of N, galligena mutants 

resistant to benzoic acid (Seng et al., 1985). 

F. Colletotrichum gloeosporioides (Penz.) Sacc. [perfect state: 

Glomerella cingulata (Stonem.) Splaud. & v. Schrenk] and 

Colletotrichum acutatum J.H. Simmonds 

These two species are involved in bitter rot of apples, which may cause 

heavy losses in warm, wet growing areas (Sutton, 1990). 

Some strains of C. gloeosporioides are capable of exhibiing both the 

sexual state (perithecia with ascospores) and the asexual state (acervuli 

with conidia), while others produce only asexual spores. Both types of 

spores can take part in disease initiation (Sutton and Shane, 1983). 

Infection takes place in the orchard during the growing season, in warm, 

wet weather, the optimal temperature for growth being about 26°C. 

Spores are washed down onto the fruit by rain, and infection can take 

place by direct penetration of germinating spores into the intact fruit 

(Brook, 1977). At a later stage, completely decayed fruits turn into 

'mummies', a source of immense quantities of conidia which serve as 

inoculum for the following year (Brook, 1977). Bitter rot is also found in 

storage following fungal penetration through wounds sustained during 

harvesting, handling and marketing. 

Bitter rot symptoms appear as circular lesions with the fungal 

reproductive structures (acervuli, perithecia or both) often in concentric 

rings on the fruit lesion (Shi et al., 1996). Isolates of the two 

Colletotrichum species exhibit a considerable variation in colony color, 

but they generally produce acervuli with typical orange to salmon-pink 

conidial masses (Jones et al., 1996). Apple isolates of C. gloeosporioides 

differ from apple isolates of C. acutatum in the morphology of their 

conidia and by a faster rate of colony growth (Shi et al., 1996). The 

frequency of occurrence of the various pathogens on apples varies among 

orchards and may be influenced by many variables including 

environmental factors, sampling date, host cultivar and management 

practices. 




G. Phytophthora cactorum (Lebert & Cohn) Schroet. and 

Phytophthora syringae (Kleb.) Kleb. 

These two species, which are the causal pathogens of Phytophthora rot, 

are soil-borne. Infection is closely related to rainfall which splashes infected 

soil onto fruits close to the ground (Edney, 1978). The asexual spores 

(zoospores), which are the main agents of infection, come into contact with 

low-hanging fruits and generally infect them via the lenticels. This process 

requires free water or wetness (Edney, 1978). Fallen apple and pear leaves 

have also been found to bear the sexual state of Phytophthora (oospores) 

and are considered to be an important source of inoculum in the orchard 

(Harris, 1979). After a resting period, and under wet conditions, the 

oospores germinate to form sporangia that release new zoospores. Fruit 

harvested in the early stages of disease development undergo rotting later, 

in storage. Decay is spread during storage by contact of infected fruit 

with sound fruit, forming 'nests' of decayed fruit (Edney, 1978). 

H. Botryosphaeria spp. 

Two species are responsible for apple and pear rots (Combrink et al., 

1984; Brown, E.A. and Britton, 1986): Botryosphaeria obtusa (Schw.) 

Shoem (imperfect state: Sphaeropsis sp.) and Botryosphaeria ribis 

Grossenb. & Duggar (imperfect state: Dothiorella sp.). 

JB. obtusa [syn. Physalospora obtusa (Schw.) Cooke], the cause of the 

black rot, and B. ribis, the cause of the white rot, may both produce the 

asexual state (pycnidia with conidia) and the sexual state (perithecia 

with ascospores). The conidia are water-borne while the ascospores may 

be dispersed to greater distances by air currents (Snowdon, 1990). Both 

stages are capable of producing the disease. The fungi may penetrate into 

the fruit via lenticels, bruises, or cracks, while in the orchard, in transit 

or in storage. Young fruits are resistant to rotting (Sitterly and Shay, 

1960), becoming susceptible on maturation. Thus, fruit harvested shortly 

after infection will rot later in storage. 

Symptoms may vary with the temperature but both fungi can be 

controlled during storage at about 0°C. 

I. Mucor piriformis Fischer 

Mucor rot can develop on various cultivars of pears and apples. The 

fungus in the sexual state, as zoospores, can survive hot dry periods 

(Michaelides and Spotts, 1986), while the asexual state (sporangia with 

sporangiospores) is of prime importance in the fungal infection. The 

fungus may penetrate the fruit through injuries of the skin (Combrink 




and Fourie, 1984) or via the stem end (Lopatecki and Peters, 1972). In 

some apple cultivars, penetration occurs through the open calyx tube 

(Sharpies and Hims, 1986; Spotts et al., 1988) resulting in core rot. 

Although the susceptibility of apple cultivars to M piriformis may differ 

among different locations or different years, Granny Smith and Braeburn 

have generally been found to be the most susceptible cultivars and Royal 

Gala the most resistant one to Mucor infection (Spotts et al., 1999). 

Fruits with unseen initial infection may be introduced into the storeroom 

where fungal development proceeds even at 0°C (Lopatecki and Peters, 

1972). According to the site of fungal penetration, lesions may appear on 

the fruit surface (Combrink and Fourie, 1984), at the stem end 

(Lopatecki and Peters, 1972), or in the core region (Spotts et al., 1988). 

The rot is soft and watery, and under humid conditions is 

characterized by the appearance of minute black heads, which are the 

sporangia of the fungus. It differs from Rhizopus watery rot in its lack of 

a sour odor (Combrink and Fourie, 1984). 

J. Rhizopus spp. 

Rhizopus stolonifer (Ehrenb. ex Fr.) Lind and, to a lesser extent, 

Rhizopus oryzae Went & Prinsen Geerligs, are the cause of Rhizopus rot 

of pome fruits. However, while Rhizopus rot is a serious postharvest 

disease of stone fruits, it is not considered a major disease of pome fruits. 

Infection is initiated by the asexual spores (sporangiospores), which 

are common components of the air spora (Barkai-Golan et al., 1977b). 

The rot is soft and watery and, at its progressive stage, releases juices 

having a sour odor. The sexual state (zygospores) has rarely been 

reported. The fungus is heterothallic and requires the presence of two 

physiologically different and compatible mycelia for sexual reproduction 

(Alexopoulos, 1961). The development of both species of Rhizopus is 

inhibited at temperatures below 5°C and the rot is, therefore, prevented 

at the storage temperatures recommended for apples. 

K. Alternaria alternata (Fr.) Keissler 

This fungus is the main cause of Alternaria rot in pome fruits. Infection 

is initiated by conidia that are produced in abundance on leaf debris and 

other plant material in the orchard. The conidia, which are very important 

components of the air spora (Barkai-Golan et al., 1977b), are disseminated 

by wind and rain, and are generally present on the fruit surface at harvest 

time. The fungus is a weak pathogen that frequently colonizes damaged or 

senescent fruit. Several different symptoms can be produced following 




Alternaria infection, including small, black corky lesions, shallow 

dark-colored rot, or black decay of the stem (Snowdon, 1990). In addition, 

the fungus can penetrate the fruit via open calyces causing core rot of 

apples (Ceponis et al., 1969; Combrink et al., 1985). It has also been 

reported as a secondary infection of bitter pit, one of the most important 

physiological disorders of apples (Ben-Arie, 1975). 

Some metabolites of A, alternata strains may be toxic to animals and 

humans. Stinson et al. (1981) reported that the main toxins produced by 

the fungus during its growth in apples were alternariol and alternariol 

monomethyl ether, with maximum concentrations of 5.8 and 0.23 mg per 

100 g tissue, respectively. Higher concentrations of both toxins were 

produced by another strain of A. alternata^ which was held at 25°C for 

several weeks (Ozcelik et al., 1990). 

L. Stemphylium botryosum Wallr. [perfect state: Pleospora 

herbarum (Pers.) Rabenh.] and Cladosporium herbarum (Pars.) 

Link 

These two species are weak pathogens that tend to attack weakened or 

senescent tissue. C. herbarum is mainly of interest because of its 

association with scald and other types of physiological disorders in 

various apple cultivars (Edney, 1983), while S. botryosum may attack 

lesions resulting from sunscald (Snowdon, 1990). Cladosporium rot has 

been found to accompany side rot caused by Phialophora malorum on 

pears (Sugar and Powers, 1986). For both pathogens the susceptibility of 

the fruit to decay increases during extended storage periods. 

M. Trichothecium roseum Link. 

Trichothecium is a weak pathogen that invades pome fruit via wounds 

or lesions incited by other pathogens. It frequently follows the scab 

fungus (Venturia inaequalis) or the black rot fungus (Botryosphaeria 

obtusa). It is also one of the fungi involved in the development of core rot 

in apple fruits (Raina et al., 1971). The fungus is common under 

non-refrigerated conditions since its growth is greatly inhibited below 

10°C. The decay is characterized by the production of pink conidia and by 

the bitter taste of the infected fruit. 

Cultivar Resistance 

Since decay pathogens are dependent on the presence of a wound to 

initiate infection, resistance of the epidermis (the skin) to breakage may 

be an important factor in the resistance of apple cultivars to decay. 




Stud)dng the force required to break the epidermis of several cultivars, 

as a criterion for resistance to wound pathogens, Spotts et al. (1999) 

found that the epidermis of Golden Delicious and Jonagold was more 

easily broken than that of other cultivars, while the epidermal tissues of 

Fuji and Granny Smith were the most resistant to puncture. 

The resistance of apples of various cultivars to core rot infection, 

initiated by the penetration of B. cinerea, P, expansum, Mucor piriformis 

and other pathogens, via the sinus between the calyx and the core cavity 

(Combrink et al., 1985; Spotts et al., 1988), was evaluated according to 

the presence of open sinuses (Spotts et al., 1999). The highest percentage 

of fruits with open sinuses (with a mean of 38%) was recorded for Fuji 

fruits; Granny Smith and Braeburn had the fewest fruits with open 

sinuses, averaging 1.0 and 0%, respectively. Various biochemical and 

physiological factors may influence the resistance of the cortex tissue of 

apple fruits to decay pathogens. These include the presence of 

glycoprotein inhibitors of pectolytic enzymes of the pathogen (Brown, 

A.E., 1984) and the accumulation of the phytoalexinic compound, benzoic 

acid (Seng et al., 1985). 


The choice of treatment for controlling postharvest diseases of pome 

fruits depends, to a large extent, on the nature of the pathogens involved, 

the source of infection and the time of infection (Edney, 1983; Eckert and 

Ogawa, 1988). The lenticel rots are caused largely by pathogens present 

in the orchard and this enables us to treat these infections by preharvest 

orchard sprays. In areas suffering from heavy losses as a result of 

lenticel rots, orchards are sprayed with fungicides in the late summer to 

suppress the production of inoculum of Gloeosporium spp. and Nectria 

galligena and to protect the fruit lenticels from quiescent infections 

(Corke and Sneh, 1979; Edney, 1983). Corke and Sneh (1979) found that 

the efficiency of preharvest systemic fungicides in reducing decay by 

Gloeosporium perennans was related to their ability to suppress fungal 

sporulation. After harvest, the fruits are treated with a systemic 

fungicide to suppress the development of quiescent infections of 

Gloeosporium in the lenticels. The control of the lenticel rotting is 

considered to be one of the greatest contributions made by systemic 

benzimidazole compounds following their introduction as postharvest 

fungicides in the late 1960s. It is their ability to reach the lenticels, 

which are located within the fruit peel, that enables these fungicides to 

control rotting (Leroux et al., 1975). However, the penetration ability of 




these fungicides contributed to their persistence in the fruit throughout 

the storage period exerting continuous pressure for the selection of 

resistant strains of Penicillium expansum and Botrytis cinerea, which 

would not be controlled by postharvest treatments (Eckert and Ogawa, 

1988). 

Various fungicides with differing modes of action are applied after 

harvest, alone or in combination, to control postharvest infections 

initiated by wound pathogens, such as P. expansum, B. cinerea, 

Alternaria alternata, Cladosporium herbarum and Aspergillus spp. (see 

the chapter on Chemical Means). Protection against Phytophthora spp., 

which can invade apple fruits before harvest, should be provided by 

orchard sprays. A postharvest treatment has to contend with infections 

aged several days, and should also be able to prevent the contact spread 

of the pathogen during storage (Edney, 1978; Edney and Chambers, 

1981). Mucor piriformis, which can also spread during storage, cannot be 

controlled by most of the fungicides; the common method for controlling 

this pathogen is the addition of chlorine or sodium or^/io-phenylphenate 

to the washing water in the packinghouse, to reduce the level of 

pathogenic fungal spores brought into the water with the fruit (Spotts 

and Peters, 1982b). 

Calcium treatments applied primarily to control physiological 

disorders in apples have also resulted in reduced fungal infection. 

Preharvest calcium sprays were found to reduce the incidence of storage 

lenticel rot caused by Gloeosporium spp. (Sharpies and Johnson, 1977) 

and bitter rot caused by Colletotrichum gloeosporioides and C. acutatum 

(Biggs, 1999). A reduction in the rate of P. expansum rot was found after 

postharvest calcium application (Conway and Sams, 1983; Conway et al., 

1987; 1994a). Although the reduction in disease development following 

calcium treatments has generally been attributed to the suppression of 

physiological diseases and the improved keeping quality of the fruit, 

direct effects of calcium on spore germination and colony growth have 

been recorded for several pathogens, such as P. expansum, B, cinerea 

(Conway et al., 1994a) and Colletotrichum spp. (Biggs, 1999). (See the 

chapter on Means for Maintaining Host Resistance - Calcium 

Application). 

Several studies have reported on the ability of antagonistic 

microorganisms to control postharvest diseases of apples. Control of both 

B. cinerea and P. expansum, the two major pathogens of apple fruit, was 

achieved with several epiphytic bacteria isolated from apple leaves 

(Sobiczewski et al., 1996). The yeast, Candida oleophila was effective in 




controlling B. cinerea, when applied to fresh wounds on apples (Mercier 

and Wilson, 1995), while Candida sake was effective against P. expansum 

when applied preharvest (Teixido et al., 1998). Preharvest application of 

Bacillus subtilis, the yeast Rhodotorula and the yeast-like fungus 

Aureobasidium suppressed postharvest decay of apples by P. expansum, B, 

cinerea and Pezicula malicorticis (Leibinger et al., 1997). Postharvest 

control of P. expansum rot in pear fruits, was achieved with antagonistic 

yeasts, used either as field sprays or as postharvest applications 

(Chand-Goyal and Spotts, 1996a). (See the chapter on Biological Control). 

Refrigeration is the most commonly used method for suppressing 

decay after harvest. Apples are generally held at temperatures above 

0°C, the optimal storage temperature varying with the sensitivity of the 

cultivar to chilling injury. Pear cultivars are able to benefit from storage 

at -1°C. Chilling the fruit in the packinghouse by refrigerated forced air 

or by hydrocooling slows ripening and reduces pathogen growth. The 

addition of chlorine to the cold water disinfects the fruit surface and 

prevents the buildup of pathogen propagules in the water (Eckert and 

Ogawa, 1988). Rapid removal of the field heat by hydrocooling to about 

4.5°C markedly reduces Monilinia growth and arrests Rhizopus and 

Colletotrichum development, although the fungi remain alive and resume 

growth when the fruit is returned to higher temperatures. 

Further extension of the storage life of apples can be achieved by 

controlled atmosphere storage, frequently accompanied by the removal of 

ethylene. This involves a careful regulation of the temperature, the 

humidity and the levels of oxygen and carbon dioxide in the atmosphere 

(see the chapter on Means for Maintaining Host Resistance - Modified 

and Controlled Atmospheres). 

STONE FRUITS 

Stone fruits are highly perishable and decay is the major problem in 

handling them, both for the fresh market and for processing (Eckert and 

Ogawa, 1988). The susceptibility of stone fruits diminishes in the order: 

cherries, nectarines, peaches, plums and apricots. The main postharvest 

pathogens of stone fruits are: Monilinia spp., Botrytis cinerea, Rhizopus 

stolonifer, Penicillium expansum, Mucor piriformis, Colletotrichum 

gloeosporioides, Alternaria alternata and Cladosporium herbarum. The 

relative importance of these pathogenic species differs in different fruits: 




Penicillium, Alternaria and Cladosporium are more common in plums 

and cherries, while Rhizopus causes heavy losses mainly in peaches, 

nectarines and apricots. 

A. Monilinia spp. 

Three species of Monilinia are involved in brown rot of stone fruits: 

M fructicola (Wint.) Honey, M fructigena (Aderh. & Ruhl.) Honey and 

M laxa (Aderh. & Ruhl.) Honey. 

Brown rot is of major importance in stone fruits in many countries 

and, if the weather is wet and cool in the spring, it can attain epidemic 

levels in peaches, nectarines and apricots (Holz et al., 1998). There are, 

however, differences in the geographical distributions of the three 

Monilinia species. While M laxa is a serious problem in apricots in the 

United States, it may attack all stone fruits to some extent in Chile and 

South Africa (Eckert and Ogawa, 1988). Monilinia fructicola is a most 

important pathogen in the Americas, South Africa, Japan, Australia and 

New Zealand, while M fructigena is a common pathogen in some stone 

fruits in Europe and Asia, but not in the United States (Snowdon, 1990). 

The fungi survive the winter as mycelium in rotted or mummified 

fruits in the orchard. In wet weather, abundant conidia are produced and 

are spread in the orchard by rain splash and wind (Tate and Corbin, 

1978) or by insects. In addition to the asexual conidia, M fructicola 

frequently produces stromata (sclerotial mat or resistant fungal tissue), 

that give rise to the sexual state - apothecia with ascospores (Biggs and 

Northover, 1985; Willetts and Hadara, 1984). In other species of 

Monilinia apothecial production is rare (Willetts and Hadara, 1984). 

Examining the conditions under which M /rwc^jco/a-infected fruit 

develop stromata and produce apothecia, Holz et al. (1998) found that 

apothecia were formed only from infected fruits that had been subjected 

to a drying and cold temperature incubation, during which a 

stromatization process takes place. In the orchard, apothecia are formed 

during the bloom, from mummies that have survived the winter. They 

commonly appear on orchard floors with natural vegetation or cover 

crops, which may create a habitat that reduces desiccation. Apothecial 

production has not been observed in infected fruits which are attached to 

trees, but only in those lying on moist soils (Hong et al., 1996; Willetts 

and Hadara, 1984). Liberated ascospores may serve as a major source of 

primary inoculum for initiating infection of blossoms (Hong et al., 1996). 

The optimal temperature for daily discharge of ascospores from the 

apothecia is IS^'C, although high discharge can occur at any temperature 




within the range of 10-25°C. Ascospore germination and germ-tube 

elongation rise with the increasing temperatures, from 7 to 15°C (Hong 

and Michaihdes, 1998). Reduction in ascospore discharge at higher 

temperatures (up to 25°C) is due to the faster disintegration of apothecia. 

However, high temperatures can also expedite drying of the apothecia in 

orchards when air humidity is low. Such information may help in the 

development of warning systems and in scheduling fungicide application 

against brown rot infection. 

Following invasion of apricot flower parts, the fungus may progress 

into young fruits, where infection remains quiescent until the fruit ripens 

(Wade and Cruickshank, 1992). Quiescent visible infections of 

M. fructicola have been described on plums (Northover and Cerkauskas, 

1994), whereas symptomless latent infections were reported on cherries 

(Adaskaveg et al., 2000). Additional infection may occur while the fruits 

are maturing via stomata, via hair sockets in peaches (Hall, 1971) or 

directly through the skin. However, the most common mode of invasion is 

through preharvest wounds caused by insects, hail or other adverse 

weather, or via injuries sustained during harvest and postharvest 

handling. 

Decay is characteristically firm (Byrde et al., 1973). In advanced 

stages, spore masses, which are often arranged in concentric rings, cover 

the surface of the affected area. Within a few days at room temperature, 

the entire fruit may be decayed. Brown rot can spread rapidly by contact 

infection, forming 'nesting' during storage and marketing. 

B. Botrytis cinerea Pers. [perfect state: Botryotinia fuckeliana (de 


This fungus is one of the major pathogens of stone fruits in all 

producing areas. It survives as sclerotia in the soil or on dead plant 

material. During cool weather the fungus sporulates and inoculum for 

infection may be readily available. Infection may be initiated 

throughout the growing season under natural orchard conditions. 

Fourie and Holz (1994) indicated that B, cinerea does not penetrate 

young nectarine and plum fruits via floral parts to establish latent 

infections as in pears and apples (DeKock and Holz, 1991; Tronsmo and 

Raa, 1977), strawberries and raspberries (Powelson, 1960; Dashwood 

and Fox, 1988), grapes (McClellan and Hewitt, 1973) and cucumbers 

(Elad, 1988); in nectarines and plums the pathogen is likely to be 

introduced by field infection of developing fruit. Conidia germinate in 

water on the surfaces of both green and mature fruits, and the germ 




tubes may give rise to appressoria when they have reached lengths of 

10-15 |am. Thin infection hyphae (or infection pegs) formed from the 

inner appressorium wall enter the substomatal cavity of the fruit via 

stomata. In the green fruit no further fungal growth occurs, probably 

because of resistance reactions characteristic of young tissue, such as 

the presence of preformed phenolic compounds or callose at the site of 

infection (Fourie and Holz, 1995). When nectarines are invaded by 

B, cinerea at an advanced stage of fruit maturity (near the picking-ripe 

stage), the majority of infection hyphae do penetrate the cuticle. In 

plum fruits at this stage of maturity, only a small number of successful 

penetrations have been recorded. In both nectarines and plums, 

however, susceptibility to infection increases with fruit maturity. 

Enhanced infection in mature nectarine fruits can also result from the 

appearance of cuticular micro-cracks on the fruit surface. These can 

provide alternative penetration routes for the pathogen (Fourie and 

Holz, 1995). The fruit can also be invaded via injuries sustained during 

harvesting and handling. 

The gray mold rot is soft and, under humid conditions, produces 

abundant surface mycelium. Under dryer conditions sporulation is 

prolific and a mass of gray conidia are formed, which constitute a ready 

source for initiation of new infections. 

C. Rhizopus spp. 

Rhizopus stolonifer (Ehrenb. ex. Fr.) Lind and Rhizopus oryzae Went & 

Prinsen Geerligs cause Rhizopus rot, one of the most serious postharvest 

diseases of stone fruits (Hall and Scott, 1989b). It inflicts heavy losses, 

especially in peaches, nectarines and cherries. Rhizopus spp. exist on dead 

material and their asexual spores (sporangiospores) are disseminated in 

the air where they form important components of the air spora and are 

responsible for disease initiation. The fungi generally infect the fruit via 

injuries sustained during harvesting and handling. Infected areas are 

water-soaked and covered with a profuse white mycelium which gives rise 

to globular sporangia with new sporangiospores. The fruit becomes very 

soft and, at the progressive stage of the disease, releases juices having a 

sour odor. During storage a rotted fruit infects sound fruit by contact (Hall 

and Scott, 1989b), causing extensive 'nesting'. 

The fungi tolerate high temperatures, having optima of 25°C for 

R. stolonifer and 35°C for R, oryzae, and infection is favored by a warm, 

moist environment (Pierson, 1966). 




D. Penicillium expansum Link. 

This fungus is a common postharvest pathogen of stone fruits in all 

producing areas, but can attack only injured or over-mature fruit. Fruits 

that have been heat treated to reduce incipient infections of brown rot 

are also sensitive to Penicillium infection (Smith, W.L. and Anderson, 

1975). During storage, conidiophores bearing blue-green conidia are 

produced in coremia, sometimes arranged in concentric circles around 

the point of infection. A white edge remains around the rotten area as it 

develops (Hall and Scott, 1989b). The infected tissue is moist and 

characterized by a musty odor. Since the growth of Penicillium is greatly 

reduced at low temperatures, rapid pre-cooling and cold storage 

markedly suppress its development. 

E. Colletotrichum gloeosporioides (Penz.) Sacc, [perfect state: 

Glomerella cingulata (Stonem.) Spauld & v. Schrenk] 

This fungus is the causal agent of anthracnose. Some strains can 

exhibit the sexual state (perithecia with ascospores), but infection is 

generally initiated by the asexual spores (conidia produced in acervuli). 

The fungus penetrates the fruit mainly via wounds caused by twig 

abrasion while on the tree (Rittenburg and Hendrix, 1983), and disease 

symptoms may appear in the orchard. If infection occurs shortly before 

harvest and the disease cannot yet be detected in the packinghouse, it 

will develop later in storage. During harvesting and handling operations, 

infections may take place through wounds and can be spread from 

diseased fruit to sound fruit by contact. 

Lesions are firm and sometimes tend to coalesce and cover the fruit 

(Ramsey et al., 1951). However, almost no progress takes place below 

5°C. Under humid conditions, a mycelium-bearing salmon-pink mass of 

spores, ready to initiate new infections, is developed. 

F. Mucor piriformis Fischer 

This fungus is generally considered a disease of minor importance in 

stone fruits. However, following its isolation from several stone fruits, 

mainly peaches and nectarines, it has been regarded as a threat since, in 

contrast to Rhizopus, this pathogen can develop in cold storage even at 

0°C (Smith, W.L. et al., 1979), and is not controlled by available fungicide 

treatments (Eckert and Ogawa, 1988). 

Infection takes place when the asexual spores (sporangiospores within 

sporangia) penetrate wounded fruits and cause a soft, watery rot. During 




storage the fungus produces a coarse, erect, white mycehum that gives 

rise to globular black sporangia with new sporangiospores. 

G. Alternaria alternata (Fr.) Keissler 

Alternaria rot is common mainly in plums and cherries. The fungus 

survives on dead material in the orchard and its spores are disseminated 

in the air. Infection is usually associated with injury that occurs during 

harvesting and handling. In cherries, fungal penetration can take place 

through cracking or splitting of the skin while the fruit is still on the tree 

(Snowdon, 1990). In apricots, the fungus can penetrate the sound fruit 

via stomata (Larsen et al., 1980). 

Lesions are firm, slightly sunken, and bear a dense mat of olive-green 

or dark conidia. 

H. Cladosporium herbarum (Pers.) Link 

Cladosporium survives on dead plant material in the soil and produces 

an abundance of conidia, which are disseminated in the air, and 

constitute the most common component of the air spora (Gregory, 1973; 

Barkai-Golan et al., 1977b). Cladosporium herbarum is a weak pathogen 

infecting fruit that has been damaged by rain or rough handling. Plums 

that have been shaken from the tree and collected from the ground 

exhibited a particularly high incidence of infection (Michaelides et al., 

1987). 

Lesions are limited in area and covered with a velvety mat of dark 

green spores. Thanks to its low minimal growth temperature of -4°C 

(Sommer, 1985), Cladosporium can grow and infect the fruit even at cold 

storage temperatures. 


Postharvest disease control of stone fruit pathogens consists of an 

integrated combination of pre- and postharvest fungicide applications, 

sanitation practices, cold storage, modified atmosphere storage and 

biological control techniques. Chilling the fruit, which is the most 

commonly used method, both slows ripening and inhibits the growth of 

most decay pathogens (Eckert and Ogawa, 1988). 

To control infection by wound pathogens, such as B, cinerea, 

P. expansum, A alternata, R, stolonifer and M piriformis, control 

measures should include the prevention of mechanical damage during 

harvesting and handling, and application of sanitation practices in the 

orchard and in the packinghouse, to minimize the level of infective 




spores. To control brown rot {Monilinia spp.) sanitation should include 

the removal of blighted flowering shoots and mummified fruits from the 

orchard (Holz et al., 1998) along with careful harvesting and handling to 

avoid injuries that would enable easy penetration of the pathogen into 

the fruit. Fungicidal sprays are generally necessary to protect the flowers 

from infection and to control brown rot in the orchard (Zehr, 1982). 

Postharvest fungicide treatments with dips, sprays or wax formulations 

containing a systemic fungicide have provided good control of 

M fructicola. However, with the appearance of resistant strains of the 

pathogen, the use of unrelated chemicals has been recommended 

(Michailides et al., 1987). Combinations of different fungicides have been 

effective in controlling both brown rot and Rhizopus rot (Wade and 

Gipps, 1973). The effectiveness of fungicides against brown rot may be 

improved by combining them with a hot water treatment. Such a 

combination has enabled the concentration of the fungicide to be reduced 

without impairing the effectiveness of the treatment (Jones and Burton, 

1973). 

Hot water alone (52°C, 2.5 min) has been found to control incipient 

infections of M fructicola and R. stolonifer in nectarines and peaches, but 

the control was accompanied by physiological injuries to the fruit. 

Combining hot water treatment with a fungicide enabled decay control to 

be achieved at a lower temperature, and no injury was observed during 

storage (Smith, W.L. and Anderson, 1975). However, Phillips and Austin 

(1982) found that some changes in peaches had already developed after 

treatment with hot water above 45°C. When stone fruits are immersed in 

a hot water fungicide bath, the time of immersion and the temperature 

should be carefully controlled to avoid fruit injury. Heat injury of the 

fruit may also lead to increased sensitivity to P. expansum infection 

(Smith, W.L. and Anderson, 1975). 

Several calcium salts, particularly calcium propionate and calcium 

silicate, were found to reduce both the incidence and the severity of 

brown rot in wound-inoculated peaches (Biggs et al., 1997). The salts 

directly reduced M fructicola growth and inhibited its pectolytic activity. 

Minimal growth and maximal inhibition of enzymatic activity occurred 

after calcium propionate application. 

Several studies have been dedicated to the control of postharvest 

diseases of stone fruits by biological means. Whereas the first biocontrol 

studies focused on the use of antagonistic bacteria, later studies used 

mainly yeast species and epiphytic fungi. The control of brown rot and 

Rhizopus rot in peaches was achieved with the bacteria. Bacillus subtilis 




and Enterobacter cloacae (Pusey and Wilson, 1984; Wilson et al., 1987b). 

Bacillus sub tilts has also controlled both brown rot and Alternaria rot in 

sweet cherries (Utkhede and Sholberg, 1986). Two species of 

Pseudomonas were used to control brown rot in peach and nectarine 

fruits (Smilanick et al., 1993), while reduction of brown rot in sweet 

cherries was achieved with two epiphytic fungi, Aureobasidium and 

Epicoccum (Wittig et al., 1997). The combination of preharvest fungicide 

treatment with postharvest yeast application gave a better control of 

brown rot in sweet cherries than the fungicide alone, although the yeast 

alone did not affect disease development. Combining the double 

treatment with modified atmosphere packaging of sweet cherries 

resulted in a further increase in decay control and almost eliminated 

brown rot during storage (Spotts et al., 1998). The advantage of a 

combined biological and chemical control over the single treatments has 

been reported also for P. expansum in harvested nectarines (Lurie et al., 

1995). (See the chapter on Biological Control). 

III. SOFT FRUITS AND BERRIES 

The title applied to this group of fruits by Snowdon (1990) indicates 

the complex of fruits involved. They include strawberries and 

raspberries, which are not true berries, and blueberries, cranberries and 

gooseberries, which are berries that grow on bushes. Snowdon (1990) 

added to these grapes and kiwifruits, which are berries that grow on 

vines. The fruits in this group are not related botanically but are all 

native to the temperate zones. They are all very delicate and should be 

treated with great care. Their storage life is greatly shortened by both 

physiological and pathological deterioration. However, decay 

development is the primary cause of loss in soft fruits, and any treatment 

capable of controlling or delaying decay and thus prolonging the storage 

life by a few days, is appreciated. 

The major postharvest pathogen of fruits in this group is Botrytis 

cinerea, the causal agent of gray mold. Species of Mucor and Rhizopus, 

the causal organisms of 'leak" disease, are major pathogens of 

strawberries and raspberries. Other fungi responsible for postharvest 

rots include Colletotrichum spp., which cause anthracnose in 

strawberries, raspberries and blueberries, and Phytophthora spp., which 

initiate leather rot in strawberries (Snowdon, 1990). Alternaria 

alternata, which is of minor importance in strawberries and raspberries. 




may be one of the main pathogens of blueberries and gooseberries 

(Dennis et al., 1976). Similarly, Phomopsis spp., which are generally of 

minor importance in strawberries, may be of greater importance in 

harvested blueberries (MilhoUand and Daykin, 1983). Other fungi 

responsible for postharvest rotting include species of Cladosporium, 

Penicillium and Pestalotia (Snowdon, 1990). The incidence of the less 

common or minor pathogens may vary from year to year, from season to 

season and from place to place, suggesting that climate factors may affect 

the amount of inoculum available or the infection resistance of the fruit 

(Dennis, 1983a). 

STRAWBERRIES AND RASPBERRIES 

A. Botrytis cinerea Pers. [perfect state: Botryotinia fuckeliana (de 


The gray mold fungus is a most important pathogen of soft fruits, 

worldwide. It survives on organic debris in the field, frequently as 

sclerotia, and under favorable conditions it sporulates, releasing quantities 

of spores that serve as a potential source of inoculum for infection of 

flowers (Braun and Sutton, 1987). Temperature and duration of humidity 

have the greatest effects on inoculum production: the optimal temperature 

for sporulation is between 17 and 18''C, but under sustained humidity the 

temperature range for profuse sporulation extends to 15-22°C 

(Sosa-Alvarez et al., 1995). Under alternating wet and dry periods, the 

total duration of high humidity and the length of wet periods determine 

the amount of inoculum produced. No sporulation occurs at 30°C. 

During the flowering and fruiting season fungal spores are common in 

the atmosphere (Jarvis, 1962), and at harvest time the fruit is already 

contaminated with Botrytis spores. Conidia are deposited on flowers by 

air or water (Braun and Sutton, 1987) and primary infection of 

strawberries takes place through senescent floral parts where the conidia 

remain quiescent in the base of the receptacle. Disease is manifested as 

stem-end rot only when the fruit ripens (Powelson, 1960), so that the 

fruits may be harvested in apparently sound condition only to decay 

during transit and marketing (Aharoni and Barkai-Golan, 1987). 

In raspberry fruits the fungus invades stigmas and styles, resulting in 

infection of individual fruitlets (Williamson and McNicol, 1986). 

Mummified fruits and receptacles entirely covered with fungal spores are 

often found on raspberry plants. After harvest, infections of strawberries 




and raspberries can arise, either from germination of spores on the 

surface of the fruit or from renewed growth of the quiescent infection 

that had been initiated in the field. Davis and Dennis (1979) found that 

the majority of strawberry infections occurring during storage appear on 

the surface of the fruit and only a small proportion are initiated from 

quiescent infections at the stem-end. Sporulation on diseased flowers and 

fruits becomes an important source of secondary inocula in annual 

production systems where flowers are continuously produced over several 

months (Legard et al., 2000). 

The decay is brown and firm, and its surface becomes covered with an 

abundance of conidiophores bearing gray-brown conidia. Under humid 

conditions, a white to gray mycelium with only a few spores 

characteristically grows over the lesions (Dennis, 1983a). Sclerotia are 

rarely found, and their development is dependent on Botrytis strains or 

isolates and environmental conditions. Having a minimal growth 

temperature of about -2°C, the gray mold is capable of development even 

at the low temperatures used for storage (Dennis and Cohen, 1976). The 

decay can spread readily by contact between rotten and healthy fruits, 

causing 'nesting' during storage (Sommer et al., 1973). Rapid reduction of 

the temperature from the field level to 1°C, and low-temperature storage 

can only delay fungal development. 

B. Rhizopus spp. and Mucor spp. 

These fungi are responsible for the "leak" disease, a destructive soft 

watery disease of soft fruits. The Mucor species involved are M piriformis 

Fischer and, occasionally, M hiemalis Wehmer. The Rhizopus species are 

R. stolonifer (Ehrenb. ex Fr.) Lind, a heterothallic species (requiring the 

presence of two physiologically compatible mycelia for sexual reproduction), 

and in some areas R, sexualis (Smith) Callen, a homothallic species 

(having a self-fertile mycelium). 

Mucor and Rhizopus are soil inhabitants but, unlike Botrytis, they do 

not sporulate on plant debris in the field. Sporulation usually occurs only 

on infected ripe fruits. The soil-borne inocula of the two fungi cause 

limited early infection in the plantation. After being established in ripe 

fruits the inoculum level increases and the latter part of the season is 

characterized by enhanced contamination and higher rates of infection by 

these fungi (Dennis, 1978). Infection is initated by the asexual spores 

(sporangiospores). Germination of sexual spores (zygospores) of i?. stolonifer 

has been observed only rarely (Alexopoulos, 1961), while those of 




R, sexualis have never been observed to germinate (Snowdon, 1990). 

Direct infection can occur if ripe fruits touch the ground or become 

contaminated by splashing rain (Harris and Dennis, 1980). Infection 

progresses rapidly, especially in injured fruits. 

Infections of Mucor and Rhizopus result in a water-soaked 

appearance. In Mucor, infections in the invaded area rapidly become 

covered with visible black sporangia containing the asexual spores 

(sporangiospores). These spores are the inocula for further infection, both 

in the field and after harvest. Under warm and dry conditions, Rhizopus 

spp. can initiate a similar infection. At their progressed stage of 

development, both Mucor and Rhizopus are capable of breaking down the 

fruit tissue and so causing juice leaks. During storage and marketing 

further rotting is caused by contact infection between infected and 

healthy fruits and through contamination with spore-laden juices. 

At 5°C or below, Rhizopus rots can be controlled. Harris and Dennis 

(1980) showed that asexual spores of J?, sexualis were inactivated at 0°C 

and did not recover their viability when the fungus was moved to a 

higher temperature. Mucor piriformis can, however, infect the fruit and 

develop even at 0°C. 

BLUEBERRIES AND GOOSEBERRIES 

The most important causes of postharvest decay of blueberries are 

Botrytis cinerea Pers., Alternaria alternata (Fr.) Keissler and, in some 

instances, Alternaria tenuissima (Kunze:Fr.) Wiltshire, Colletotrichum 

spp. and Phomopsis vaccinii Shear. In many cases the stem scar is the 

predominant site of infection for various pathogens of blueberries 

(Ceponis and Cappellini, 1983; MilhoUand and Daykin, 1983). Mucor 

piriformis Fischer, the cause for "leak" disease in strawberries and 

raspberries, has been reported by Dennis and Mountford (1975) as an 

important pathogen of blueberries as well. Species of Monilinia, which 

cause the brown rot of stone fruits, are important field pathogens of 

blueberries (Barta, 1987). Monilinia invades the berries through the 

flowers, giving rise to diseased fruits that either fall to the ground and 

turn into ^mummies' or are harvested along with healthy berries. This is 

why Monilinia species are also included among the postharvest 

pathogens of blueberries. 

The major postharvest pathogens of gooseberries are JB. cinerea, 

M. piriformis and A, alternata. Botrytis invades the fruit in the field via 




the senescent calyx. Infection of the fruit by Mucor frequently occurs 

following contamination by soil splash after heavy rain (Dennis, 1983a). 

Infection by Alternaria (and sometimes also by Stemphylium) via the 

calyx end similarly occurs in the field prior to harvest, and may result in 

premature fall of infected berries from the bushes. The fungus initially 

colonizes the seeds. However, when the fruits are stored at ambient 

temperatures, the fungus can progress and begin to infect the pericarp. 


Chemical control of postharvest diseases of soft fruits has mainly 

focused on the suppression of B. cinerea, the most important pathogen of 

these fruits. Since infection originates in the field, fungicidal sprays 

during the flowering period are the first step in controlling the disease. 

Preharvest fungicidal application of benzimidazole successfully 

controlled decay in the early 1970s (Jordan, 1973). Following the 

emergence of 5. cinerea strains resistant to these chemicals, they have 

been replaced by fungicides of the dicarboximide group, and preharvest 

spraying of strawberries with iprodione during the flowering period 

provided good control of the gray mold disease during storage (Aharoni 

and Barkai-Golan, 1987). However, these fungicides, similarly to the 

benzimidazoles, are not effective against Phycomycetes fungi, such as 

Rhizopus spp. and Mucor spp. (Eckert and Ogawa, 1988), and B, cinerea 

strains resistant to the dicarboximides have also developed (Hunter et 

al., 1987). Postharvest fungicidal applications are not practical for ripe 

berries because of their sensitivity to wetting. 

A rapid reduction of the temperature from the field level to below 5°C 

is important for retarding decay development in strawberries during 

shipment (Harris and Harvey, 1973). This is currently achieved by 

forced-air cooling, although hydrocooling, which removes field heat from 

the berries more rapidly than air cooling, cleans them and does not cause 

moisture loss. The latter has not been recommended because wetting 

may lead to excessive decay (Mitchell, 1992). In a recent study of the 

decay hazards associated with hydrocooling, Ferreira et al. (1996) 

concluded that hydrocooling, with the addition of proper chlorination, has 

promise as a method for rapid cooling and cleaning of strawberries as 

well as for reducing the level of S. cinerea inoculum on the fruit surface. 

These investigators considered that if cooled, good-quality berries were 

subsequently stored at the low temperatures recommended (0 to 1°C), 

the risk associated with hydrocooling would be minimal. 




Elevating the CO2 content of the atmosphere to 20-30% is a known 

procedure for suppressing decay in strawberries during transit. A level 

of 20% CO2 both reduces decay in strawberries and retards their 

softening, without injuring the berries or impairing their flavor (Harris 

and Harvey, 1973). Reduced decay can also be achieved by wrapping 

strawberries in polyvinyl chloride (PVC) film which leads to the 

accumulation of a high level of CO2 within the package (Aharoni and 

Barkai-Golan, 1987). Low O2 concentrations can reduce both B, cinerea 

and R. stolonifer decay in stored strawberries, but excessively low O2 

results in the development of persistent off flavors in the fruit as a 

result of the accumulation of alcohols and aldehydes in the tissue 

(Sommer, 1982). Storing blueberries in 15-20% CO2 at 2°C for 7-14 days 

reduced the decay caused mainly by B. cinerea, Alternaria spp. and 

Colletotrichum spp. and prolonged the shelf life of the fruit (Ceponis 

and Cappellini, 1985). 

Several investigators have studied the reduction of postharvest 

diseases of strawberries by antagonistic organisms. Preharvest spraying 

of strawberry flowers with antagonistic non-pathogenic isolates of 

Trichoderma species reduced the incidence of gray mold in storage 

(Tronsmo and Dennis, 1977). Storage diseases of both B. cinerea and 

R, stolonifer were reduced after application of the yeast-like fungus, 

Aureobasidium pullulans to strawberries grown under plastic tunnels 

(Lima et al., 1997). The antagonistic effect was more pronounced when 

the fungus was applied at the flowering stage, and under these 

conditions the antagonistic fungus was more effective than the fungicidal 

treatment (vinclozolin). Moline et al. (1999) found that of the many 

bacteria isolated from the surface of non-treated strawberries, the most 

effective ones were species of Pseudomonas and Chryseobacterium, which 

were capable of reducing the incidence of gray mold under field 

conditions. 

GRAPES 

The major postharvest problems of grapes are desiccation, bruising 

and decay (Capellini et al., 1986), decay being directly related to 

bruising. The main decay pathogens of cold-stored grapes at -1°C are 

Botrytis cinerea, Cladosporium herbarum, Alternaria spp., Rhizopus 

stolonifer, Aspergillus niger and Penicillium spp. (Hewitt, 1974; Nelson, 

1985). 






The gray mold rot caused by B, cinerea is the most important disease 

of grapes, and can cause very heavy losses in all grape-producing 

countries, especially after wet seasons (Pearson and Goheen, 1988). 

The fungus persists in the soil and can be found in infected vines and 

on decasdng plant material. The pathogen exhibits both the sexual state 

(apothecia with ascospores) and the conidia, which are the asexual state 

(Snowdon, 1990). Both sporulation and infection take place under wet 

conditions. When late-harvested grapes have been exposed in the field to 

high humidity, dew and rainfall, a blossom infection may occur; the 

fungus develops a quiescent infection in the young developing fruits that 

cannot be eradicated by the SO2 treatment (Eckert and Ogawa, 1988). 

The fungus resumes activity when the fruit matures (Marais, 1985). 

Thus, grapes which were apparently sound when harvested and packed 

may develop rotting only at a later stage (Nelson, 1956). Secondary 

infection spreads by contact during storage, resulting in the formation of 

'nests' of decay among the grapes. 

Decay is manifested by a brown discoloration, which later becomes 

covered with an abundance of gray-brown spores (Marais, 1985). Under 

humid conditions, a sporeless mycelium is developed. 

B. Cladosporium herbarum (Pers.) Link 

This species is the cause of Cladosporium rot in stored grapes. It is an 

important cause of spoilage because of its ability to develop in grapes in 

cold storage. 

The pathogen has frequently been reported as the most important 

airborne fungus (Gregory, 1973). Primary infections occur before harvest. 

The fungus is capable of penetration through the intact fruit or at the 

blossom end of the berry and infection is favored by wet seasons (Hewitt, 

1974). The disease is characterized by circular black spots or lesions 

which, after removal to shelf-life conditions, become covered with 

olive-green conidia-bearing mycelium. The decay is shallow and does not 

extend to the seeds. The affected tissue is attached to the skin and can be 

removed with it (Harvey and Pentzer, 1960). 

C. Alternaria alternata (Fr.) Keissler 

This fungus develops on dying flowers that then become a source of 

airborne spores. Infection can occur on any part of the fruit but is often 

initiated at the stem end. Disease initiation and development are favored 




by wet weather (Hewitt, 1974). The lesion is dark brown and firm and 

the margins of the infected area are less distinct than those of 

Cladosporium rot (Harvey and Pentzer, 1960). 

D. Aspergillus niger v. Tieghem 

This pathogen can be a serious problem when grapes are marketed at 

ambient temperatures (Mandal and Dasgupta, 1983). The fungus 

survives on plant debris in the soil at high temperatures (25-30°C) and 

airborne spores infect berries via injuries caused by insect punctures, 

splits and stem-end fractures. Infection occurs only in mature berries; 

young berries are resistant to infection even when wounded (Abdelal et 

al., 1980). Infection can also take place through injuries incurred during 

handling. Since the fungus does not grow at temperatures below 5°C, 

cold storage is effective in suppressing disease development (Matthee et 

al., 1975). 

E. Penicillium spp. 

Several species of Penicillium, including P. citrinum Thom, P. cyclopium 

West, and P. expansum Link, can infect harvested grapes causing the 

blue mold rot (Barkai-Golan, 1974). Although infection can sometimes be 

initiated before harvest, the disease is usually associated with wounds 

and cracks in the skin at harvest (Harvey and Pentzer, 1960). Infection is 

induced in injured berries by conidia, which are dispersed by air 

currents, winds and insects. Decay continues to develop, although at a 

very slow rate, during the prolonged cold storage of grapes at 0°C 

(Harvey and Pentzer, 1960), and it may spread through the bunch. 

F. Rhizopus stolonifer (Ehrenb. Ex Fr.) Lind and R. oryzae Went & 

Prinsen Geerligs 

Rhizopus rot, incited by both of these species, can become a problem 

for grapes that are marketed at ambient temperatures (Mandal and 

Dasgupta, 1983). 

The fungi exist and sporulate in the soil and on plant debris. Asexual 

spores (sporangiospores) are spread by air currents and primary infection 

is via injuries (Barbetti, 1980). R, oryzae is also capable of penetrating the 

intact skin of mature berries in the presence of exuded grape juice. 

Infection continues to spread during storage, when rotted berries infect 

adjacent healthy berries by contact. The infected tissue rapidly becomes 

covered with sporangia (white when young, later turning black), which are 

borne on sporangiophores that arise in clusters on the white mycelium. 




Both species have high optimal growth temperatures (30-35°C for 

R. oryzae and 20-25°C for K stolonifer) and their development can thus 

be suppressed, similarly to that of A, niger, by refrigeration. Growth is 

resumed, however, on transfer of the fruits to shelf-life conditions. 


Minimizing the amount of debris in the vineyard by sanitation 

measures, and injury prevention by gentle handling of the berries are 

important for reducing postharvest diseases initiated by wound 

pathogens. Thinning of bunches may also result in reduced cracking and 

splitting (Barbetti, 1980). 

Cold storage after rapid removal of the field heat is the basic means for 

decay suppression in grapes, while keeping them in a cool room 

throughout marketing will suppress decay at the end of the marketing 

chain (Beattie and Dahlenburg, 1989). The classic fungicidal means for 

decay control is periodical fumigation of the fruit with sulfur dioxide gas 

(Eckert and Ogawa, 1988), but this treatment may result in damage to the 

berries and in deposition of sulfite residues on their surface (Marois et al., 

1986). Looking for alternatives to SO2, Forney et al. (1991) found that 

vapor-phase hydrogen peroxide suppressed germination of B. cinerea 

conidia on grapes and reduced the incidence of decay in non-inoculated 

fruits, without affecting their color or soluble solids contents. Fumigation 

of grapes with low concentrations of acetic acid was suggested by Sholberg 

et al. (1996) as a suitable alternative to SO2 for controlling decay in cold 

storage. At 0.27% (vol/vol), acetic acid-treated grapes did not show any 

phytoxicity and there were no differences between SO2 and acetic acid 

treatments regarding fruit color and composition. In addition to table 

grapes, wine grapes could also benefit from fumigation with acetic acid. 

Acetaldehyde vapors have also been considered as a possible treatment for 

controlling postharvest decay in grapes (Avissar and Pessis, 1991). 

However, acetic acid was found to be much more inhibitory to B. cinerea 

spores than acetaldehyde, and the latter has been reported to have some 

adverse effects on fruit quality, such as damage to the berries and an 

off-flavor in Thompson Seedless grapes (Pesis and Frenkel, 1989). 

KIWIFRUIT 

Several fungi are responsible for storage decay of kiwifruit. They 

include Botrytis cinerea, Penicillium spp., Phomopsis actinidiae, 




Alternaria alternata and species of Colletotrichum, Botryosphaeria and 

Phoma (Hawthorne et al., 1982; Opgenorth, 1983; Pennycook, 1985; 

Sommer et al., 1994). 



The gray mold rot caused by B, cinerea is the most serious disease of 

kiwifruit in storage (Sommer et al., 1994). Botrytis conidia are capable of 

surviving on the surface of kiwifruit, where they remain viable and 

infectious throughout the growing season (Walter et al., 1999). Under 

moist conditions invasion may take place via senescent floral parts at the 

blossom end, after which the fungus remains quiescent for some time in 

storage before resuming activity (Opgenorth, 1983; Sommer et al., 1983). 

Botrytis may also penetrate the fruit through the cut stem or through 

wounds in the skin (Sommer et al., 1983). Conidia on the leaves and 

those accumulated on the hairy surface of kiwifruit are the major sources 

of inoculum for infection of the picking wound at harvest or after it 

(Pennycook and Manning, 1992). 

Infection results in soft rot at the site of penetration, and it continues 

to develop, although very slowly, at 0°C. During prolonged storage the 

fungus can spread by contact from decayed to sound fruit, causing a 

typical 'nesting' of gray mold rot. In addition to the direct losses caused 

by B. cinerea, infected fruit produce ethylene in cold storage, which 

accelerates softening of healthy fruits (Brook, 1991). 

B. Other Pathogens 

Species of Phoma, Colletotrichum and Botryosphaeria are capable of 

colonizing dying flower parts and entering the young fruits on the tree. 

However, they remain quiescent at the site of penetration until the fruit 

ripens after harvest. Hence, the name 'ripe rot' frequently given to these 

rots (Snowdon, 1992). Phomopsis actinidiae (perfect state: Diaporthe 

actinidiae Sommer & Beraha) may initiate stem-end rots when the fruit 

ripens (Hawthorne et al., 1982). Although the fungal development is 

suppressed in cold storage, growth is resumed when the fruit is 

transferred to shelf-life conditions. Decay can also be caused by several 

species of Penicillium, which penetrate the fruit via injuries. Alternaria 

species may grow on the fruit surface but are not considered primary 

pathogens (Eckert and Ogawa, 1988). 





Cold storage suppresses decay development, although S. cinerea grows 

slowly even at 0°C. In order to prevent softening of kiwifruit during 

storage, the environment must be kept free of ethylene, which enhances 

the ripening process (see the chapter on Factors Affecting Disease 

Development - Effects of Ethylene). Care must be taken, therefore, to 

exclude damaged, diseased or over-mature fruits that produce ethylene, 

and to avoid storage together with ethylene-producing commodities, such 

as apples and pears (Snowdon, 1992). Storage life can be further 

extended by the use of controlled atmosphere (Arpaia et al., 1987) or 

modified atmosphere with ethylene removal (Ben-Arie and Sonego, 

1985). 

Reduction of postharvest decay can also be achieved by curing 

kiwifruit after harvest. Looking for the optimum conditions for curing, 

Bautista-Bafios et al. (1997) found that holding the fruit at 10-20°C in 

relative humidity higher than 92% for no more than 3 days, resulted in 

the lowest subsequent disease incidence, without lowering fruit quality 

during cold storage. 

Postharvest decay, which often results from latent infection initiated 

in senescent floral parts or stem-end scars (receptacles) in the vineyard, 

can be reduced by preharvest fungicidal sprays (such as iprodine or 

vinclozolin) beginning at the end of the blossom period (Beever et al., 

1984). Michailides and Morgan (1996) found a direct relationship 

between the incidence of B, cinerea in the fruit sepals and receptacles 

and the incidence of gray mold after several months of storage; they 

therefore suggested that the predicted incidence of decay in storage 

based on the rate of latent infections in the vineyard could be used to 

determine when preharvest fungicidal sprays are needed and justified. 

IV. SOLANACEOUS FRUIT VEGETABLES 

Truit vegetables' of the family Solanaceae include tomatoes, peppers 

and eggplants. The main pathogens of these fruits are Botrytis cinerea, 

Alternaria alternata, Erwinia carotovora, Rhizopus stolonifer, Mucor 

spp., Geotrichum candidum, Fusarium spp., Phytophthora spp. and 

Rhizoctonia solanL Of these, B. cinerea, A, alternata and E, carotovora 

are the most important pathogens of fruits from any region. In general, 

B, cinerea is the major cause of loss in temperate countries, while in 

hotter areas the common causes of decay are R, stolonifer, G, candidum 




and E. carotovora (McColloch et al., 1982), which are characterized by 

high optimal growth temperatures. Other pathogens may be of local or 

occasional importance. Various fungi, such as Trichothecium roseum and 

species of Phomopsis, Colletotrichum, Phoma, Pythium, Sclerotinia, 

Stemphylium, Cladosporium, Penicillium or Aspergillus, may also be 

involved in fruit decay but are considered of minor importance. 



This fungus is generally regarded as the major pathogen in 

greenhouse-grown tomatoes and peppers, while Alternaria is of greater 

importance in field-grown crops (Dennis, 1983b). B. cinerea can persist in 

the soil or on plant debris as sclerotia and, under cool, moist conditions, 

it produces an abundance of asexual spores (conidia) which are dispersed 

by air currents. The pathogen can penetrate young fruits through 

senescent flower parts (Lavy-Meir et al., 1988), through growth cracks 

and wounds or via the stem end, before or during harvesting. However, if 

the fruits are weakened by exposure to chilling in the field or during 

storage, they become susceptible to direct penetration, and lesions may 

develop anywhere on their surface (McColloch et al., 1982). In addition to 

penetrating chill-injured fruit, the fungus can directly penetrate the 

cuticle of immature fruit but, in this case, further growth is arrested and 

*ghost spots' are formed on the growing fruit (Verhoeff, 1974). 

The infected tissue is soft and water-soaked. Under humid conditions, 

an abundance of gray-brown conidia, sometimes accompanied by black 

sclerotia, are formed and serve as inoculum for new infections. 

A considerable proportion of the rots caused by B. cinerea spread during 

storage by contact between infected and sound fruits. 

B. Alternaria alternata (Fr.) Keissler 

This is a very common pathogen of solanaceous fruits (Barkai-Golan, 

1981), which can limit their commercial life after harvest. The fungus 

survives on plant debris and its conidia are very important components 

of the air spora. 

Alternaria is a weak pathogen that requires injured or weakened 

tissue for penetration and development. When free water forms on the 

surface of ripening fruit from rain, dew or overhead irrigation, spores 

germinate in response to water-soluble nutrients on the fruit surface 

(Pearson and Hall, 1975). The fungus can enter via growth cracks, insect 

injuries or mechanical damage, but infection is often initiated at the 




calyx scar (Dennis 1983b; Fallik et al., 1994b). Incipient infections, which 

may appear at the margin of the stem scar, remain quiescent unless the 

fruits are subjected to weakening conditions, including chilling injury, 

sunscald or over-maturing. The enhancing effects of chilling and of 

high-temperature treatments on the susceptibility of tomato fruits to 

Alternaria injection has been exhibited at both the mature-green and 

mature development stages (Barkai-Golan and Kopeliovitch, 1989). 

Prolonged storage can also increase fruit susceptibility. Although the 

optimal growth temperature of the fungus is 28°C, it may continue 

growing during storage, thanks to the relatively high temperatures 

recommended for these cold-sensitive fruits. 

A. alternata infections result in firm lesions, slightly sunken and 

covered in dense, olive-green to black masses of conidia (McCoUoch et al., 

1982). Pearson and Hall (1975) described incipient infections of 

A. alternata on green fruits, which failed to resume activity when the 

fruits ripened. This phenomenon resembles the *ghost spotting' described 

by Verhoeff (1974) in green tomatoes infected by JB. cinerea. 

The development of A. alternata in tomatoes may be associated with 

the production of several non-specific toxic metabolites in the infected 

fruits. Stinson et al. (1981) found that the main toxin in infected tomato 

fruits was tenuazonic acid; others, such as alternariol, alternariol 

monomethyl ether and altenuen were found in much smaller amounts. 

However, working with another strain of A, alternata, Ozcelik et al. 

(1990) reported that the major toxin in infected tomatoes was alternariol, 

followed by alternariol methyl ether. Fungal growth, along with toxin 

production, occurred at temperatures between 4 and 25°C. 

C. Erwinia carotovora ssp. carotovora (Jones) Dye and Erwinia 

carotovora ssp. atroseptica (van Hall) Dye 

These soft rot bacteria are among the most common postharvest 

pathogens of tomatoes and peppers, and are found in the soil and plant 

debris wherever they are grown. These pathogens are spread by 

splashing of contaminated soil, by wind, by insects and by postharvest 

washing (Bartz, 1982). They are wound pathogens, requiring moisture to 

initiate infection, and entering the fruit under warm, wet conditions, 

through breaks in the skin, the cut stem or any injury incurred during 

picking and packing (Volcani and Barkai-Golan, 1961; Sherman et al., 

1982). Growth and multiplication of the bacteria are favored by wet 

conditions and temperatures of 24-30°C. Under these conditions, the 

entire fruit may rot in a few days. 




The infected tissue becomes soft and water-soaked and, later, the skin 

may spht hberating an infecting liquid. Decay spreads rapidly from fruit 

to fruit, forming nests of decay, especially under shelf-life conditions at 

temperatures above 20°C. Keeping the fruits under refrigeration during 

transport and storage suppresses bacterial growth. 

D. Rhizopus stolonifer (Ehrenb. Ex Fr.) Lind 

This species is the cause of Rhizopus rot, which can occur wherever 

tomatoes, peppers and eggplants are grown. In tropical countries another 

species, R. oryzae Went & Prinsen Geerligs, is common (Snowdon, 

1992). 

R. stolonifer is widely distributed in the soil and in the atmosphere and, 

although it may produce sexual spores (zygospores), it commonly exists in 

the asexual state as sporangia and sporangiospores. Spore germination 

occurs in a warm, moist environment and penetration takes place via 

injuries or damaged tissues (Barkai-Golan and Kopeliovitch, 1981). The 

infected area of tomatoes and peppers appears water-soaked through the 

distended skin, which often ruptures and releases an abundance of liquid. 

Sporulation occurs on the surface of infected fruits and the disease is 

spread to adjacent fruit, resulting in extensive 'nesting' during storage 

(Barkai-Golan and Kopeliovitch, 1981). However, the disease develops 

slowly in fruits stored at the recommended temperatures. 

E. Geotrichum candidum Link ex Pers. 

This fungus is responsible for the sour rot in stored tomatoes. It is a 

soil-borne pathogen whose spores are transmitted mainly by insects. 

Flies are known to transfer spores from rotten fruits to healthy fruits and 

also to create wounds that enable fungal penetration (Butler, 1961). The 

fungus is a wound pathogen, infecting both mature-green and ripe fruits 

via the stem scar or any wound on the fruit surface. If fruits are infected 

shortly before harvest, symptoms will not be visible at harvest and the 

decay can develop during storage or marketing. 

G. candidum has a high optimum growth temperature (about 30°C) 

and can develop under warm, moist conditions. The infected tissues in 

ripe fruits are soft and watery whereas those in mature-green fruits are 

characteristically water soaked. In both cases decay is accompanied by a 

sour odor. At an advanced stage, the infected skin splits and a 

white-to-creamy mycelium begins to develop on the exposed tissue and 

forms conidia (oidia or arthrospores) by fragmentation of the hyphae 

(Butler, 1961). 




F. Fusarium spp. 

Several Fusarium spp. may cause Fusarium rot of tomatoes, peppers 

and eggplants. The most frequently recorded are F. equiseti (Corda) 

Sacc, F. avenaceum (Corda ex Fr.) Sacc, F. moniliforme Sheldon, F. solani 

(Mart.) Sacc. and F, oxysporum Schlecht (Snowdon, 1992). 

The Fusarium spp. are common inhabitants of the soil. The conidia are 

dispersed by wind or water, and fruits arriving into storage may already 

be contaminated with them. The fungi are weak pathogens that infect 

only wounded fruits or those that have been weakened by chilling 

temperatures. Fruits stored at too low temperatures for a long time are 

particularly susceptible to infection. 

The decayed area is usually water-soaked and becomes covered by a 

mycelium which may be white, yellow or pinkish, according to the 

Fusarium involved. The rot extends into the center of the fruit and the 

infected tissue appears pale brown (Dennis, 1983b). 

G. Phytophthora infestans (Mont.) de Bary 

This fungus induces late blight of potatoes and tomatoes and may cause 

serious losses in many areas during wet seasons. Infection is initiated by 

the asexual state (sporangia and zoospores), although P. infestans can also 

exhibit the sexual state (oospores) (Hermansen et al., 2000). 

Disease development depends on weather conditions prior to 

harvesting. The optimal temperature for infection is 15-18°C, and wet 

conditions are essential for infection capability of the zoospores 

(McCoUoch et al., 1982). Infection of leaves, stems and fruits may be by 

germinating sporangia or may follow the release of zoospores from the 

sporangium (Alexopoulos, 1961). The zoospores, which gain mobility from 

their flagella, swim in the film of rainwater on the host. Infection 

frequently takes place at the edge of the stem-scar although, under 

optimal conditions, direct penetration of the fruit skin can also occur 

(Eggert, 1970). Tomatoes harvested from infected fields may be in the 

initial stages of infection, when lesions are not yet visible. Such fruits are 

packed and decay can occur during transit or at the market. 

The infected area is brown, or rusty-tan, hard and with irregular 

margins. There is little spreading from diseased to healthy fruit in 

transit, in the ripening room or in storage. 

H. Phytophthora spp. 

Several species of Phytophthora induce Phytophthora rot of tomatoes, 

peppers and eggplants, the most common species being P. nicotianae var. 




parasitica (Dastur) Waterh. and P. capsici Leonian (Snowdon, 1992). 

These species are soil inhabitants and can survive in seeds as 

thick-walled oospores (the sexual state) or chlamidospores. Infection 

takes place in the field in warm, wet conditions, which favor the 

production of thin-walled sporangia, the asexual state (Bhardwaj et al., 

1985) giving rise to numerous zoospores, which are swimming spores 

that must have free water to survive and cause fruit infection. They are 

capable of penetrating both unripe and ripe uninjured fruit. Infection is 

generally confined to fruits growing low down on the plant, near to or in 

contact with the soil (Springer and Johnston, 1982). When disease 

development is interrupted by harvest, growth continues afterwards and 

may be spread to healthy fruits by contact during storage. 

In peppers, infection frequently originates at the stem end, whereas in 

eggplants lesions often appear near the distal end. Phytophthora rot of 

tomatoes is usually called TDuckeye rot' and is characterized by the 

formation of concentric, brown rings around the point of disease 

initiation. In humid conditions, an off-white mycelium, giving rise to 

numerous sporangia, is developed on the surface of the infected area 

(McColloch et al., 1982). The occurrence of the disease depends upon 

weather conditions, regardless of geographical location. 

I. Rhizoctonia solani Kuhn [perfect state: Thanatephorus 

cucumeris (Frank) Donk] 

This fungus causes soil rot or Rhizoctonia rot, which is of importance 

mainly in tomatoes (Baker, K.F., 1970). Rhizoctonia is a soil-inhabiting 

fungus, which produces both the sexual state (a hymenium giving rise to 

basidiospores) and the sterile state (sclerotia). Infection occurs under wet 

conditions by splashing of contaminated soil onto the low-hanging fruits 

(Baker, K.F., 1970), but fruits in contact with the ground are generally 

infected by penetration of the fungus from the soil through injuries 

(Murphy et al., 1984). The disease develops on both green and ripe fruits 

and is common in tomatoes intended for processing, since they are 

generally grown without stakes and many of them remain in contact with 

the soil until fully ripe. The rate of infection depends upon weather 

conditions (McColloch et al., 1982): the optimal growth temperature is 

26-27°C and there is little development below 10°C. Tomatoes that were 

infected in the field but showed no evidence of decay, or were overlooked 

during grading and packing, may decay during transit. 

Typical spots on ripe fruits are firm and reddish brown, while the 

infected flesh is soft or even watery. At an advanced stage of 




development, a white or gray mycelium may cover the lesion, and the 

fungus can spread from diseased to healthy fruit by contact (Strashnov et 

al., 1985). 

J. Other Pathogens 

Phomopsis spp. (perfect state: Diaporthe spp.) 

These fungi are of minor importance in tomatoes and peppers, but 

infection of eggplants by Phomopsis vexans (Sacc. & Sydow) Harter may 

result in destructive decay during transit or storage. In fact, Phomopsis 

rot is included among the common diseases of eggplants. The fungus may 

exhibit the sexual state (perithecia and ascospores) (Gratz, 1942) 

although infection is usually induced by the asexual conidia which are 

borne within pycnidia. The fungus reaches the host in rainwater, 

germinates and penetrates directly through the healthy tissue of leaves, 

stems and fruits (Divinagracia, 1969). Fruits infected in the field just 

before harvest may seem healthy when packed but will decay later in 

storage. The optimal temperature for fungal growth is 26-30°C; there is 

httle growth at or below TC (McCoUoch et al., 1982). 

The lesions are brown, tough and characterized by the development of 

minute black bodies: the conidia-bearing pycnidia. 

Colletotrichum spp. (perfect state: Glomerella spp.) 

Several Colletotrichum spp., such as C. coccodes (Wallr.) Hughes, 

C. capsici (Sydow) Butler and Bisby, and C. gloeosporioides (Penz.) Sacc, 

may cause anthracnose in solanaceous fruits (Adikaram et al., 1982; 

Batson and Roy, 1982). 

In addition to the asexual state (acervuli with conidia), some of the 

species exhibit the sexual state (perithecia with ascospores). The fungus 

may survive from season to season on plant debris (McCoUoch et al., 1982). 

Infection occurs during warm, wet weather, when conidia are splashed onto 

immature fruits by wind-driven rain, and germinate to form appressoria 

that adhere to the wax or the cuticle of the fruit. The infection hs^hae that 

form from the appressoria pierce the cuticle and the fungus remains 

quiescent until the fruit ripens. Quiescent infection of peppers is associated 

with the production of phsâlexins in the young infected tissue (Adikaram 

et al., 1982). Anthracnose is primarily a disease of ripe fruits and, therefore, 

becomes a problem mainly in tomatoes and peppers grown for canning and 

left to ripen on the plant. Manandhar et al. (1995) reported on the ability of 

C. gloeosporioides to cause anthracnose on pepper fruits of all ages; disease 

incidence was correlated with cuticle and exocarp thickness. 




With progress of the disease, the lesions become dark, characterized by 

salmon-pink masses of conidia in the center. In some cases black 

sclerotia are also formed. 

Cladosporium herbarum (Pars.) Link 

Fungal conidia are generally found in the soil and are very common in 

the atmosphere. However, Cladosporium rot of tomatoes and peppers is 

of importance only during long transit or extended storage. The fungus is 

a weak pathogen, infecting fruits via mechanical damage, or following 

sunscald or chilling injury (Barkai-Golan, 1981). Lesions are circular, 

firm and with definite borders; at an advanced stage and under humid 

conditions, they become covered with a velvety olive-green mycelium 

bearing new conidia. 

Stemphylium botryosum Wallr. [perfect state: Pleospora 

herbarum (Pers.) Rabenh.] 

This fungus may exhibit both the asexual state (conidiophores bearing 

conidia) and the sexual state (perithecia with ascospores). Infection 

occurs under moist conditions, being initiated at growth cracks, the edge 

of the stem scar or at any injury on the fruit surface. Lesions are similar 

to those formed by A alternata, but Stemphylium is much less common. 

When the fungus exhibits the sexual state, the development of 

perithecia, which appear as minute black fruit bodies, may serve to 

distinguish between the two fungi (McCoUoch et al., 1982). 


To reduce infection by wound pathogens such as species of Alternaria, 

Goetrichum, Fusarium, Cladosporium, Stemphylium, as well as B, cinerea 

that may also gain entry through wounds, care should be taken to 

prevent injury to the fruit during harvesting. Fruits showing growth 

cracks, sunscald or chilling injury in the field, which predispose the fruit 

to pathogen penetration, should be rejected at the packing stage. 

To reduce the source of inoculum in the field, plant debris on which 

the fungi can proliferate, should be removed. Good sanitation practices 

around the packinghouses and regular disinfection of picking containers, 

are also advisable. Since infection by soft rot bacteria depends on wet 

conditions (Volcani and Barkai-Golan, 1961), the fruit should be picked 

while dry. 

Fungicidal treatments to control Botrytis rot on staked tomatoes start 

with sprays during flowering, to protect the flowers and the young fruits 




from preharvest infection (Eckert and Ogawa, 1988). Preharvest sprays 

with chlorothalonil are also effective in controlling A. alternata during 

storage (Davis et al., 1997). Sprays with two fungicides (metalaxyl and 

mancozeb) combined effectively control P. infestans in the field, reduce 

the inoculum level and minimize infections that lead to decay 

development in the markets (Eckert and Ogawa, 1988). 

Use of chlorinated water at 38-43°C to wash tomatoes in the 

packinghouse prevents the buildup of inoculum in the water. The water 

should be properly chlorinated and frequently changed (Bartz, 1982) and 

the fruit has to be dried before being packed. After removal of the surface 

water, the fruit may be treated with postharvest fungicides. Postharvest 

fungicidal dips (using imazalil) were found to be effective against 

A. alternata in tomatoes and peppers (Spalding, 1980) and against 

B. cinerea in tomatoes (Manji and Ogawa, 1985). Most of the chemical 

compounds used are ineffective against G. candidum (Eckert and Ogawa, 

1988). 

Rotting is often retarded in storage since the pathogens grow very 

slowly at or below 10°C. However, the correct storage temperature 

should be carefully maintained, to prevent chilling injury (Kader, 1986), 

which enhances susceptibility to infection (Barkai-Golan and 

Kopeliovitch, 1981, 1989). While ripe tomatoes can be held for a few days 

at 7-10°C, mature green fruits, which are more susceptible to low 

temperature injury, should usually be held at or above 13°C. Relatively 

high storage temperatures are also recommended for the chill-sensitive 

eggplants (8-12°C) and peppers (7-10°C). 

Reduction in the incidence of B, cinerea, R, stolonifer and A. alternata 

could also be achieved by biological means (Chalutz et al., 1991): 

application of a known antagonistic yeast (Pichia guilliermondii) to 

wounds on the surface of harvested ripe tomato fruits, prior to 

inoculation with the pathogens, reduced decay by 90%. The yeast had no 

effect on the pathogens in culture and its inhibitory effect on pathogens 

on the fruit was attributed to competition for nutrients. The efficacy of 

the antagonist in reducing decay was affected by the concentrations of 

both the yeast cells and the fungal spore suspension used for inoculation. 

Fruits of the non-ripening tomato mutants, nor (non-ripening) and rin 

(ripening inhibitor), which are devoid of typical carotenoids of tomato and 

fail to soften (Tigchelaar et al., 1978), have been found to be less 

susceptible to postharvest diseases than normal tomato fruits 

(Barkai-Golan and Kopeliovitch, 1981; Lavy-Meir et al., 1989). Increased 

resistance toward B, cinerea has also been exhibited by the hybrid fruit 




of the nor mutant (Barkai-Golan and Kopeliovitch, 1989; Lavy-Meir et 

al., 1989), which is characterized by color development and retarded 

softening (Tigchelaar et al., 1978). This resistance was, however, 

partially broken when the fruit was exposed to chilling temperature or 

hot water treatment, which favor fungal penetration (Barkai-Golan and 

Kopeliovitch, 1989). 

Some cultivars of solanaceous fruits are available, which are resistant 

to certain postharvest pathogens. They include tomato and pepper 

cultivars resistant to various Phytophthora spp. causing fruit rot, tomato 

cultivars resistant to P. infestans and eggplant cultivars resistant to 

Phomopsis (Snowdon, 1992). 



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Acetaldehyde, 7, 8, 44, 177-179, 318 

Acetic acid, 171, 172, 318 

Acetobacter 

— pineapples, 289 

Acremonium 

— bananas, 223, 279 

Acremonium strictum, 223 

Active oxygen, 53, 90, 91 

Aflatoxin, 63 

Aloe vera gel, 183, 184 

Altenuene, 62, 322 

Alternaria, 4, 10, 60, 69 

— apples, 96 

— avocados, 286 

— bananas, 96 

— gooseberries, 314 

— grapes, 315 

— lemons, 230 

— mangoes, 96 

— papayas, 148, 283 

— stone fruits, 304 

— strawberries, 199 

— sweet peppers, 117 

— sweet potatoes, 117 

Alternaria alternata, 10, 17-19, 22, 58, 61, 

62, 65, 75, 123, 136, 141, 183, 191, 206, 

229, 328 

— apples, 20, 23, 26, 38 


— blueberries, 310, 311, 313 

— celery, 79 

— eggplants, 180 

— gooseberries, 311, 313 


— kiwifruit, 319 

— mangoes, 17, 19, 20, 72, 254, 282 

— pears, 26, 229 

— peppers, 63, 180 

— persimmons, 20, 138, 290 

— pome fruits, 293, 299, 302 

SUBJECT INDEX 


395 

— raspberries, 179 

— solanaceous fruit vegetables, 320-322, 

327, 328 

— stone fruits, 303, 308 

— strawberries, 179, 310 

— tomatoes, 20, 23, 26, 50, 58, 71, 117, 

231, 236, 322 

— zucchini, 23 

Alternaria alternata f. tenuis, 62 

Alternaria alternata pv. citri, citrus fruits, 

271 

Alternaria citri, 206 

— citrus fruits, 25, 137, 166, 271, 272 

Alternaria cucumerina, watermelons, 105 

Alternaria rot 

— pome fruits, 299 


— tomatoes, 216 

Alternaria solani, 65 

Alternaria tenuissima 


— blueberries, 313 

Alternariol, 62, 300 

Alternariol methyl ether, 63 

Alternariol monomethyl ether, 62, 322 

Anthracnose, 326 

— avocados, 25, 57, 102, 137, 148, 253, 

260, 286-288 

— bananas, 80, 148, 159, 279 


— citrus fruits, 25, 273, 274, 278 

— guavas, 291 

— mangoes, 25, 137, 148,159, 201, 213, 

281, 282 

— papayas, 25, 148, 159, 284, 285 

— rambutans, 240 


— solanaceous fruit vegetables, 326 


— tangerines, 49, 257, 273


396 Subject Index 

— tropical and sub-tropical fruits, 13, 25, 

148, 155, 159 

Antioxidants, 253, 260, 288 

— with prochloraz, 260, 288 

Apples {see also Pome fruits) 

— Alternaria alternata, 20, 23, 26, 38 

— Alternaria spp., 96 

— Aspergillus niger, 21 

— Aureohasidium pullulans, 229, 241, 

242, 262, 303 

— Bacillus subtilis, 229 

— bitter pit, 23 

— bitter rot, 144, 297 

— black rot, 23 

— blue mold rot, 20, 22, 159, 216, 244, 

249, 261, 294 

— Botryosphaeria obtusa, 23 

— Botryosphaeria ribis, 15 

— Botrytis cinerea, 21, 26, 54, 130, 159, 

176, 224, 225, 229, 231, 244, 245, 249, 

262,301 

— brown rot, 100 

— Candida oleophila, 227, 302 

— Candida sake, 224, 303 

— Cladosporium herborum, 23 

— Colletotrichum acutatum, 144, 297, 302 

— Colletotrichum gloeosporioides, 144, 

297, 302 

— Colletotrichum spp., 96 

— core rot, 21 

— Cryptococcus, 236, 237 

— Diplodia spp., 96 

— Fusarium spp., 21 

— Gloeosporium album, 20 

— Gloeosporium perennans, 17, 20 

— Gloeosporium spp., 26, 142, 149, 155, 

159,199,204 

— Glomerella cingulata, 297 

— gray mold, 159, 249 

— lenticel rot, 159 

— Monilinia fructicola, 67 

— Mucor piriformis, 21, 301 

— Nectriagalligena, 13, 80, 107, 297 

— Nectria galligena, resistance to, 18 

— Penicillium expansum, 20, 22, 23, 26, 

38, 63, 96, 142, 143, 159, 176, 177, 199, 

216, 224, 225, 229, 231, 242, 244, 245, 

261, 262, 301 


— Penicillium funiculosum, 21 

— Pestalotia spp., 96 

— Pezicula malicorticis, 229, 303 

— Phomopsis mali, 21 

— Pichia guilliermondii, 229, 232, 261 

— pink mold, 23 

— Pleospora herbarum, 21 

— Pseudomonas cepacia, 234, 249 

— Pseudomonas syringae, 242, 244 

— scab fungus, 23 

— Rhodotorula glutinis, 229 

— Sclerotinia fructicola, 67, 100 

— Sclerotinia fructigena, 100 

— Sporobolomyces roseus, 242, 244 

— Stemphylium botryosum, 23, 26 

— Trichothecium roseum, 21, 23 

— Trichothecium spp., 96 

— Venturia inaequalis, 23 

Apricots {see also Stone fruits) 

— Monilinia fructicola, 9, 26 

— Rhizopus stolonifer, 26 

Aspergillus, 4, 22, 63 


— guavas, 291, 292 

— pome fruits, 294, 302 

Aspergillus flavus, 54, 55, 63 

Aspergillus niger, 38, 62, 81, 206 


— grapes, 178, 225, 227, 231, 315, 317 

Ascorbic acid, 43 

Aspire {Candida oleophila), 247, 277 

Attack mechanisms, 52, 54-65 

Aureobasidium 



Aureobasidium pullulans, 226 

— apples, 229, 241, 242, 262, 303 

— grapefruits, 225 




Avocados, 13, 16, 18, 19, 71, 74 

— Alternaria, 286 

— Alternaria alternata, 287 

— anthracnose, 25, 57, 102, 137, 148, 253, 

260, 286-288 


— Botryodiplodia theobromae, 286, 287


Subject Index 397 

— Botryosphaeria ribis, 20, 287 

— Colletotrichum gloeosporioides, 13, 16, 

18, 25, 49, 50, 54, 57, 68, 71, 74, 91, 

102, 222, 241, 253, 260, 261, 286 

— Colletotrichum magna, 261 

— Dothiorella aromatica, 222 

— Dothiorella gregaria, 20, 286, 287 

— Erwinia carotovora, 286 

— fruit rot, 286 

— Fusarium solani, 96, 97, 103 



— Penicillium spp., 286 

— Pestalotiopsis, 222 

— Pestalotiopsis versicolor, 286 

— Phomopsis spp., 222, 286, 287 

— Physalospora rhodina, 287 

— Pseudocercospora purpurea, 286 

— Pseudomonas syringae, 286 


— stem-end rot, 286-287 

— Thyronectria, 222 

— Trichothecium roseum, 286 

Bacillus spp., 222, 234 

Bacillus subtilis, 184, 223, 233 



— peaches, 246 



Bacterial soft rot, 235, 327 

— cabbages, 24 

— carrots, 24, 237 

— celery, 24, 140 

— citrus fruits, 273 

— lettuces, 24, 140 

— peppers, 237, 322, 323 

— potato tubers, 20, 22, 23, 44, 86, 142 

— tomatoes, 23, 34, 127, 237, 322, 323 

— squash, 24 

Bananas, 71, 105, 184 

— Acremonium spp., 279 


— anthracnose, 80, 148, 159, 279 

— Botryodiplodia theobromae, 105, 279, 

280 

— Cephalosporium spp., 279 


— Ceratocystis paradoxa, 279 

— Colletotrichum gloeosporiodes, 159 

— Colletotrichum musae, 14, 25, 80, 223, 

279 


— crown rot, 279, 280 

— Diplodia spp., 96 

— finger rot, 280 

— Fusarium pallidoroseum, 279 

— Gloeosporium musae, 25 

— Lasiodiplodia theobromae, 227, 281 

— Penicillium expansum, 96 

— Pestalotia spp., 96 

— Trichoderma spp., 227 

— Trichoderma viride, 227, 281 


— Verticillium theobromae, 279 

Benomyl, 149, 159, 200, 201, 216, 230, 277, 

290 

Benzaldehyde, 179 

Benzimidazole compounds, 148, 149, 159, 

160, 161, 165, 275, 276, 278, 301, 314 

— resistant fungal strains, 160, 161 

Benzoic acid, 18, 69, 80, 81, 297, 301 

Benzyl alcohol, 179 

Benzylisothiocyanate, papayas, 71 

Biochemical changes following infection, 

103-107 

Biological control (biocontrol), 221-251, 

276, 277, 281, 288, 302, 309, 310, 315, 

328 

— integration into postharvest strategies, 

246-251 

— isolation and selection of antagonists, 

222-226 

- characteristics of potential 

antagonists, 224-226 

- epiphytic microorganisms, 221-223, 

227-229 

- methods and media for isolation, 223, 

224 

- mixtures of antagonists, 229, 242, 

243 

— mode of action, 233-242 

- antibiotic compounds, 233-235 

- competition for nutrients, 235-237 

- effects on the pathogen and its 

enzymatic activity, 237-240



- induction of defense mechanism, 

240-242, 261, 262 

— pre- and postharvest application, 

228-233,246 

— with calcium treatments, 244, 248 

— with chemical treatments, 250, 310 

— with 2-deoxy-D-glucose (see sugar 

analogs), 244, 245 

— with glycochitosan, 245 

— with modified atmosphere packaging 

(MAP), 250 

— with nitrogenous compounds, 244 

Biphenyl (diphenyl), 155, 156 

— resistant fungal strains, 156 

BioSave^^ (Pseudomonas syringae), 247, 

277 

Bitter pit, apples, 23 

Bitter rot, apples, 144, 297 

Black rot 


— pineapples, 159, 288, 290 

Black spot 

— mangoes, 282, 283 

— persimmons, 138 

Blight see Late blight 

Blue-green mold, melons, 24 

Blue mold rot 

— apples, 20, 22, 159, 216, 242, 244, 249, 

261, 294 

— citrus fruits, 24, 26, 268 

— grapes, 142, 317 

— pears, 132, 149, 229, 249, 294 

Blueberries 

— Alternaria alternata, 310, 311, 313 

— Alternaria tenuissima, 313 

— anthracnose, 310 

— Botrytis cinerea, 310, 313 


— gray mold, 310 

— leak' disease, 313 

— Monilinia, 313 

— Mucor piriformis, 313 

— Phomopsis spp., 311 

— Phomopsis vaccinii, 313 

Botryodiplodia, papayas, 148 

Botryodiplodia theobromae, 188 

— avocados, 286, 287 

— bananas, 105, 279, 280 




— litchis, 292 

— mangoes, 281, 282 

— papayas, 283, 284 


Botryosphaeria 



Botryosphaeria obtusa 



Botryosphaeria ribis 


— avocado, 20, 287 



Botryotinia fuckeliana 








Botrytis, 4 

— carrots, 41 

— grapes, 13, 142, 233 



— sweet peppers, 117 



Botrytis cinerea, 9, 10, 54, 65, 67-69, 81, 

123, 136, 144, 160, 163, 183, 191, 206, 

235, 237, 238, 255 

— apples, 21, 26, 130, 159, 176, 224, 225, 

227, 229, 231, 244, 245, 249, 262, 301 

— blueberries, 310, 313 

— cabbages, 9, 38 

— carrots, 4, 41, 87, 219 

— celery, 26, 38, 78, 79, 140 



— grapes, 22, 36, 158, 178, 225, 227, 315, 

316 

— kiwifruit, 318-320 

— lettuce, 38



— nectarines, 44, 46, 216, 305, 306 

— pears, 26, 41, 149, 229, 231, 305 

— peppers, 79,180, 320, 321 

— plums, 44, 46, 216, 305, 306 

— pome fruits, 293, 295, 301-303 

— raspberries, 179, 305, 310, 311 

— solanaceous fruit vegetables, 320-322, 

327, 328 

— stone fruits, 303, 305, 308 

— strawberries, 9, 12, 13, 22, 24, 26, 34, 

50, 118, 131, 148, 177, 179, 189, 223, 

228, 230, 305, 310, 311, 314, 315 

— tomatoes, 13, 16, 22, 23, 33, 34, 51, 66, 

95, 96, 98, 100, 101, 225, 231, 232, 236, 

321 

Botrytis rot 



Broccoli, 132, 133 

— Escherichia coli, 153 

Brown rot, 11,13,55,70 


— citrus fruits, 11, 33, 147, 166, 272, 273 

— nectarines, 201 

— peaches, 19, 200, 201 

— stone fruits, 13, 36, 145, 159, 304, 305, 

309, 310 

— sweet cherries, 132, 201, 250, 310 

Brown spot, rambutans, 240 

Buckeye rot, solanaceous fruit vegetables, 

325 

Butanal, 178 

Cabbages, 9 

— bacterial soft rot, 24 

— Botrytis cinerea, 9, 38 

— decay rate at different relative 

humidities, 41 

— Erwinia spp., 24 


— Sclerotinia sclerotiorum, 12 

— Sclerotinia spp., 24 

— soft watery decay, 12 

— white watery rot, 24 

Caffeic acid, 55, 70 

Calcium application, 142-145, 174, 175, 

248, 249, 302, 309 

— calcium propionate, 145, 174 


— reduced fungal growth and pectolytic 

activity, 144, 145 

— reduced storage disorders and decay, 

142-145 

— with biocontrol, 146, 244, 248 

— with heating, 146, 204 

— with polyamine inhibitors, 146 

Camphor, 182 

Candida famata, citrus fruits, 241, 261, 

276 

Candida oleophila 

— apples, 227, 302 



Candida saitoana, 244 

— apples, 244, 245 


— oranges, 246 

— peaches, 244 

Candida sake, apples, 224, 303 

Candida sp., 206 


Y-Caprolactone, 179 

Capsenone, peppers, 80 

Capsicannol, peppers, 80 

Capsidiol, peppers, 79, 80 

Captan, 154, 157, 200 

Carbendazim, 149, 159 

Carbonate and bicarbonate salts, 172, 173 

Carrots, 12, 41, 42, 75, 76, 87 

— bacterial soft rot, 24, 237 

— Botrytis, 41 

— Botrytis cinerea, 41, 42, 87, 219 



— Rhizopus, 41 

— Rhizopus stolonifer, 9, 117 

— Sclerotinia sclerotiorum, 12, 42, 219 



— UV illumination, 255 


Cauliflowers, 41 

— Sclerotinia sclerotiorum, 12 


Celery 


— bacterial soft rot, 24, 140



— Botrytis cinerea, 26, 38, 78, 79, 140 

— columbianetin, 78, 79 

— Erwinia cartovora, 26, 140 


— fungal soft rot, 140 

— Sclerotinia sclerotiorum, 12, 26, 78, 79, 

130,140 


— watery soft rot, 12, 130 


Cellulases, 58 

Cellulolytic enzymes, 55 

Cephalosporium, 63 


Ceratocystis fimbriata 

— sweet potato roots, 51, 78 

Ceratocystis paradoxa 



— pineapples, 105, 159, 288 

Cercospora, papayas, 285 

Chemical control, 147-184, 249, 275-277, 

280, 283, 292, 293, 314, 318, 328 

— generally recognized as safe (GRAS) 

compounds, 170-176 

— natural chemical compounds, 6-8, 

177-184, 234, 249 

— postharvest treatments, 154-170 

— preharvest treatments, 147-150 

— with antioxidant, 288 

— with biological control 310 

— with heating, 157, 159, 200-203, 282, 

283, 286, 292, 293, 309 

— with modified atmosphere packaging, 

310 

Chili fruits, Colletotrichum 

gloeosporioides, 105 

Chilling injury, 109, 110, 117, 118, 201, 

268, 269, 278, 303, 328 

Chilling injury retardation, 118-121, 

196-198, 201, 303 

Chinese cabbages, 41 

Chitinase, 53, 92, 183, 219, 241, 256-259, 

263, 264 

Chitosan, 92, 182, 183, 258, 259 

Chitosanase, 92, 258, 259, 262 

Chlorine, 140, 141, 151-154, 174, 175, 302, 

314, 328 


Chlorine dioxide, 153, 175 

Chlorogenic acid, 17, 55, 69, 70, 107 

Chocolate spot t3rpe lesions, papaya, 284 

Chlorothalonil, 167, 328 

Chryseobacterium, strawberries, 315 

Chryseobacterium indologenes, 

strawberries, 223 

Cineole, 182 

Citral, 6, 49, 73, 81, 138 

Citrinin, 63 

Citrus fruits, 5, 6, 7, 8, 81, 88, 155, 255, 

268 

— Alternaria alternata pv. citri, 271 

— Alternaria citri, 25, 137, 166, 271, 272 

— anthracnose, 25, 273, 274, 278 


— blue mold rot, 24, 26, 268 

— Botryodiplodia theobromae, 271 

— Botryosphaeria ribis, 271 

— brown rot, 11, 33, 147, 166, 272, 273 

— Candida famatay 241, 261, 276 

— Candida oleophila, 247 

— Candida saitoana, 245 

— Colletotrichum gloeosporioides, 13,25, 

273, 278 

— Diaporthe citri, 271 

— Diplodia natalensis, 13, 95, 137, 159, 

166, 271, 272, 277 

— Dothiorella gregaria, 271, 272 


— Geotrichum candidum, 22, 26, 95, 166, 

167, 231, 268, 270, 276, 278 


— green mold rot, 24, 25, 41, 159, 167, 

268, 270 

— Penicillium digitatum, 5, 8, 21, 24-26, 

41, 43, 94, 95, 101, 105, 149, 159, 160, 

163-167, 199, 201, 213, 230, 247, 268, 

269,275-277 

— Penicillium italicum, 21, 24-26, 95, 

159, 160, 163, 165, 166, 230, 268, 269, 

276 

— Phomopsis citri, 13, 25, 137, 148, 159, 

166, 271, 272, 277 


— Phytophthora citrophtora, 11, 147, 199, 

272 

— Phytophthora hibernalis, 272



— Phytophthora nicotianae var. 

parasitica, 272 

— Phytophthora parasitica, 278 

— Phytophthora spp., 166, 167, 278 

— Pichia guilliermondii, 246, 247, 261 

— Pseudomonas syringae, 94, 226, 247, 

276 

— sour rot, 22, 26, 166, 167, 270, 276 

— stem-end rot, 13, 137, 138, 156, 157, 

159, 166, 271, 277 

— Trichoderma, 274, 275, 279 

— Trichoderma rot, 274, 275, 279 

— Trichoderma viride, 21A 

Cladosporium, 4, 22, 38 

— melons, 23 



— solanaceous fruit vegetables, 321, 327 



Cladosporium herbarum, 10, 123, 191, 206, 

229 


— grapes, 315, 316 




Cladosporium rot 


— peppers, 327 


Cold storage, 108-116 

— recommended conditions for fruits, 

110-113 

— recommended conditions for vegetables, 

114-116 

Colletotrichins, 62 

Colletotrichum, 38, 62, 303 







— papayas, 24 







Colletotrichum acutatum 



Colletotrichum capsici, solanaceous fruit 

vegetables, 326 

Colletotrichum coccodes, solanaceous fruit 


Colletotrichum gloeosporioides, 39, 49, 50, 

54, 71, 273 


— avocados, 13, 16, 18, 25, 49, 50, 57, 68, 

71, 74, 91, 102, 222, 241, 253, 260, 261 


— chili fruits, 105 




— mangoes, 25, 137, 159, 199, 201, 204, 

213, 268, 281, 282 

— papayas, 13, 20, 25, 159, 200, 283, 284 

— pome fruits, 293, 297, 302, 303 






— tangerines, 49, 51, 257, 273 

— tropical and subtropical fruits, 13, 148, 

155, 159 

Colletotrichum magna, 74 


Colletotrichum musae, 9, 49, 161 

— bananas, 14, 25, 80, 223, 279 

Columbianetin, celery, 78, 79 

Controlled atmosphere (CA), 122-130, 249, 

288, 315, 320 

— effects on disease development, 

126-130 

— effects on the pathogen, 122-125 

— with carbon monoxide, 130, 131 

— with removal of ethylene, 126, 303 

Corynebacterium, 223 

Crown rot, bananas, 279, 280 

Cryptococcus, 236, 237 


— cherries, 236



— pears, 229, 236 

Cryptococcus infirmo-miniatis 

— pears, 229 

— sweet cherries, 250 

Cryptococcus laurentii, pears, 249, 250 

Cucumbers 

— Cladosporium, 23 

— Penicillium, 23 

— Pythium aphanidermatum, 105 

— Sclerotinia sclerotiorum^ 12 


Curing, 86-90, 196, 276, 320 

Curvularia verruculosa, pineapples, 105 

Cuticle, 52, 54 

— thickness, 66 

Cutinase, 54, 55 

Cylindrocarpon fruit rot, pome fruits, 296 

Cylindrocarpon malt, pome fruits, 296 

Dark dry decay, 22 

Deharyomyces, 276 

Debaryomyces hansenii, citrus fruits, 227 

Defense mechanisms, 52, 53, 64-69 {see 

also Active oxygen; Cold storage; 

Modified and controlled atmosphere; 

Induced resistance; Pathogenesis 

related proteins; Phytoalexins; 

Preformed antifungal compounds; 

Wound healing) 

Detoxification of defense compounds, 52, 

65 

Diaporthe spp., solanaceous fruit 


Diaporthe actinidiae, kiwifruit, 319 

Diaporthe citri, citrus fruits, 271 

Dicarboximide compounds, 149, 157, 162, 

314 

— resistant fungal strains, 149, 162, 314 

Dicloran (botran), 155, 157, 200, 224 

— with heating, 157 

Diene and monoene compounds, 16, 46, 71, 

72, 74, 253, 260, 261 

3,4-Dihydroxybenzaldehyde, 16 

Diplodia 

— apples, 95, 96 

— bananas, 95, 96 

— mangoes, 95, 96, 283 

Diplodia natalensis, 47, 48, 206 


— citrus fruits, 13, 95, 137, 159, 166, 271, 

272, 277 



— tropical and subtropical fruits, 25 

Dothiorella aromatica 


Dothiorella gregaria 

— avocados, 20, 286, 287 

— citrus fruits, 271, 272 


Dothiorella spp. 

— mangoes, 282, 283 


Eggplants, 141, 180 {see also Solanaceous 

fruit vegetables) 


— Botrytis cinerea, 180 

— Phomopsis, 326, 329 

— Phomopsis vexans, 326 

Enterobacter cloacae^ stone fruits, 310 

Environmental effects 

— on spore germination, 5-10 

— on fungal development, 37-45 

Enz5anatic activity, 52, 54 

— cellulolytic enzymes, 55, 58, 59 

— cutinase, 54, 55 

— cutin esterase, 240 

— pectolytic enzymes, 15, 41, 45, 46, 

55-57, 102, 103, 266, 269 

- inhibitors of, 48, 57, 66-69, 145, 240, 

258, 264, 301, 309 

- source, 102, 103 

Epicatechin, 68, 72, 91, 253, 254 

Epicoccum 



Epicoccum purpurescens, cherries, 229 

Erwinia, 223, 237 





— lettuces, 24 


— potato tubers, 22 

— squash, 24




Erwinia carotovora, 4, 56 

— avocadoes, 286 


— celery, 26, 140 


— potato tubers, 20, 77 


— tomatoes, 127, 237 

Erwinia carotovora ssp. atroseptica 




Erwinia carotovora ssp. carotovora, 322 





Escherichia coli 

— broccoli, 153 

— lettuce, 153 

Esculetin, 78 

Essential oils, 181, 182 

Etaconazole, 165, 166 

Ethanol, 7, 8, 180, 202, 203 


Etheric oil, from citrus fruit, 5,6, 44, 73 

Ethyl benzoate, 179 

Ethylene, 7, 47-51, 122, 126, 134, 139, 

208, 257, 265, 266, 270, 272, 274, 275, 

278, 320 

— degreening of citrus fruits, 48, 49, 272, 

274,278 

— effect on abscission enzjrmes 47, 48 

— effect on fruit susceptibility, 49. 51 

— evolution following infection, 84, 

94-100, 140, 319 

— manipulation of biosjoithesis {see 

genetic resistance in tomatoes), 265, 

266 

— production by fungi, 100, 101 

— source, 100-102 

— wound ethylene, 84, 99 

Extensin, 93 

Falcarindiol, in carrots, 75 

Falcarinol, in carrots, 76, 83 

Fenpropimorph, 167 


Ferulic acid, 69, 107 

Finger rot, banana, 280 

Fixed copper, 147 

Flutriafol, 167 

Formaldehyde, 154, 275 

Formic acid, 171, 172 

Fosetyl aluminum (fosetyl al), 167, 278 

Fresh herbs, 132 

Fruit rot, avocados, 286 

Fruitlet core rot, pineapples, 288, 289 

Fumaric acid, 62 

Fumonisins, 63 

Fungal dry rot, potato tubers, 86 

Fungal soft rot, celery, 140 

Fusaric acid, 62 

Fusarium, 4, 63 


— avocados, 222, 286 




— onions, 24 



— solanaceous fruit vegetables, 320, 324, 

327 

Fusarium dry rot, 243 (see also Potato dry 

rot) 

Fusarium avenaceum, solanaceous fruit 


Fusarium equiseti, solanaceous fruit 


Fusarium moniliforme, 62, 63 

Fusarium moniliforme var. subglutinans, 

pineapples, 289 

Fusarium oxysporum, 62, 69, 324 


Fusarium oxysporum f. sp. lycopersici, 

tomatoes, 65 

Fusarium pallidoroseum, bananas, 279 

Fusarium roseum, 10, 123 

Fusarium sambucinum, 11 

— potato tubers, 77, 85, SQ, 93, 106, 243 

Fusarium solani, avocados, 96, 97, 103 

Gamma radiation see Ionizing radiation 

Genetic resistance in tomatoes, 265, 266 

— *gene for gene' system, 266



Geotrichum, 56 


Geotrichum candidum, 152, 206 


268, 270, 276, 278 


— lemons, 8, 117 

— limes, 270 


— melons, 22, 166 


327,328 

— tomatoes, 22, 23, 26, 95, 98, 101, 323 

Geotrichum candidum var. citri-curantii, 

136 

5-Geranoxy-7-metoxy coumarin, 73 

Ghost spots, solanaceous fruit vegetables, 

321 

Gibberellic acid (GA3), 78, 138-141, 291 

{see also Gibberelin under Growth 

regulators) 

— with chemicals, 140, 141 

Gliocephalotrichum microchlamydosporum, 

rambutans, 240 

Gliocladium roseum., 230 

Gloeosporium, 23 

— apples, 26, 142, 149, 155, 159, 199, 

204 


— pears, 26 



Gloeosporium album 



Gloeosporium musae, bananas, 25 

Gloeosporium perennans 

— apples, 17, 20 


Gloeosporium rot, pome fruits, 296 

Glomerella, solanaceous fruit vegetables, 

326 

Glomerella cingulata 










|3-l,3-glucanase, 53, 92, 183, 219, 241, 

256-259, 262-264 

Glucosinolates, 182 

Gooseberries 


— Alternaria alternata, 311, 313 


— Mucor piriformis, 313 

— Stemphylium, 314 

Grapefruits, 5, 8, 81, 88, 119, 219, 229, 244 

(see also Citrus fruits) 

— Aureobasidium pullulans, 225 

— Geotrichum candidum, 270 

— green mold, 189, 229, 244 

— Penicillium digitatum^ 184, 189, 229, 

244 

— Penicillium italicum, 104 

— Pichia guilliermondii, 229 

— sour rot, 270 

Grapes, 175, 227, 318 



— Aspergillus niger, 178, 225, 227, 231, 

315,317 


— blue mold rot, 142, 317 

— Botryotinia fuckeliana, 316 

— Botrytis, 13, 142, 233 


227, 315, 316 

— Candida sp., 206 

— Cladosporium herbarum, 315, 316 

— Cladosporium rot, 316 

— Geotrichum, 142 

— gray mold, 13, 228, 316 

— Penicillium citrinum, 317 

— Penicillium cyclopium, 317 

— Penicillium expansum, 142, 317 


— Rhizopus, 24, 233 

— Rhizopus oryzae, 317, 318 

— Rhizopus rot, 231, 233, 317 

— Rhizopus stolonifer, 178, 225, 227, 315, 

317,318 

— Trichoderma harzianum, 240



Gray mold, 9, 12, 235 

— apples, 159, 249 



— grapes, 13, 228, 316 

— kiwifruits, 319 

— pears, 149, 229 


— raspberries, 310-312 

— strawberries, 9, 12, 13, 26, 118, 148, 

310-312,315 


Green mold, 5 


270 

— grapefruit, 189, 229, 244 


— oranges, 149, 216 

Green onions, 132, 133 

Growth regulators (plant hormones), 78, 

137-141, 272 

— abscissic acid, 139 

— auxin 137, 139, 141 

— cytokinin, 139 

— 2,4-dichlorophenoxyacetic acid (2,4-D), 

48, 137, 138, 272, 277 

— gibberehn (GA3), 138, 139 (see also 

Gibberellic acid) 

— h-naphtalene acetic acid (NAA), 141 

- with chemicals and modified 

atmosphere packaging (MAP), 141 

Guavas, 105, 291 


— Aspergillus spp., 291, 292 


— Ceratocystis paradoxa, 291 

— Colletotrichum gloeosporioides, 291 




— Gloeosporium, 292 


— Mucor spp., 291 


— Pestalotia spp., 201, 291, 292 

— Phoma spp., 291, 292 


— Rhizopus spp, 291 

— stem-end rot, 291 

Guazatine, 166 

Gum materials, 88, 90 


Hanseniaspora, grapes, 233 

Heat treatments, 189-195, 198, 199, 254, 

255, 282, 285, 286, 307, 309 

— accumulation of phytoalexins, 196, 254 

— enhanced resistance to infection, 195, 

196 

— induction of heat shock proteins, 196, 

197, 254 

— resistanse to chilling injury, 197 

— short- and long-term treatments, 190, 

193,197 

— with calcium, 204 

— with chemicals, 157, 159, 200-204, 282, 

283, 286, 292, 293, 309 

— with ethanol, 202, 203 

— with ionizing radiation, 204, 205, 

211-213,216 

— with rinsing, 205 

— with sucrose, 204 

— with ultraviolet illumination, 216 

Helminthosporium, 60, 62 

Helminthosporium solani, potato tubers, 

20, 174 

Hevein, 184 

1-Hexanol, 180 

Z-3-Hexen-l-ol, 180 

E-2-Hexenal, 180 

Hinokitiol, 180, 181 

Host-pathogen interactions, 51 

— pathogen attack and host defense 

mechanisms, 52, 53 

Host tissue acidity, effects on disease, 42, 43 

Host tissue growth stimuli, 43-45 

Human antibiotic compounds, 235 

Hydrogen peroxide, 170, 171, 318 

p-Hydroxybenzoic acid, 83 

H5êrsensitive response, 53, 91, 257-259 

H5T)obaric (low) pressure, 135-137, 254 

{see also Controlled Atmosphere) 

— effects on disease development, 137 

— effects on fungal growth, 136, 137 

Hypochlorite, 151, 153 

— calcium hypochlorite, 151, 175 

— sodium hypochlorite, 151, 175



Hypochlorous acid, 151, 152 

Imazalil, 163-165, 181, 201, 245, 247, 

276-278 


Induced resistance, 53, 195-197, 240, 

253-262 

— biological elicitors, 260-262 

— chemical elicitors, 257-260 

— physical elicitors, 253-257 

Inducible preformed compounds, 73-76, 241 

Inoculum level 

— effects on disease development, 35-37 

— reduction of, by sanitation, 150 

Ionizing (gamma) radiation, 205-217, 255, 

283 

— decay suppression in fruit, 209-211 

— effects on the pathogen, 206-209 

— induction of phytoalexins, 208, 209 

— sprouting inhibition and decay 

susceptibility, 211 

— with chemicals, 216 

— with heating, 211-213, 216 

— with heating and chemicals, 216 


(MAP), 217 


(MAP) and biocontrol agents, 217 

— with ultraviolet illumination, 216 

Iprodione, 149, 162, 163, 203, 228, 230, 

250, 314, 320 


Isonitrosoacetophenone, 208 

Isopimpenellin, 73 

Isopropyl alcohol, 154 

Isothiocyanate, 182 

Iturin, 184, 233 

Kiwifruit 


— Botryosphaeria, 319 


— Botrytis cinerea, 318-320 

— Colletotrichum, 319 

— Diaporthe actinidiae, 319 


— Penicillium spp., 318, 319 

— Phoma, 319 


— Phomopsis actinidiae, 318, 319 

— ripe rot, 319 

Kloeckera, grapes, 231 

Kumquats, 8, 81 

— Penicillium digitatum, 82, 218 

— ultraviolet treatment, 82, 218 

Lasiodiplodia theobromae, 239, 240 

— bananas, 227, 281 

Late blight 

— potato tubers, 11, 324 


Latex, 184 

Leak disease 


— raspberries, 310, 312 


Leather rot, strawberries, 310 

Lectins, 184-188 

— binding to different fungal cell walls, 

186, 187 

— interference with fungal growth, 187, 

188 

Lemons, 5, 8, 81, 88, 138, 270 {see also 

Citrus fruits) 


— Geotrichum candidum, 8, 117, 270 

— Gliocladium roseum, 230 

— green mold, 232 

— Paecilomyces variotii, 230, 231 

— Penicillium digitatum, 73, 81, 196, 202, 

232,245 

— Phytophthora citrophthora, 105, 199 

— sour rot, 117, 270 

— Trichoderma harzianum, 229, 230 


Lenticel rot, apples, 155, 159 

Lettuces 

— bacterial soft rot, 24, 140 



— Escherichia coli, 153 

— Sclerotinia sclerotiorum, 12, 140 




Lignin, lignification, 85, 87, 88, 91, 93, 

106, 107, 262



Limes 



Limetin, 73, 81 

Limonene, 7, 8, 44, 182 

Lipoxygenase, 17, 72, 74, 253, 260 

Litchis 

— Botryodiplodia theohromae, 292 



— Rhizopus spp., 292 

Lubimin, in potato tubers, 77, 209, 211 

Maceration, 56, 57, 104, 258 

Malformins, 62 

Mancozeb, 148, 328 

Mangoes, 13, 119, 215 

— Alternaria alternata, 17, 19, 20, 72, 

254,282 


— anthracnose, 25, 137, 148, 159, 201, 

213, 282 

— black spot, 282, 283 

— Botryodiplodia theobromae, 281, 282 


159, 199, 201, 204, 213, 268, 281, 282 


— Diplodia natalensis, 137 

— Diplodia spp., 95, 96, 283 

— Dothiorella spp., 282, 283 


— Pestalotia, 96 



— resorcinols in, 18, 19, 72, 75 

— stem-end rot, 137, 281-283 

— Trichoderma viride, 283 


Marmesin, 78 

Melons 

— blue-green mold, 24 

— Cladosporium spp., 23 

— Geotrichum candidum, 22, 166 

— Penicillium spp., 23, 24 

— pink mold, 24 



Metalaxyl (ridomil), 77, 166, 167, 278, 328 



— with etaconazole, 166 

Methyl saHcylate, 179 

6-Methoxymellein, 76, 83, 219, 255 

Metschnikowia pulcherrima, apples, 237 

Modified atmosphere (MA), 121, 122, 253, 

254, 320 

Modified atmosphere packaging (MAP), 

131-135, 250, 253, 254, 315 

— with biological control, 132 

— with chemical treatment, 132 

— with ethylene-scavenging materials, 134 

Moniliformin, 63 

Monilinia, 303 




Monilinia fructicola, 22, 55, 70, 130, 145, 

157, 191, 203, 206, 235 


— apricots, 9, 26 

— cherries, 132, 229, 233 

— nectarines, 26, 199, 204, 216, 309 

— peaches, 19, 26, 55, 66, 95, 176, 199, 

200, 233, 246, 309 

— plums, 26, 199, 216, 305 


— stone fruits, 13, 20, 36, 145, 159, 304 

Monilinia fructigena, 81 



Monilinia laxa 



Mucor, 4, 23 


— pears, 195, 248 





— strawberries, 26, 310, 312, 313 


Mucor hiemalis 

— soft fruits, 312 

Mucor piriformis 

— apples, 21, 301 

— blueberries, 313



— gooseberries, 313, 314 


— softfruits, 312, 313 



Mycosphaerella caricae, papayas, 148, 284 

Mycotoxins, 63 

— aflatoxin, 63 

— altenuene, 62, 322 

— alternariol, 62, 300 

— alternariol methyl ether, 63 

— alternariol monomethyl ether, 62, 322 

— citrinin, 63 

— fumonisin, 63 

— moniliformin, 63 

— patulin, 63, 64, 294, 295 

— penicillic acid, 63 

— tentoxin, 62 

— tenuazonic acid, 62, 63, 322 

— trichothecene, 63 

P-Myrcene, 7, 182 

Myrothecium, 63 

Natural antifungal compounds see 

Preformed antifungal compounds 

Nectarines, 204 {see also Stone fruits) 

— Botrytis cinerea, 44, 46, 216, 305, 306 


— Candida oleophila, 224 

— Monilinia fructicola, 26, 199, 204, 216, 

309 



309 

Nectria fruit rot, pome fruits, 296 

Nectria galligena 

— apples, 13, 18, 80, 107, 297 


Neryl-acetate, lemon peel, 73 

Nigrospora, pome fruits, 294 

Non-ripening tomato mutants, 98-101, 

103, 264, 328 

Nonanal, 6 

2-Nonanone, 180 

Onions 

— Fusarium, 24 

— ionizing (gamma) radiation, 211 



Ophiobolins, 62 

Oranges, 5, 7, 8, 81, 88 {see also Citrus 

fruits) 


— Candida saitoana, 245, 246 

— curing, 87 


— green mold, 149, 216 

— Penicillium digitatum, 149, 201, 204, 

216, 245 

— Phomopsis citri, 148 

— Phytophthora parasitica, 33 


Oxadixyl, 167 

Oxalic acid, 62 

Oxylubimin, in potato tubers, 77 

Paecilomyces variotii 

— lemons, 230, 231 


Papayas, 13, 88, 105, 119, 184, 205 

— Alternaria, 148, 283 

— anthracnose, 25, 148, 159, 283-285 

— benzylisothiocyanate in unripe, 71 

— Botryodiplodia, 148 

— Botryodiplodia theobromae, 283, 284 

— Cercospora, 285 

— ^chocolate spot' type lesions, 284 



25, 159, 200, 283, 284 

— Fusarium, 283 


— Mycosphaerella caricae, 148, 284 

— Phoma caricae-papayae, 283, 284 

— Phomopsis, 24, 283, 284 

— Phytophthora, 148, 200 

— Phytophthora palmivora, 283-285 


— Rhizopus rot, 23, 148, 285, 286 

— Rhizopus stolonifer, 24, 148, 285 

— Stem-end rot, 200, 283-285 

— Stemphylium, 283, 285 

Pathogenesis related (PR) proteins, 53, 92, 

93, 183, 219, 256, 258, 259 

Pathogens 

— list of main pathogens, diseases and 

hosts, 27-32



— penetration of, 11-24, 33, 34, 272, 285, 

289, 290-301, 304-309,311-314,316, 

317, 319-327 

Patulin, 63, 64, 294, 295 

Peaches, 70, 204 {see also Stone fruits) 


— brown rot, 19, 200, 201 

— Candida saitoana, 244 

— Monilinia fructicola, 19, 26, 55, 66, 95, 

176, 199, 200, 233, 246, 309 

— phenolic acids, 55 


— Rhizopus stolonifer, 26,149, 199, 306, 

309 


Pears, 67, 68, 132 {see also Pome fruits) 

— Alternaria alternata, 26, 229 {see also 

Side rot) 

— blue mold, 132, 149, 229, 249, 294 


305 

— Cladosporium herharum, 229 {see also 

side rot) 

— Cryptococcus spp., 229 

— Cryptococcus infirmo-miniatus, 229 

— Cryptococcus laurentii, 249, 250 

— Gloeosporium spp., 26 

— gray mold, 149, 229 

— Mucor, 195, 248 

— Penicillium expansum, 26, 41, 63, 132, 

149, 229, 231, 249 

— Phialophora, 195 

— Phialophora malorum, 229, 231, 249, 

250, 300 {see also side rot) 

— Pseudomonas cepacia ^ 249 

— side rot, 229, 249, 250, 300 

— Rhodotorula glutenis, 229 

— Stemphylium botryosum, 26 

Penicillic acid, 63 

Penicillium, 4, 56, 63 


— cucumbers, 23 



— kiwifruit, 318, 319 

— melons, 23, 24 






Penicillium citrinum, grapes, 317 

Penicillium crustosum, pome fruits, 294 

Penicillium cyclopium 



Penicillium digitatum, 5-9, 44, 56, 58, 69, 

87, 90, 130, 136, 155, 156, 163, 164, 

183, 192, 201, 206, 256, 262, 269, 270 

— citrus fruits, 5, 8, 21, 24-26, 41, 43, 94, 

95, 101, 105, 149,159, 160,163, 

165-167, 196, 199, 201, 213, 230, 247, 

268, 269, 275, 276, 277 

— grapefruits, 184, 189, 229, 244 

— kumquats, 82, 218 

— lemons, 73, 81, 196, 202, 232, 245 

— oranges, 149, 201, 204, 216, 245 

Penicillium expansum, 22, 63, 80, 142, 144, 

162, 183,191, 206, 237, 238 

— apples, 20, 22, 23, 26, 38, 63, 96, 142, 

143, 159, 176, 177, 199, 216, 224, 225, 

229, 231, 242, 244, 245, 261, 262, 301 


— grapes, 142, 317 



— pears, 26, 41, 63, 132, 149, 229, 231, 

249 

— pome fruits, 63, 293-295, 301-303 

— stone fruits, 63, 303, 307-310 

— sweet cherries, 250 

Penicillium funiculosum 


— pineapples, 229, 289 

Penicillium italicum, 6, 56, 58, 130, 155, 

156, 206, 269 

— citrus fruits, 21, 24-26, 95, 159, 160, 

163, 165,166, 230, 268, 269, 276 


Penicillium verrucosum, pome fruits, 294 

Peppers {see also Solanaceous fruit 

vegetables) 


— bacterial soft rot, 237, 322, 323 


— capsenone, 80 

— Cladosporium rot, 327


410 


— Erwinia carotovora ssp. atroseptica, 

322 

— Erwinia carotovora ssp. carotovora, 322 


— Phytophthora capsici, 79 


Peracetic acid, 172 

Peroxidase, 85, 93, 106, 107, 182, 241, 256, 

262 

Persimmons 

— Alternaria alternata, 20, 138, 290 

— black spot, 138 




— Mucor, 290 


— Phoma, 290 


Pestalotia 



— guavas, 201, 291, 292 


Pestalotiopsis, avocados, 222 

Pestalotiopsis versicolor, avocados, 286 

Pezicula alba , pome fruits, 296 

Pezicula malicorticis 



Phenolic compounds, 16, 55, 85, 87, 90, 93, 

107, 182, 258, 259, 306 

Phenylalanine ammonia liase (PAL), 91, 

195, 208, 240, 241, 256, 261 

Phialophora, pears, 195 

Phialophora malorum, 229 

— pears, 229, 231, 249, 250, 300 

Phoma 

— guavas, 291, 292 




Phoma caricae-papayae 

— papayas, 283, 284 

Phomopsis 

— avocados, 222, 286, 287 



Subject Index 

— eggplants, 326, 329 



— papayas, 24, 283, 284 



Phomopsis actinidiae 

— kiwifruit, 318, 319 

Phomopsis citri, 206 


272, 277 



Phomopsis mali 


Phomopsis vaccinii 


Phomopsis vexans 


Physalospora obtusa 


Physalospora rhodina 




Phytoalexins, 15, 18, 52, 76-84, 107, 196, 

208, 209, 211, 218, 219, 241, 255, 256, 

258, 261, 297, 301, 326 

— benzoic acid, 18, 69, 80, 81, 297, 301 

— capsenone, 80 

— capsicannol, 80 

— capsidiol, 79, 80 

— chlorogenic acid, 107 

— columbianetin, 78, 79 

— esculetin, 78 

— p-hydroxybenzoic acid, 83 

— isonitrosoacetophenone, 208 

— lubimin, 77, 209, 211 

— marmesin, 78 

— 6-methoxymellein, 76, 83, 219, 255 

— oxylubimin, 77 

— phytuberin, 77 

— phytuberol, 77 

— psoralen, 78 

— rishitin, 76, 77, 209, 211 

— rishitinol, 77 

— scoparone, 81, 82, 196, 208, 218, 241, 

254, 255, 261,



scopeletin, 78, 208,241,255,261 

scopolin, 208 

solavetivone, 77 

umbelliferone, 78 

Phytophthora, 11, 12, 23 

citrus fruits, 166, 167, 278 

papayas, 148, 200 

pome fruits, 293,302 

solanaceous fruit vegetables, 320,324, 

329 

stone fruits, 36 

strawberries, 310 

Phytophthora cactorum, pome fruits, 298 

Phytophthora capsici, solanaceous fruit 


Phytophthora capsici, peppers, 79 


citrus fruits, 11, 147,199,272 

lemons, 8, 105, 199 

oranges, 166 

Phytophthora hibernalis, citrus fruits, 272 

Phytophthora infestans, 11,328 

potato tubers, 11, 23, 76, 77, 167, 324 

tomatoes, 324, 328,329 

Phytophthora nicotianae var. parasitica 

citrus fruits, 272 

solanaceous fruit vegetables, 324 

Phytophthora palmivora, papayas, 

283-285 

Phytophthora parasitica, 167 


oranges, 33 

Phytophthora rot, solanaceous fruit 

vegetables, 324, 325 

Phytophthora syringae, pome fruits, 298 

Phytuberin, in potato tubers, 77 

Phytuberol, in potato discs, 77 

Pichia 


grapes, 233 

Pichia guilliermondii, 237-240 

apples, 261 

citrus fruits, 246,247, 261 

grapefruits, 229, 236,244 

peaches, 261 

tomatoes, 232, 236,328 

Pineapples 

Acetobacter, 289 


black rot, 159, 288, 290 

m Ceratocystis paradoxa, 105, 159,288 

Cladosporium, 289 

Curvularia verruculosa, 105 

Erwinia, 289 

m fruitlet core rot, 288, 289 

Fusarium moniliforme var. 

subglutinans, 289 

Penicillium funiculosum, 229,289 

Pseudomonas, 289 

Thielaviopsis paradoxa, 288 

Trichoderma, 289 

(~-Pinene, 7, 182 

~-Pinene, 182 

Pink mold 

apples, 23 

melons, 24 

Plant extracts, 181, 182 

Pleospora herbarum 

apples, 21 

pome fruits, 300 

solanaceous fruit vegetables, 327 

Plums (see also Stone fruits) 

Botrytis cinerea, 44, 46, 216, 305,306 

Monilinia fructicola, 26,199, 216, 305 

Rhizopus stolonifer, 26,216 

Pome fruits (see also Apples and Pears) 

Alternaria alternata, 293,299, 302 

Alternaria rot, 299 

Aspergillus spp., 294, 302 

Bacillus subtilis, 303 

blue mold rot, 294 

Botryosphaeria obtusa, 298 

Botryosphaeria ribis, 298 

Botryosphaeria spp., 293,298 

Botryotinia fuckeliana, 295 

Botrytis cinerea, 293,295,301-303 

Candida oleophila, 302 

Candida sake, 303 

Cladosporium herbarum, 293,300, 302 

Colletotrichum gloeosporioides , 293 

Colletotrichum spp., 303 

Cylindrocarpon fruit rot, 296 

Cylindrocarpon mali, 296 

Dothiorella, 298 

Epicoccum, 294 

Fusarium, 294 

Gloeosporium album, 293,296



— Gloeosporium perennans, 293, 296, 301 

— Gloeosporium rot, 296 

— Gloeosporium spp., 294, 301, 302 



— Monilinia fructicola, 295 

— Monilinia fructigena, 295 

— Monilinia laxa, 295 

— Monilinia spp., 303 

— Mucor piriformis, 298, 301, 302 


— Nectria fruit rot, 296 

— Nectria galligena, 293, 296, 301 

— Nigrospora, 294 

— Penicillium crustosum, 294 

— Penicillium cyclopium, 294 

— Penicillium expansum, 63, 293-295, 

301-303 

— Penicillium verrucosum, 294 

— Pezicula alba, 296 

— Pezicula malicorticis, 229, 296, 303 


— Physalospora obtusa, 298 

— Phytophthora cactorum, 298 


— Phytophthora syringae, 298 

— Pleospora herbarum, 300 

— Rhizopus oryzae, 299 

— Rhizopus spp., 293, 299, 303 


— Rhodotorula, 303 

— Sphaeropsis, 298 

— Stemphylium botryosum, 293, 300 

— Trichoderma spp., 294 

— Trichothecium roseum, 293, 300 

Pomelo, 88 {see also Pummelo) 

Potassium sorbate, 174 

Potato dry rot, 77, SQ, 93 

Potato tubers, 86, 211, 214 

— bacterial soft-rot, 20, 22, 23, 44, 86, 142 

— Erwinia carotovora, 20, 77 

— Erwinia carotovora pv. atroseptica, 142 


— Fusarium dry rot, 243 (see also Potato 

dry rot) 

— Fusarium sambucinum, 77, 85, 93, 106, 

243 

— Helminthosporium solani, 20, 174 


— ionizing (gamma) radiation, 211, 214 

— late blight, 11,324 

— potato dry rot, 77, 86, 93 

— Phytophthora infestans, 11, 23, 76, 77, 

167, 324 

— silver scurf, 20 

— Trichoderma harzianum, 230 

Prangolarin, 44 

Preformed antifungal compounds, 15, 18, 

52, 69-73, 182, 195, 241, 279 

— benzylisothiocyanate, 71 

— caffeic acid, 55, 70 

— chlorogenic acid, 17, 55, 69, 70 

— citral, 6, 73, 138 

— diene and monoene compounds, 16, 46, 

71, 72, 74, 253, 260, 261 

— 3,4-dihydroxybenzaldehyde, 16 

— falcarindiol, 75 

— falcarinol, 76, 83 

— ferulic acid, 69, 107 

— 5-geranoxy-7-metoxy coumarin, 73 

— isopimpenellin, 73 

— isothiocyanates, 182 

— limetin, 73, 81 

— phenolic compounds, 16, 55, 85, 87, 90, 

93, 107, 182, 258, 259, 306 

— phytuberin, 77 

— resorcinols, 17, 19, 46, 51, 72, 75, 254, 

282 

— tannins, 46, 67, 71 

— tomatidine, 71 

— tomatine, 16, 46, 64, 70, 71 

Preharvest and harvesting effects, 33-35 

Prochloraz, 141, 165, 180, 181 260, 277, 

288 

Propanal, 178 

Propionic acid, 172 

Pseudocercospora purpurea, avocados, 286 

Pseudomonas, 223, 249 




Pseudomonas cepacia, 184, 233 

— apples, 234, 249 


— pears, 249 

Pseudomonas fluorescence, lemons, 235 

Pseudomonas putida, strawberries, 223



Pseudomonas syringae, 64 

— apples, 242, 244 



Pseudomonas syringae pv. syringae, citrus 

plants, 226 

Pseudomonas syringae pv. tomato, 

tomatoes, 266 

Psoralen, 78 

Pummelo, 89 (see also Pomelo) 

Pyrrolnitrin, 184, 234, 235, 249 

Pythium aphanidermatum, cucumbers, 

105 

Pythium, solanaceous fruit vegetables, 321 

Quaternary ammonium compounds, 154 

Quiescent infections (latent infections), 

12-19, 25, 69, 70, 90, 148, 155, 159, 

229, 260, 271, 279, 281, 282, 284-287, 

290, 293, 295-297, 301, 305, 311, 312, 

316, 319, 320, 326 

— mechanisms of quiescence, 14-19 

Rambutans, 240 



— brown spot, 240 


— Gliocephalotrichum microchlamydosporum, 

240 


— Trichoderma harzianum, 240 

Raspberries {see also Soft fruits) 







— gray mold, 310-312 

— leak' disease, 310, 312 

— Mucor, 310 

— Mucor hiemalis, 312 


— Rhizopus, 310, 312, 313 

— Rhizopus sexualis, 312 


Resorcinols, 17, 19, 46, 51, 72, 75, 254, 282 


Respiration following infection, 94-97 

Rhizoctonia rot, solanaceous fruit 


Rhizoctonia solani, solanaceous fruit 

vegetables, 320, 325 

Rhizopus, 4, 56, 62, 303 



— grapes, 24, 233 






— raspberries, 310, 312, 313 


— strawberries, 24, 199, 310, 312, 313 


Rhizopus oryzae 

— grapes, 317, 318 




Rhizopus rot 

— grapes, 231, 233, 317 

— papaya, 23, 148, 285, 286 

— soft fruits, 310, 312, 313 



Rhizopus sexualis 


Rhizopus stolonifer, 9, 10, 22, 38, 123, 183, 

191, 203, 206, 224, 312 

— apricots, 26 


— carrots, 9, 117 

— grapes, 178, 225, 227, 315, 317, 318 

— nectarines, 26, 199, 216, 306, 309 

— papayas, 24, 148, 285 

— peaches, 26, 149,199, 306, 309 

— plums, 26, 216 




328 

— stone fruits, 157, 303, 306, 308, 309 

— strawberries, 26, 117, 118, 177, 189, 

228, 312, 315



— sweet potatoes, 86, 157 

— tomatoes, 26, 95, 100, 103,106, 117, 

225, 231, 236 

Rhodotorula 

— pears, 233 


Rhodotorula glutinis 


— pears, 229 

Ripe rot, kiwifruit, 319 

Ripening stage, effect on disease 

development, 45-47 

Rishitin 

— in potato tubers, 76, 77, 209, 211 

— in tomato fruits, 209 

Rishitinol, in potato tubers, 77 

Sabinene, 7 

Saccharomyces cerevisiae, lemons, 232 

Salicylic acid, 259, 260 

Sanitation, 150-154, 275, 286, 303, 308, 

318,327 

Sanosil-25, 171 

Scab fungus, apples, 23 

Sclerotinia, 4, 56 







Sclerotinia fructicola, apples, 67 

Sclerotinia fructigena, apples, 81, 100 

Sclerotinia sclerotiorum, 62, 69, 70, 183, 

255 


— carrots, 12, 42, 219 

— cauliflowers, 12 

— celery, 12, 26, 78, 79, 130, 140 


— lettuces, 12,140 

Sclerotium rolfsii, 62, 239 

Scoparone, 81, 82, 196, 208, 218, 241, 254, 

255,261 

Scopeletin, 78, 208, 241, 255, 261 

Scopolin, 208 

Sec-butylamine, 155-157 


Septoria lycopersici, 65 

Side rot, pears, 229, 249, 250, 300 

Siderophores, 9 

Silver scurf, potato tubers, 20 

Sodium carbonate, 275 

Sodium or^/io-phenylphenate (SOPP), 155, 

156, 275, 290, 302 


Soft fruits, 310-313 




— Rhizopus rot, 310, 312, 313 

— Rhizopus sexualis, 312 


Soft scald, 23 

Soft watery rot/decay (watery soft rot) 


— carrots, 9, 12 

— cauliflowers, 12 

— celery, 12, 130 







Solanaceous fruit vegetables, 320-329 (see 

also Tomatoes, Peppers, Eggplants) 

— Alternaria alternata, 320-322, 327, 328 


— Aspergillusy 321 


— Botrytis cinerea, 320-322, 327, 328 

— buckeye rot, 325 

— Cladosporium, 321, 327 

— Cladosporium herbarium, 327 

— Colletotrichum, 321, 326 

— Colletotrichum capsici, 326 

— Colletotrichum coccodes, 326 


— Diaporthe, 326 


— Erwinia carotovora ssp. atroseptica, 

322 

— Erwinia carotovora ssp. carotovora, 322 

— Fusarium avenaceum, 324 

— Fusarium equiseti, 324



— Fusarium oxysporum, 324 

— Fusarium rot, 324 

— Fusarium spp., 320, 324, 327 

— Geotrichum candidum, 320, 323, 327, 

328 

— ghost spots, 321 

— Glomerella spp., 326 



— Phoma, 321 

— Phomopsis, 321, 326 

— Phytophthora capsici, 325 

— Phytophthora infestans, 328, 329 

— Phytophthora nicotianae var. 

parasitica, 324 

— Phytophthora rot, 324, 325 



— Pleospora herharum, 327 

— Pythium, 321 

— Rhizoctonia rot, 325 

— Rhizoctonia solani, 320, 325 


— Rhizopus rot, 323 

— Rhizopus stolonifer, 320, 323, 328 

— Sclerotinia, 321 

— soft rot bacteria, 322, 327 

— Stemphylium, 321, 327 

— Stemphylium hotryosum, 327 

— Thanatephorus cucumeris, 325 


Solavetivone, in potato tubers, 77 

Sour rot 

— citrus fruits, 22, 26, 166, 167, 270, 276 


— lemons, 117, 270 

— hmes, 270 


— tomatoes, 22, 23 

Sphaeropsis spp., pome fruits, 298 

Sporobolomyces roseus, apples, 242, 244 

Squash 





Stachybotrys, 63 

Staphylococcus, avocados, 223 


Steam, 34, 154, 275 

Stem-end rot, 48 

— avocados, 286-288 

— citrus fruits, 13, 137, 138, 156, 157, 

159, 166, 271, 277 


— mangoes, 137, 281-283 


— papayas, 200, 283-285 



Stemphylium, 4, 200 


— papayas, 283, 285 


Stemphylium botryosum, 206 

— apples, 23, 26 

— pears, 26 



Stone fruits, 26 



— Alternaria rot, 308, 310 

— Aureobasidium, 310 

— Bacillus subtilis, 309, 310 


— Botrytis cinerea, 303, 305, 308 

— brown rot, 13, 36, 145, 159, 304, 305, 

309, 310 


— Cladosporium herbarum, 303, 308 

— Colletotrichum gloeosporioides, 303, 

307 

— Enterobacter cloacae, 310 

— Epicoccum, 310 

— Gloeosporium, 36 


— Monilinia fructicola, 13, 20, 36, 145, 

159,304 

— Monilinia fructigena, 304 

— Monilinia laxa, 175, 304 

— Monilinia spp., 303, 304, 309 

— Mucor piriformis, 303, 307, 308 


— Penicillium expansum, 63, 303, 

307-310 

— Pseudomonas, 310





— Rhizopus rot, 306, 309 


309 

— soft watery rot, 26, 157 

Storage conditions, 37-42 

— atmosphere gas composition, effect on 

disease, 42 {see also Controlled 

atmosphere and Modified atmosphere) 

— minimal temperatures for postharvest 

fungi, 39 

— relative humidity, effects on disease, 

40-42 

Strawberries, 9, 209, 223 (see also Soft 

fruits) 




— Aureobasidium pullulans, 228, 315 


— Botrytis, 199, 230 

— Botrytis cinerea, 9, 12, 13, 22, 24, 26, 

34, 50, 118, 131, 148, 177, 179, 189, 

223, 228, 230, 305, 310, 311, 314, 315 

— Botrytis rot, 34 

— Chryseobacterium, 315 

— Chryseobacterium indologenes, 223 

— Cladosporium, 199, 311 



— Gliocladium roseum, 230 

— gray mold, 9,12, 13, 26, 118, 148, 

310-312, 315 


— leather rot, 310 

— Mucor, 26, 310, 312, 313 



— Paecilomyces variotii, 230 

— Phytophthora spp., 310 

— Pseudomonas, 315 

— reducing ethylene, 51 

— Rhizopus, 24, 199, 310, 312, 313 

— Rhizopus sexualis, 312, 313 


189, 228, 312, 315 

— soft watery rot, 26, 312 


— Trichoderma, 228, 230, 315 

— Trichoderm harzianum, 229, 230 


— wrapping, 131 

Suberin, suberization, 85-87, 90, 93 

Sucrose, with hot water, 204 

Sugar analogs, 175, 176, 244 

Sulfur dioxide, 158, 159, 178, 202, 318 


Sweet cherries (see also Cherries) 

— brown rot, 132, 201, 250, 310 

— Cryptococcus, 236, 237 

— Cryptococcus infirmo-miniatus, 250 

— Cryptococcus laurentii, 250 

— Monilinia fructicola, 132, 229, 233 


Sweet peppers (see also Solanaceous fruit 

vegetables) 





Sweet potatoes, 78, 86 



— Ceratocystis fimbriata, 51, 78 

— Mucor, 117 


— Rhizopus stolonifer, 86, 157 

— soft watery rot, 157 


Tangerines, 8, 257 (see also Citrus fruits) 

— anthracnose, 49, 51, 257, 273 


257, 273 

Tannins, 46, 67, 71 

Terpene compounds, 7, 8, 43 

Tentoxin, 62 

Tenuazonic acid, 62, 63, 322 

Thanatephorus cucumeris, solanaceous 

fruit vegetables, 325 

Thiabendazole, 140, 141, 149, 159, 164, 

200, 247, 249, 277, 287, 290 

Thiophanate-methyl, 159 

Thielaviopsis paradoxa, pineapples, 288 

Thyronectria, avocado, 222 

Tomatidine, 71



Tomatine, 16, 46, 64, 70, 71 

Tomatoes, 63, 211 (see also Solanaceous 

fruit vegetables) 

— Alternaria alternata, 20, 23, 26, 50, 58, 

71, 117, 231, 236, 322 

— Alternaria rot, 216 


— bacterial soft rot, 23, 34, 127, 237, 322, 

323 


— Botrytis cinerea, 13, 16, 22, 23, 33, 34, 

51, 66, 95, 96, 98, 100, 101, 225, 231, 

232, 236, 321 

— Botrytis rot, 327 

— Cladosporium rot, 327 


- Erwinia carotovora, 121, 237 

- Erwinia carotovora ssp. atroseptica, 

322 

- Erwinia carotovora ssp. carotovora, 

322 

— Fusarium oxysporum f. sp. lycopersici, 

65 

— Geotrichum candidum, 22, 23, 26, 95, 

98, 101, 323 


— late blight, 324 

— non-ripening mutants, 98-101, 103, 

264,328 

— Phytophthora infestans, 324, 328, 329 



106, 117, 225, 231, 236 

— soft watery rot, 26 

— sour rot, 22, 23 


Toxins, 52, 59-64 

— host-specific, 59-61 

— non-host-specific, 61-64 {see also 

mycotoxins) 

- colletotrichin, 62 

- fumaric acid, 62 

- fusaric acid, 62 

- malformin, 62 

- ophiobolin, 62 

- oxalic acid, 62 

Transgenic resistant plants, 262-265 

— source of genes, 263, 264 


Trichoderma, 63, 232, 239 


— citrus fruits, 275, 279 



— strawberries, 228, 230, 315 

Trichoderma harzianum, 229, 239, 240, 

245 


— lemons, 229, 230 




Trichoderma longibrachiatum, 245 

Trichoderma rot, citrus fruits, 274, 275, 

279 

Trichoderma viride, 206, 239, 240 

— bananas, 227, 281 


— lemons, 229, 230 



Trichothecenes, 63 

Trichothecium, 22 




Trichothecium roseum, 206 

— apples, 21, 23 





Tropical and subtropical fruits 

— anthracnose, 13, 25, 148, 155, 159 

— Colletotrichum gloeosporioideSy 13, 148, 

155, 159 


— Dothiorella gregaria, 25 

— Phomopsis citri, 25 

Ultraviolet (UV) illumination, 81, 82, 

217-220, 255-257 

— inducement of disease resistance, 217, 

218, 220, 255, 256 

— inducement of pathogenesis related 

(PR) proteins, 219


418 

— inducement of phytoalexins, 218, 219, 

255 

— with ionizing radiation, 216 

Umbelliferone, 78 

Y-Valerolactone, 179 

Venturia inaequalis, apples, 23 

Verticillium theohromae, bananas, 279 

Vibrio, 223 

VinclozoHn, 149,162,163, 228, 287, 315, 320 


Watermelons, Alternaria cucumerina, 105 Zucchini, Alternaria rot, 23 

Subject Index 

Watery soft rot see Soft watery rot/decay 

White watery rot 






Wound healing, 52, 84-90, 220 

Xanthomonas, 223

Post harvest diseases fruits and vegetables - Xavier University ...

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