Sweetpotato guide to major pests, diseases and disorders

Sweetpotato:
Major Pests,
Diseases, and
Nutritional
Disorders
T. Ames, N.E.J.M. Smit, A.R. Braun,
J.N. O’Sullivan, and L.G. Skoglund

ISBN 92-9060-187-6

Sweetpotato: Major
Pests, Diseases, and
Nutritional Disorders

T. Ames, N.E.J.M. Smit, A.R. Braun,
J.N. O’Sullivan, and L.G. Skoglund

International Potato Center (CIP)

C O N T E N T S

The International Potato Center (CIP) is a
scientific, nonprofit institution dedicated to the
increased and more sustainable use of potato, Page
sweetpotato, and other roots and tubers in the Foreword vii
developing world, and to the improved
management of agricultural resources in the Acknowledgments viii
Andes and other mountain areas. CIP is part
of the global agricultural research network
known as the Consultative Group on Introduction 1
International Agricultural Research (CGIAR). CGIAR
Insect Pests of Sweetpotato and
Their Management 3
International Potato Center
Apartado 1558 Storage Root Feeders 4
Lima 12, Peru Sweetpotato Weevils (Cylas spp.) 4
West Indian Sweetpotato Weevil (Euscepes
ISBN 92-9060-187-6 postfasciatus) 10
Press run: 1000 Rough Sweetpotato Weevil (Blosyrus sp.) 12
Printed in Lima, Peru
August, 1997
Clearwing Moth (Synanthedon spp.) 14
Peloropus Weevil (Peloropus batatae) 14
Cover: Photo of chlorotic spots with and without purple margins induced White Grubs 15
by SPFMV (taken by S. Fuentes).
Stemborers and Feeders 16
T. Ames, N.E.J.M. Smit, A.R. Braun, J.N. O’Sullivan, and L.G. Skoglund. Clearwing Moth (Synanthedon spp.) 16
1996. Sweetpotato: Major Pests, Diseases, and Nutritional Disorders.
International Potato Center (CIP). Lima, Peru. 152 p.
Sweetpotato Stemborer (Omphisia anastomasalis) 18
Striped Sweetpotato Weevil (Alcidodes dentipes and
1. Sweetpotato – Insect pests. 2. Sweetpotato – Diseases. A. erroneus) 22
3. Sweetpotato – Nutritional disorders. 4. Sweetpotato – Integrated manage- Sweetpotato Weevils (Cylas spp.) 24
ment. I. International Potato Center.
Minor Stemborers and Feeders 25
Peloropus Weevil (Peloropus batatae) 25
Sweetpotato Bug (Physomerus grossipes) 26
Long-Horned Beetle 26

Foliage Feeders 28
Sweetpotato Butterfly (Acraea acerata) 28
Tortoiseshell Beetles (Aspidomorpha spp. and others) 30
Sweetpotato Hornworm (Agrius convolvuli) 34
Armyworms (Spodoptera eridania, S. exigua, S. litura) 36

iii

Page Page
Leaf Folders (Brachmia convolvuli, Herpetogramma Sweetpotato Sunken Vein Virus (SPSVV)
hipponalis, and others) 40 (Whitefly-transmitted closterovirus) 68
Strobiderus Beetle (Strobiderus aequatorialis) 43 Sweetpotato Virus Disease (SPVD) 69
Rough Weevil (Blosyrus sp.) 43 Sweetpotato Mild Mottle Virus (SPMMV)
Sweetpotato Weevils (Cylas spp.) 43 (Whitefly-transmitted potyvirus) 70

Minor Leaf Feeders 44 Other Viral Diseases 71
Grasshoppers and Locusts (Zonocerous variegatus,
the variegated grasshopper and Attractomorpha Bacterial Diseases 72
psitticina, the slant-faced grasshopper, Bacterial Stem and Root Rot (Erwinia chrysanthemi)72
and others) 44 Bacterial Wilt (Pseudomonas solanacearum) 74
Soil Rot (Streptomyces ipomoea) 76
Virus Transmitters 46
Aphids (Aphis gossypii and others) 46 Foliar and Stem Diseases Caused
Whiteflies (Bemisia tabaci) 48 by Fungi 78
Leaf and Stem Scab (Elsinoe batatas, Sphaceloma
Mites 50 batatas) 78
Erinose caused by Eriophyid mites, Aceria sp. 50 Alternariosis, Anthracnose, Blight (Alternaria bataticola) 80
Eriophyes gastrotrichus 50 Phomopsis Leaf Spot (Phyllosticta Leaf Spot)
(Phomopsis ipomoea-batatas
Natural Enemies 52 (Phyllosticta batatas)) 82
Earwigs 53 Minor Leaf Spot Fungi 84
Spiders 53 Chlorotic Leaf Distortion (Fusarium lateritium) 86
Pheidole and Other Predacious Ants 54 Fusarium Wilt (Fusarium oxysporum f. sp. batatas) 88
Beetles 56 Violet Root Rot (Helicobasidium mompa) 90
Flies and Parasitic Wasps 58 Sclerotial Blight and Circular Spot (Sclerotium rolfsii) 92
Viruses 61 Black Rot (Ceratocystis fimbriata) 94
Fungal Pathogens (Metarrhizium anisopliae and
Beauveria bassiana) 62 Storage Root and Postharvest
Diseases 96
Diseases and Pathogens of Sweet- Foot Rot (Plenodomus destruens) 96
potato and Their Management 65 Java Black Rot (Lasiodiplodia theobromae (Diplodia
gossypina)) 98
Viral Diseases 66 Charcoal Rot (Macrophomina phaseolina) 100
Sweetpotato Feathery Mottle Virus (SPFMV) Soft Rot (Rhizopus stolonifer, Mucor sp.) 102
(Aphid-transmitted potyvirus) 66

iv v

Foreword

Page This field guide presents information on common pests,
Diseases Caused by Nematodes 104 diseases and nutritional disorders of sweetpotato. It is
Root-Knot Nematode (Meloidogyne spp.) 104 intended primarily as a tool for correct identification of
Brown Ring (Ditylenchus destructor, D. dipsaci) 106
these problems and ailments, as an essential first step
Reniform Nematode (Rotylenchulus reniformis) 108
Lesion Nematode (Pratylenchus spp.) 110 in their control.

Disorders of Unknown Origin 112 The principal entries in the guide are accompanied
Fasciation 112 by photographs or illustrations, and pinpoint where specific
problems occur. Additional information is provided on
Nutritional Disorders and Their symptoms and recommended control practices, with
Management 115 emphasis on integrated crop management.
Causes of Nutritional Disorders 116 We believe that researchers, extension agents, stu-
Diagnosing Nutritional Disorders 117
Correcting Nutritional Disorders 118 dents and farmers alike will find this guide useful, as
Nutrient Requirements of Sweetpotato 120 it can assist them in controlling pests and diseases while
Nitrogen Deficiency 122 safeguarding the natural environment needed for sus-
Phosphorus Deficiency 124 tainable agriculture.
Potassium Deficiency 130
Magnesium Deficiency 132 Wanda Collins
Boron Deficiency 134 Deputy Director General for Research
Iron Deficiency 136 International Potato Center
Acid Soils and Aluminum Toxicity 138
Salinity 140

Literature Consulted 142

About the Authors 146

Photo Credits 147

Index 148

vi vii

Acknowledgments Introduction

The information on nutritional disorders of sweetpotato The purpose of this field guide is to aid researchers
presented in this field guide was derived from a research and extensionists throughout the world in the identification
project of the Australian Centre for International Agricultural of common pests, diseases, and nutritional disorders of
Research (ACIAR). The author of the section on nutritional sweetpotato (Ipomoea batatas). This guide is based on
disorders, J.N. O’Sullivan, is a member of the project based the authors’ experiences with the crop in various regions
at the University of Queensland, Australia. of the world since 1990.

We would like to thank virologist Richard W. Gibson In contrast to most other major staple food crops,
of the Natural Resources Institute for contributing to the sweetpotato is able to produce a comparatively high yield
section on virus diseases. under relatively adverse conditions; however, a number
of pests, diseases, and nutritional disorders affect the
crop. Among the pest and disease constraints, sweetpotato
weevils (Cylas spp.) and virus diseases probably contribute
the most to yield losses, although leaf-feeding insects,
such as the sweetpotato butterfly (Acraea acerata), can
cause significant losses during outbreaks. Nutritional
disorders can cause slight to complete yield loss, and
are the main factor limiting most unfertilized crops. They
may also reduce tolerance of pests and diseases.

The basis for successful management of sweetpotato
pests, diseases, and nutritional disorders is integrated crop
management. This implies prevention of insect infestation
and infection by pathogens through the use of adequate
cultural practices, and the conservation of natural enemies.
Adequate cultural practices include the selection of healthy
planting material from well-adapted varieties, rotation, good
field sanitation, and maintenance of soil fertility. Conser-
vation of natural enemies involves avoidance of pesticide
use, enhancement of natural enemy action through favorable
cultivation practices, and the introduction of natural enemies
if necessary.

viii 1

Insect Pests of Sweetpotato
and Their Management

Integrated crop management for sweetpotato is cov- Many insect species attack sweetpotato and the im-
ered comprehensively in the Farmer Field School Guide portance of different species varies between agroecological
for Sweetpotato Integrated Crop Management. This pub- zones. Within a zone, the importance of a species depends
lication can be obtained from the CIP Regional Office for on the season; many insect pests are a problem mainly
East and Southeast Asia and the Pacific or from CIP in dry periods.
headquarters in Lima.1 Nutritional disorders are covered
comprehensively in the publication Nutritional Disorders In this guide, we divide pest species into three groups
of Sweet Potato,2 available from the Australian Centre for according to whether the damage is caused to leaves,
International Agricultural Research. stems, or roots. Defoliation reduces yield depending on
the severity of infestation and the growth stage of the
The photographs in this publication were assembled sweetpotato crop in which it occurs. In some areas,
from the collections of the authors and their colleagues, sweetpotato is grown for its foliage and leaf feeders can
and are acknowledged as such. be a problem. In addition to feeding, certain insects, such
as aphids and whiteflies, transmit viruses. Extensive stem
damage can result in wilting or even in death of the plant.
Damage to the vascular system caused by insect feeding
and tunneling and pathogen invasion of the injured tissue
can reduce the size and number of storage roots. Damage
to storage roots, the plant part generally consumed by
humans, is of two kinds. External damage results in a
loss of quality. Although roots with external damage may
bring a lower market price or be unsaleable, they can
often still be consumed in the farm household. Internal
damage often causes complete loss.
1 To order copies of the
Farmer Field School Guide for Sweetpotato Integrated
Crop Management, contact the CIP-ESEAP Regional Office, Fax: 62-251-316264;
email: cip-bogor@cgnet.com or CIP headquarters, Fax: 51-1-435-1570; email:
cip@cgnet.com .

2 Nutritional Disorders of Sweet Potato is available from the Australian Centre

for International Agricultural Research (ACIAR), GPO Box 1571, Canberra 2601,
Australia, Fax: 61-6-2170501, email: aciar@aciar.gov.au .

2 3

Storage Root Feeders

Sweetpotato Weevils
Cylas spp. (Coleoptera: Curculionidae)

Description. Three species of the genus Cylas are
pests of sweetpotato; they are commonly called sweetpotato 1 2
weevils. All three species—Cylas formicarius, C. puncticollis,
and C. brunneus—are found in Africa. C. formicarius is
present in Asia and in parts of the Caribbean. The elongated
ant-like adults of the three species can be distinguished
from each other.

Cylas puncticollis is the easiest to distinguish because
the adult is all black and larger than the other two (Fig. 3 4
1). C. formicarius has a bluish black abdomen and a reddish
brown thorax. C. brunneus adults are small and not uniform
in coloring. The most common type can easily be confused
with C. formicarius.

In all species, the eggs are shiny and round (Fig. 2).
The legless larvae (Fig. 3) are white and curved, and the
pupae are white (Fig. 4).
6
Damage. Damage symptoms are similar for all three
species. Adult sweetpotato weevils feed on the epidermis
of vines and leaves. Adults also feed on the external surfaces
of storage roots, causing round feeding punctures, which
can be distinguished from oviposition sites by their greater
depth and the absence of a fecal plug (Fig. 5). The developing
larvae of the weevil tunnel in the vines and storage roots,
causing significant damage. Frass is deposited in the 5
tunnels. In response to damage, storage roots produce
toxic terpenes, which render storage roots inedible even
at low concentrations and low levels of physical damage.
Feeding inside the vines causes malformation, thickening,
and cracking of the affected vine (Fig. 6).

4 5

Distribution and importance. Cylas weevils At 27°C, C. puncticollis has a total development time
are serious pests of sweetpotato worldwide, especially of about 32 days, whereas C. brunneus takes 44 days.
in drier agroecological zones. They are often the most Adults of the first species live an average of 100 days,
significant sweetpotato pest. whereas the latter dies after about 2 months. C. puncticollis
females lay 90–140 eggs in their lifetime, whereas C.
Distribution. C. formicarius is an important pest brunneus females lay 80–115.
in India and Southeast Asia, Oceania, the United States,
and the Caribbean. In Africa, it has been found only in Control. When sweetpotato weevil populations are
Natal, South Africa, and at one location in coastal Kenya. high, no single control method provides adequate pro-
C. puncticollis and C. brunneus are confined to Africa. tection. The integration of different techniques, with
emphasis on the prevention of infestation, provides
Biology. All sweetpotato weevil species have a sustainable protection.
similar life history. The adult female lays eggs singly in
cavities excavated in vines or in storage roots, preferring Cultural control. Cultural practices have proved
the latter. The egg cavity is sealed with a protective, gray to be effective against the sweetpotato weevil and should
fecal plug. The developing larvae tunnel in the vine or be the main basis of control. Cultural practices include:
storage root. Pupation takes place within the larval tunnels.
A few days after eclosion, the adult emerges from the •Use of uninfested planting material, especially vine tips.
vine or storage root. Because the female weevil cannot •Crop rotation.
dig, she finds storage roots in which to lay her eggs by •Removal of volunteer plants and crop debris
entering through soil cracks. Alternate hosts of sweetpotato (sanitation).
weevils are Ipomoea spp. weeds. •Flooding the field for 24 hours after completing a
harvest.
Adults of all species may be conveniently sexed by •Timely planting and prompt harvesting to avoid a dry
the shape of the distal antennal segment, which is filiform period.
(thread-like, cylindrical) in males and club-like in females. •Removal of alternate, wild hosts.
The males have larger eye facets than the females. •Planting away from weevil-infested fields.
•Hilling-up of soil around the base of plants and filling
At optimal temperatures of 27–30°C, C. formicarius in of soil cracks.
completes development (from egg to egg) in about •Applying sufficient irrigation to prevent or reduce soil
33 days. Adult longevity is 2 1/2 to 3 1/2 months and cracking.
females lay between 100 and 250 eggs in this period.
At suboptimal temperatures, development takes longer.

6 7

Treatment of planting material. Dipping planting
material in a solution of Beauveria bassiana or in an in-
secticide (such as carbofuran or diazinon) for 30 minutes
prior to planting (Fig. 7) can control sweetpotato weevils
for the first few months of the growing season.

Less-susceptible varieties. Varieties with im-
munity or a high level of resistance are not available.
Some varieties have low to moderate levels of resistance.
Others escape weevil damage because their storage roots
are produced deep in the soil or because they mature
quickly and can be harvested early.

Sex pheromones. The species-specific pheromones
of all three Cylas species that are released by female
7
weevils and attract males have been identified. Pheromone
lures for C. formicarius are commercially available. Phero-
mone traps are used as monitoring, training, and man-
agement tools. Many effective traps have been designed
by farmers using locally available materials (Fig. 8). Traps
are so sensitive that their failure to catch weevils is a
good indication that the pest is not present.

Microbial control Promising biological control
control.
agents for sweetpotato weevils appear to be the fungi
B. bassiana and Metarrhizium anisopliae and the nema-
todes Heterorhabditis spp. and Steinernema spp.The fungi
attack and kill adult weevils, whereas the nematodes kill
the larvae.

Predators. Ants, spiders, carabids, and earwigs are 8
important generalist predators that attack weevils.They
are described more fully in the section on "Natural Enemies."

8 9

West Indian Sweetpotato Weevil
Euscepes postfasciatus
(Coleoptera: Curculionidae)

Description. Adult weevils are reddish brown
to blackish gray, and are covered with short, stiff, erect
bristles and scales (Fig. 9). Eggs are grayish yellow to
yellow. Larvae are white. Pupae are whitish and sedentary.

Distribution. E. postfasciatus is widespread in the
Caribbean and South America, and is usually present in
sweetpotato fields and storage.

Damage. Adults feed on sweetpotato stems and
storage roots, and emerge by chewing exit holes (Fig. 9
10). Larvae feed deep in the plant tissues. Internally,
flesh and stem tissues are severely damaged. Affected
roots are not edible by humans or animals.

Control. Integrated pest management includes
removal of infested sweetpotato vines and storage
roots from the field after harvest, removal of alternate
hosts, and use of uninfested planting material. Biological
control with B. bassiana and the use of early-maturing
varieties also reduces damage.

10

10 11

Rough Sweetpotato Weevil
Blosyrus sp. (Coleoptera: Curculionidae)

Description and biology. Adult weevils are
blackish or brownish and the surface of the elytra is ridged
(Fig. 11). This makes them look like a lump of soil. Larvae
are whitish and C-shaped. Adult weevils lay eggs under-
neath fallen leaves (Fig. 12). The larvae develop in the
soil and pupate there. Adult weevils are found on the
ground underneath foliage during the day.

Damage. Adult weevils feed on foliage, but the larvae
cause greater damage. While feeding under the soil surface,
they gouge shallow channels on the enlarging storage
roots (Fig. 13). These "grooves" reduce marketability. When
11 12
extensively damaged, the skin of the storage root has
to be thickly peeled before eating, because the flesh discolors
just under the grooves.

Distribution and importance. This weevil is
a common pest of sweetpotato in East Africa, and causes
serious problems in some localities.

Control. Some of the cultural control measures used
to control Cylas should be effective in reducing incidence
of this pest, especially rotation and sanitation. The pos-
sibility of biological control is under investigation.

13

12 13

Clearwing Moth White Grubs
Synanthedon spp. (Lepidoptera: Sesiidae)
White grubs, the larvae of various species of scarabid
The larvae can tunnel through the vine into the storage beetles, live in the soil. In the larval stage, they are large
roots. Usually, only the tip of the storage root is affected. and fleshy with swollen abdomens, well-developed head
This pest is discussed in detail in the section on "Stem- capsules, and large jaws and thoracic legs (Fig. 14). They
borers and Feeders." usually adopt a C-shaped position. When they feed, white
grubs gouge out broad, shallow depressions in sweet-
potato roots. Most species attack a wide range of host
plants. Control is not usually necessary. Handpicking of
Peloropus Weevil exposed grubs during land preparation and weeding is
useful. Light trapping can be used to control white grubs
Peloropus batatae when they become a chronic problem in a localized area.
The larvae of the Peloropus weevil can tunnel down the
vines to the storage roots. This species is discussed in
the section on "Minor Stemborers and Feeders."

14

14 15

Stemborers and Feeders

Clearwing Moth
Synanthedon spp.(Lepidoptera: Sesiidae)

Description and biology. Adults lay batches of
70–100 yellowish eggs on vines and leaf stalks. The larvae
burrow into the vines soon after they hatch and tunnel
downward. They are whitish, with transparent skin and
a brown head capsule. They can grow up to 2.5 cm long.
Pupation occurs in the main stem just above ground level
and the grayish brown pupal case (Fig. 15) can be seen
protruding from the swollen stem. The adult moth has
distinctive transparent wings (Fig. 16). 15

Damage. The larvae burrow into the vines and some-
times into the storage roots. The vine base is charac-
teristically swollen and traversed with feeding galleries.
With heavy infestation, the vine breaks off easily at the
base.

Distribution and importance. Three closely
related species of Synanthedon are regularly found in sweet-
potato, but they are prominent pests only in some local-
ities in East Africa.

Control. Frequent earthing-up around the plant base
reduces the incidence of this pest. Other cultural control
measures, such as those practiced for sweetpotato weevil
species, also help to control clearwing moths. 16

16 17

Sweetpotato Stemborer
Omphisia anastomasalis
(Lepidoptera: Pyralidae)

Description and biology. Most eggs are laid
individually along the underside of the leaves, along the
leaf margins. Some are laid on the stem. The egg, larval,
and pupal stages last an average of 55–65 days. There
are six larval instars. A newly emerged larva has a brown
head and a reddish or pinkish body. After a few days,
it turns creamy with black markings. Full-grown larvae
are 30 mm long (Fig. 17). Infested plants usually have
a pile of brownish frass around their base. Before pupating,
the larva makes an exit hole that is covered with the
epidermis of the stem. Pupation lasts about two weeks 17
and takes place in a web-covered cocoon within the
tunnel (Fig. 18). The adults emerge by breaking through
the dry papery covering of the exit hole. They live 5–
10 days and the females lay an average of 150–300 eggs.
The moths are 15 mm long and have reddish brown heads
and bodies, and light brown wings (Fig. 19).

Damage. The larva bores into the main stem shortly
after hatching and sometimes penetrates the neck of the
storage root. Larval feeding results in enlargement and
lignification of the stems at the base of the plant and
in the formation of hollow cavities filled with frass. Plants
may wilt and die. Attack during the early stages of plant
growth may inhibit the formation of storage roots.
18

18 19

Distribution and importance. The stemborer
is one of the most destructive pests of sweetpotato in
trop-ical and subtropical Asia and the Pacific. It is widespread
in the Philippines, Indonesia, India, Sri Lanka, Malaysia,
Taiwan, Hawaii, and Vietnam, where it is a severe pest
in the central region of the country. It also occurs in China,
Japan, Cambodia, Laos, Burma (Myanmar), and Thailand.
Infestation during the establishment phase of the crop
can result in yield losses of 30–50% or more.

Control. Using planting material infested with
stemborer eggs or planting a new field next to an infested
one are often the main means of disseminating this pest.
Treatment of planting material and crop rotation are valuable
means of control. Hilling-up, often practiced to reduce
19
damage from sweetpotato weevil, also contributes to the
containment of a stemborer infestation. Hilling-up is effec-
tive when the holes, made to provide the adults with
a means of exiting the stems, are covered with soil. Earwigs
and ants may attack the larvae developing within
sweetpotato vines. Sources of resistance have been
identified by the Asian Vegetable Research and Devel-
opment Center, Taiwan.

20 21

Striped Sweetpotato Weevil
Alcidodes dentipes and A. erroneus
Description and biology. Adult A. dentipes
is about 1.4 cm long and has conspicuous white stripes
longitu-dinally along the elytra (Fig. 20). Adult A.erroneus
is bigger than the former, and is brownish black with
an irregular yellowish patch on each elytron. Larvae of
both species are white, with an orange-brown head capsule,
and are C-shaped. Larvae and pupae (Fig. 21) are found
inside the sweetpotato vine, most often at the base. The
adults eat their way out of the vine. The life cycle is
very similar to that of sweetpotato weevils. Larvae and
pupae also resemble those of immature Cylas weevils, 20
but the Alcidodes later instars are much bigger.

Damage. The larvae bore into the vines and sometimes
into the storage roots. The vine base swells up. Adult
weevils girdle the vines, causing wilting.

Distribution and importance. Alcidodes spp.
are minor pests throughout most of East and Central Africa.

Control. Control is not usually required. Frequent
earthing-up around the plant base reduces the incidence
of this pest. Other cultural control measures, such as the
ones practiced for sweetpotato weevil species, also help
to control Alcidodes spp.
21

22 23

Sweetpotato Weevils Minor Stemborers and Feeders
Cylas spp. larvae can do considerable damage to
sweetpotato vine bases by tunneling. In East Africa, this Peloropus Weevil
damage can prevent a crop from establishing. This species Peloropus batatae (Coleoptera:
is discussed in greater detail in the section on "Storage Curculionidae)
Root Feeders."
This reddish brown, compact, 3–4 mm weevil has
been found inside stems and storage roots at some
locations in East and Central Africa. The last instar of the
white larva is longer than one would expect considering
the small size of the adult. The larva makes long tunnels
in the stem and can go down to the storage root via
the storage root "neck." Pupae and adults are found at
the end of the tunnels. The life cycle is long compared
with that of other sweetpotato weevils—2 months or
more. Because the larva enters via the storage root neck,
storage roots that seem undamaged on the outside could
be inedible because of several tunnels on the inside.
Control is not usually required.

24 25

Sweetpotato Bug
Physomerus grossipes (Hemiptera: Coreidae)
Description and biology. The sweetpotato bug
lays groups of eggs on the undersides of leaves or on
the stem. The mother bug guards her eggs (Fig. 22A,B)
and the young gregarious nymphs (Fig. 23). The egg stage
lasts about 15 days. There are 5 nymphal stages and
total development takes about 85 days for males and
88 days for females. The adult is 20 mm long.

Damage. The nymphs and adults pierce the stems
and petioles of the sweetpotato and suck the plant sap,
thus causing wilting and stunting.
22A 22B
Distribution and importance. The sweetpotato
bug is found in Southeast Asia, where it is a minor pest.

Control. Large numbers of bugs are usually found
feeding together, making handpicking of the bugs or re-
moval of the infested plants a feasible means of control.

Long-Horned Beetle

A species of long-horned beetle (Coleoptera:
Cerambycidae) has been found to attack stem bases in
some localities in East Africa. The larvae are large, with
big heads, and they are found inside the stem base. They
cause severe swelling. Control is seldom necessary. 23

26 27

Foliage Feeders

Sweetpotato Butterfly
Acraea acerata (Lepidoptera: Nymphalidae)

Description and biology. Pale yellow eggs (Fig.
24) are laid in batches of 100–400 on both surfaces of
the leaves. The caterpillars are greenish black and covered
24
with branching spines. These larvae are concentrated in
a protective webbing during the first 2 weeks after hatch-
ing. They then become solitary and hide from the
sunlight on the ground during the day (Fig. 25). The pupae
are yellowish and hang singly on the underside of leaves
or on another support. The attractive adult butterfly has
orange wings with brown margins (Fig. 26). The life cycle
takes 27–50 days depending on temperature.

Damage. The caterpillars feed on the leaves. Young
25
caterpillars feed on the upper leaf surface, whereas older
larvae eat the whole leaf except for the primary midribs.
Complete defoliation may result from severe attacks (Fig. 27).

Distribution and importance. The sweetpotato
butterfly is a pest in East and Central Africa. It is an important
production constraint in some localities. Outbreaks are
sporadic and seasonal and usually occur at the beginning
of the dry season.
26
Control. Sweetpotato fields should be observed for
sweetpotato butterfly adults and damage early in the dry
season. Webs containing young caterpillars should be
collected and destroyed weekly. Early planting and
harvesting enable the crop to escape severe attacks. In
case of severe outbreaks, chemical control can be carried
out with carbaryl, pyrethrum, etc.; however, effects on
natural enemies should be considered. Various species
of tachinid, braconid, and ichneumonid parasites attack
the larval stage. Beauveria bassiana infects the sweet- 27
potato butterfly.
28 29

Tortoiseshell Beetles
Aspidomorpha spp. and others
(Coleoptera: Chrysomelidae)
Description and biology. Eggs are laid on the
underside of the sweetpotato leaves or other
Convolvulaceae in batches cemented to the leaves. The
eggs of some species are concealed in a papery oothecum
(Fig. 28). Larvae are characteristically flattened and spiny. 28
In some species, the tail is held up over the back (Fig.
29A,B) and the larva may carry excreta and previous cast
skins (Fig. 30). The pupa is less spiny than the larva, and
is fixed inert to the leaf. The adults are broadly oval and
may be bright and patterned (Fig. 31). Larvae, pupae, and
adults are found on both sides of the foliage. Development
from egg to adult takes 3-6 weeks depending on the
species.
29A 29B

Damage. Both adults and larvae eat large round holes
in the leaves. Attacks are sometimes sufficiently severe
to completely skeletonize the leaves and peel the stems.

Distribution and importance. Four species of
Aspidomorpha and eight other Chrysomelidae have
been recorded in Kenya on sweetpotato. Several species
occur in Southeast Asia including Cassia circumdata and
C. obtusata, the green tortoiseshells; A. miliaris, the spotted
30
tortoiseshell; A. elevata, the golden tortoiseshell (Fig. 32A,
B), and A. amabilis, with reddish brown elytra. Tortoiseshell
beetles are widely distributed and often common. Although
their damage is quite conspicuous, they seldom if ever
cause yield losses.

31

30 31

Control. Control is rarely warranted. Removal of
convolvulaceous weeds in the surrounding area may reduce
their numbers. Several natural enemies including egg and
larval parasites (Tetrastichus sp., Eulophidae; Chalcidae) and
predators (Stalilia sp., Mantidae) have been reported.

32A

32B

32 33

Sweetpotato Hornworm
Agrius convolvuli (Lepidoptera: Sphingidae)
Description and biology. The small, shiny eggs
are laid singly on any part of the plant. The larvae have
a conspicuous posterior "horn." They vary in color from
green to brown and are marked with distinct striped patterns.
The last instar caterpillars reach 9.5 cm in length (Fig. 33).

Hornworms are found mainly on young shoots. The
larval period lasts 3–4 weeks. Pupation takes place in
the soil and takes 5–26 days, depending on the tem-
perature. The large, reddish brown pupa (Fig. 34)is
characterized by a prominent proboscis, which is curved
33
downward. Adults are large, gray hawkmoths with black
lines on the wings. Wingspan is 8–12 cm.

Damage. Yield losses can occur if heavy defoliation
takes place when the crop is young. A large caterpillar
can defoliate a plant and a large population of late instar
larvae can defoliate a field overnight. The larvae feed on
the leaf blades, causing irregular holes, and may eat the
entire blade, leaving only the petiole.

Distribution and importance. A. convolvuli
occurs worldwide. It is not usually a serious pest, although
severe outbreaks have been reported in Vietnam.

Control. Handpicking the larvae from the leaves is
usually sufficient. Plowing the land between crops exposes 34
the pupae. Light trapping can be used to monitor the
population of adults. When a large increase in adult numbers
occurs, manual removal of small larvae can prevent the
buildup of an outbreak population of the voracious late
instar larvae. Pesticide use disrupts the action of the egg
and larval parasites of the hornworm.

34 35

Armyworms
Spodoptera eridania, S. exigua, S. litura
(Lepidoptera: Noctuidae)

Description. Adult female moths of S. eridania are
light brown with dark spots on their front wings (Fig.
35). Males are smaller with a black spot or a bar on the
center of the front wings. Larvae in their first stages are
gregarious and black and velvety with lateral yellow lines.
In later stages, larvae are gray to olive green with two
parallel dorsal lines (Fig. 36) and they disperse all over
the plant.

The white eggs of S. exigua are laid in round or oval
clusters and are covered with a layer of a felt-like substance 35 36
(Fig. 37). The larvae are initially a grass-green color; they
then become green or dark brown with yellowish stripes
in later instars (Fig. 38). Pupation occurs in the soil and
development from egg to adult takes about 23 days. S.
exigua adults can lay 1000 eggs.

The eggs of S. litura are laid in clusters containing
as many as 350 eggs. These are of variable shape and
size and are covered with “felt.” The caterpillars hatch
after 3–5 days and take about 2 weeks to reach the pupal
stage. The larvae (Fig. 39) possess two characteristic black
crescents on the fourth and tenth abdominal segments,
bordered by yellow lateral and dorsal stripes. The larvae
prefer moist sites and may hide in the soil during the
day, attacking plants at night. Pupation occurs in the soil.
Female moths mate several times and produce a sex 37 38
pheromone. The male moths are very sensitive to the
pheromone on the fourth day after emergence. The female
adults (Fig. 40) can lay as many as 2000–3000 eggs.

Damage. Early instar larvae feed by scraping and
scarifying the leaf surface. From the third instar on, they

36 37

consume the parenchymal leaf tissue, leaving only the
veins (Fig. 41). The late instars of S. litura are very voracious
and may bore into sweetpotato roots when these are
exposed.

Distribution and importance. Armyworms are
widespread and feed on many host plants. S. litura is
confined to Asia, the Pacific, and Australia.

Control. Weedy hosts should be eliminated. Ipo-
moea reptans (kankung) and several weeds (Amaranthus 39
sp., Passiflora foetida, Ageratum sp.) are common alternate
hosts in Asia. Collection of egg clusters or leaves infested
with gregarious young larvae can be an effective means
of control. Spot applications of insecticide or Bacillus
thuringiensis can be made in the early larval stage when
larvae are still gregarious. Formulations of nuclear poly-
hedrosis viruses are available. The green muscardine
fungus, Nomuraea rileyi, is highly pathogenic to S. litura,
and the virus Borrelinavirus litura can cause mortality
after an incubation period of 4–7 days. Predatory bugs, 40
carabid beetles, vespid wasps, and spiders attack the
larvae, and more than 40 species of scelionid, braconid,
ichneumonid, and tachinid parasites are known.

41

38 39

Leaf Folders
Brachmia convolvuli (Lepidoptera:
Gelechiidae), Herpetogramma hipponalis
(Lepidoptera: Pyralidae), and others

Description and biology. The larvae of the black
leaf folder, B. convolvuli (Fig. 42), and the green leaf folder,
H. hipponalis (Fig. 43), feed inside a folded leaf, leaving
the lower epidermis intact. In most cases, only one larva
is found per leaf fold. The black leaf folder lays yellowish
white oval eggs singly on the leaves. The eggs hatch
after 3–5 days and the five larval instars last 2–5 days
each. The average total larval period is 11 days. The larva
has prominent bland and white markings on the thorax
42
and abdomen. The pupal period is 4–7 days. The female
moth lives an average of 5 days. The green leaf folder
lays eggs in groups on the upper surface of the leaf near
the midrib. The eggs are shiny green, oblong, and covered
with a scale-like gelatinous material. The eggs hatch after
3–5 days and there are five larval instars. The larva is greenish
yellow with sparse brown setae and a dark brown head
and prothoracic plate. The pupal period lasts 4–8 days.
The adult is a yellowish brown moth with dark brown markings
on its wings. The female moth lives about 3 days.

Damage. The leaf margin is folded once by B. con-
volvuli. H. hipponalis folds the leaf margin twice and prod-
uces some webbing. Leaf folder feeding results in a lace-
like appearance of the leaf, with the main leaf veins left
intact. 43

Distribution and importance. Leaf folders are
widespread throughout Asia and Africa.

Control. A high rate of parasitism by braconid wasps
is common. Earwigs and other generalist predators are
also important in maintaining natural control. If natural

40 41

enemy action is not disrupted by pesticide use, control Strobiderus Beetle
is rarely needed. The use of uninfested planting material Strobiderus aequatorialis
is an effective means of reducing the incidence of leaf
folders. (Coleoptera: Chrysomelidae)
This is a small, yellowish beetle, 5–7 mm long ocurring
The brown leaf folder, Ochyrotica concursa (Lepidoptera:
in East Africa. The adults perforate the leaves and cause
Pyralidae), and the pink-striped leaf folder, Anticrota ornatalis
damage similar to that of tortoiseshell beetles. Control
(Lepidoptera: Pyralidae), have been reported in the Phil-
measures are not usually necessary.
ippines. The green larvae of the pyralid leafroller, Tabidia
aculealis, scarify the mesophyll from the inner side of
the rolled leaf. They prefer full-grown leaves and the pupae
Rough Weevil
are found in leaf cases. Reported in Indonesia, they
occasionally cause serious damage. Blosyrus sp. (Coleoptera: Curculionidae)

The adults feed on the leaves of sweetpotato in Africa,
but their important pest stage is the larva, which affects
the storage roots (see the section on "Storage Root Feed-
ers").

Sweetpotato Weevils
Cylas spp. (Coleoptera Curculionidae)

The adults feed on the leaves of sweetpotato, but
are much more important as pests of the storage roots
and stems (see the section on "Storage Root Feeders").

42 43

Minor Leaf Feeders
Grasshoppers and Locusts
Zonocerous variegatus
(Orthoptera: Pyrgomoriphidae),
the variegated grasshopper and
Attractomorpha psitticina
(Orthoptera: Acrididae),
the slant-faced grasshopper, and others

In Africa, both adults and nymphs of Z. variegatus can
defoliate sweetpotato (Fig. 44). Outbreaks seldom occur
and control is rarely needed.

An Asian species, the polyphagous slant-faced grass-
hopper is bright green and characterized by a pointed
conical head and short antennae. It measures 30–40 mm
in length.

The female taro grasshopper, Gesonula zonocena
mundata (Orthoptera: Acrididae), is found in Southeast
Asia. It bores through the petiole of the host plant to
lay its eggs. These are covered with a reddish brown
gummy substance. The pale brown to green adult is 30
mm long and has black stripes running from its eyes
to the tips of its wings. The hind legs are black and the 44
tibia are bluish with white-tipped spines. Taro and water
hyacinth are also hosts.

44 45

Virus Transmitters

Aphids
Aphis gossypii and others
(Homoptera: Aphididae)

Description and biology. Aphids are soft-bodied
insects, 1–2 mm long, yellowish green to black, with or
without wings (Fig. 45). Aphids can multiply asexually,
resulting in fast population buildup. Several generations
occur per year.

Damage. Aphids damage plants by sucking sap from
growing shoots. Symptoms of aphid attack are wrinkling,
cupping, and downward curling of young leaves. During
heavy infestation, plant vigor is greatly reduced.

As aphids feed and move from plant to plant in the
field, they transmit viruses. The most important aphid-
transmitted virus is sweetpotato feathery mottle virus.
Winged forms may travel long distances and introduce
viruses into new areas. A. gossypii has a wide host range,
including cotton, cucur-bits, and many legumes.

Distribution and importance. Aphids are cos-
mopolitan. Their main impact in sweetpotato is as vectors 45
of virus diseases.

Control. Control is rarely necessary. Predators such
as ladybird beetles, lacewings (Chrysoperla sp.), and syr-
phids naturally reduce aphid populations. In case of heavy
outbreaks, farmers tend to apply insecticides; however,
these should be used with great caution since they reduce
natural enemy populations and can contribute to further
aphid outbreaks.

46 47

Whiteflies
Bemisia tabaci (Homoptera: Aleyrodidae)

Description and biology. The female of B.
tabaci lays eggs on the undersides of leaves. All the
nymphal stages (Fig. 46) are greenish white, oval in
outline, scale-like, and somewhat spiny. The adult
(Fig. 47) is minute and covered with a white, waxy
bloom. Development of one generation takes 3–4
weeks.

Damage. High whitefly populations may cause
yellowing and necrosis of infested leaves. The pest is
more important as a transmitter of viruses, especially
sweetpotato mild mottle virus. B. tabaci has a wide host 46
range, including cotton, tomato, tobacco, and cassava.

Distribution and importance. The main impact
of B. tabaci, a cosmopolitan species, on sweetpotato is
as a vector of virus diseases.

Control. Control measures are not usually needed.
Controlling whiteflies is not usually an effective means
of limiting the incidence of the viruses they transmit.

47

48 49

Mites
Erinose caused by Eriophyid mites, Aceria sp.
(Acari: Eriophyidae)

Description. Vines and leaves become excessively
hairy, beginning at the shoot tip (Fig. 48).

Biology. Erinose is present in East Africa and the
United States. The problem is more pronounced at lower
altitudes where the climate is hot and dry. Research suggests
that yields may be reduced.

Control. Control is through selection of mite-free
48
planting material and good field sanitation. This might
not be effective enough, however, because mite popu-
lations can build up rapidly.

Eriophyes gastrotrichus (Acari: Eriophyidae)

Description. Infested vines present pocket-like galls
on leaves, petioles, and stems (Fig. 49). Initially, galls are
light green, but they become brown afterwards. Several
mites in every stage of development live together inside
each gall. When heavy infestations occur, leaves become
crinkled and lose their shape.

Biology. Erinose caused by the gall mite is present 49
in the Philippines and Papua New Guinea.

Control. Mite-free planting material should be used
together with good field sanitation and destruction of
weeds that can act as hosts.

50 51

Natural Enemies
Earwigs
The natural enemies of sweetpotato pests have re-
ceived limited research attention. What little is known Earwigs (Dermaptera: Forficuliidae) have a character-
of their biology and ecology relates mostly to generalist istic hind pair of forceps-like pincers that are used for
predators, such as ants, and to fungal pathogens of the defense. Adults can live for several months and are most
sweetpotato weevils. active at night. They enter stemborer tunnels in search
of larvae. Occasionally they climb the foliage to prey on
Generalist predators are often the most important leaf folder larvae. They can consume 20–30 prey daily.
group of biological control organisms in agricultural systems
because they can switch from one prey type to another.
Their flexible food habits allow them to respond as one Spiders
pest re-places another in terms of relative abundance.
Herbivorous species that cause little or no economic damage
The importance of spiders as predators has been clear-
play an important ecological role in agricultural systems
ly demonstrated for rice, but their role has not been studied
by providing food to maintain populations of beneficial
adequately in many other crops and little is known about
species at levels that can prevent damaging pest outbreaks.
their contribution to biological control of sweetpotato
pests.
In Asia, where sweetpotato is often grown in rotation
with rice, many of the generalist predators commonly
The lynx spider Oxyopes sp. and the wolf spider Lycosa
found in rice fields persist in the sweetpotato crop that
sp. are abundant in sweetpotato fields. These do not
follows (for more information on these, see Shepard et
spin webs but rather hunt prey directly. Web-spinning
al. 1987).
spiders are also common.

52 53

Pheidole and Other Predacious Ants

Cuban farmers practice augmentation of Pheidole
megacephala (Myrmicinae) and Tetramorium guinensis
(Myrmicinae) to control the sweetpotato weevil, C.
formicarius. Several ants—Pheidole sp. (Fig. 50),
Iridomyrmex anceps (Dolichoderinae), and Anoplolepis
longipes (Formicinae)—have been confirmed as predators
of the sweetpotato weevil in Indonesia. Although their
role as predators of other sweetpotato pests is unknown,
Pheidole ants are known to attack small prey stages such
as eggs and early instar insect larvae.

A number of other ant species are reported from
sweetpotato fields in Indonesia (Tetramorium sp., 50
Monomorium sp., Odontoponera transversa, Dolichero-
derus thoracicus, Polyrachis sp., Camponotus maculatus,
Diacamma sp., Myrmicaria sp., Leptogenys sp., Pachy-dondyla
sp., and Odontomachus simillimus), Uganda (Myr-micaria
sp.), and Vietnam (Solenopsis geminata and Paratrechina
sp., found in tunnels made by Omphisia anas-tomasalis).
Although these ants have not been confirmed as predators
of sweetpotato pests, the literature contains references
suggesting a predacious behavior for several species. An
example is Myrmicaria sp., often found attack-ing caterpillars
in agricultural fields and dragging them to their nests.

54 55

Beetles

Carabids are one of the most important families of
predatory beetles. The majority of carabids in agricultural
systems are ground-dwelling species that feed on other
insects that live or pupate in or on the soil. An example
of a predatory carabid identified from sweetpotato fields
in Indonesia is Pheropsopus sp. A few carabids climb
foliage and can be found within the chambers made by
leaf folders.

The staphylids are another common family of scav-
engers and generalist predators. Of these, Paederus spp.
(Fig. 51) are common in many crop environments, including 51
sweetpotato. P. fusciceps occurs in Indonesia.

The Coccinellidae, or ladybird beetles, are a large family,
nearly all of which are predacious. Their main prey are
aphids, mealybugs, and scale insects, but they also feed
on insect eggs or on the slow-moving early instars of
some Lepidoptera. Both the larvae (Fig. 52) and the adults
are predacious. Adult females deposit distinctive yellow
cigar-shaped eggs in groups of 5–30 on plants, mostly
near colonies or groups of prey. Ladybird larvae forage
on the plant and pupate on the foliage.

52

56 57

Flies and Parasitic Wasps

Hover flies (Syrphidae) are brightly colored and fre-
quently visit flowers to feed on pollen and nectar. The
adult flies (Fig. 53) lay their elongated white eggs singly
on plants, often near aphid colonies, which serve as prey
for their white, maggot-like offspring. Syrphid larvae are
active mostly at night and feed mainly on aphids. Syrphids,
cocinellids, and parasitic wasps form a natural enemy
complex that plays an important role in regulating aphid
populations.

Tachinid flies (Fig. 54) mostly attack medium-to-late
instar caterpillars. Some species deposit their eggs directly
on a host. Others lay eggs on the plant close to where 53
the host is feeding. The eggs are consumed by the host
but remain intact and develop within the host’s body.
Other species lay incubated, fully developed eggs on the
leaves; the newly hatched larvae search for a host and
penetrate its cuticle. Caterpillars parasitized by tachinids
can be recognized by the presence of dark spots on the
cuticle, or by discoloration and deformation. Adult tachinids
visit flowers for nectar and survive for several weeks.

The tachinid Zygobothria ciliata is a larval parasite
of the sweetpotato hornworm. Cuphocera varia, a
larviparous species, and Blepharella lateralis are
common parasites of armyworms, and are widespread
throughout Southeast Asia. The larvae of C. varia are
deposited on leaves near the feeding sites of their
hosts and rapidly bore into the side of a host between 54
the segments. The host caterpillar attempts to dis-
lodge the attached parasite by twisting and turning
violently. B. lateralis lays it eggs on the leaves and
the flies often emerge from the host pupa. Carcelia
kockiana, a highly polyphagous species, parasitizes
armyworms in Indonesia. Carcelia normula parasitizes
the sweetpotato butterfly in Uganda.

58 59

The wasp Telenomus spodopterae (Scelionidae) is a Viruses
common egg parasite of S. litura. The encyrtids Anastatus
dasyni and Oencyrtus malayensis are egg parasites of Nuclear polyhedrosis viruses are common on army-
Physomerus grossipes. Development of A. dasyni takes worms. The larvae become infected by eating virus-con-
16 -18 days and the females live for about one month. taminated foliage. As infection develops in a larva, it
As many as 8 adults of O. malayensis emerge from an becomes sluggish and stops feeding. Later the larva turns
egg of P. grossipes. The wasps are able to attack eggs whitish and then black and hangs from the foliage by
within one day of emergence, and live for about one its prolegs. The fluid oozing from its body contaminates
month. The eulophid Tetrastichus sp. has been reported foliage and continues the disease cycle.
as a pupal parasite of the green tortoiseshell beetle.
Brachymeria sp., a chalcid, parasitizes the green leaf folder. Granulosis viruses attack lepidopteran larvae. Horn-
Trichogramma minutum (Trichogrammatidae) is an egg par- worm species are often affected. Host larvae that eat
asite of the sweetpotato hornworm. Charops sp. contaminated foliage move sluggishly and then stop
(Ichneumonidae) has been reported from the sweetpotato feeding. After 1-2 weeks, the body becomes constricted,
butterfly. The adult is slender with a long, stalked abdomen, giving a segmented appearance. Infected larvae turn yellow,
which is flattened laterally. The adult also has a charac- pink, or black and become soft.
teristic stalked and barrel-shaped pupal cocoon.

Many of the host insects of braconid parasites live
in protected sites such as tunnels, mines, and folded
leaves (Fig. 55), or under webbing. Some examples are
Macrocentrus sp., a parasite of the black leaf folder;
Microbracon cylasovorus and Bassus cylasovorus, parasi-
tes of C. formicarius; and Meteorus sp., a parasite of
sweetpotato butterfly larvae.

55

60 61

Fungal Pathogens
Metarrhizium anisopliae and
Beauveria bassiana

Sweetpotato weevils are among the insect species
attacked by M. anisopliae. Spores germinate on the body
of a host insect under conditions of prolonged high hu-
midity. The fungus penetrates the insect and uses its
internal body contents as a substrate for proliferation. When
the host dies, the fungus emerges through joints in the
insect exoskeleton, appearing first as a white growth.
When spores are formed, the fungus turns green. Spores
emerging from the dead host are spread to new hosts
by wind or water.
56

Beauveria bassiana (Fig. 56) attacks stemborers, leaf
folders, and bugs among others, and is a confirmed patho-
gen of sweetpotato weevils and the sweetpotato butterfly.
Like other fungal diseases, it requires conditions of pro-
longed high humidity for the air-or waterborne spores
to germinate. The fungus invades the soft tissues and
body fluids of the host and grows out of the body to
sporulate. Affected insects appear to be covered with
a powdery, white substance.

Other pathogens that may play a role in the biological
control of pests in sweetpotato fields include the fungi
Hirsutella spp. and Nomuraea rileyi, and the nematodes
Heterorhabditis spp. and Steinernema spp.

62 63

Diseases and Pathogens of
Sweetpotato
and Their Management
A number of pathogenic organisms affect sweetpotato.
Most appear to be widespread, but damage levels vary.

In this guide, we include viral, fungal, and bacterial
diseases, as well as those caused by nematodes. Each
group of pathogens is covered in a separate section. Patho-
genic bacteria, although not very common, are responsible
for important economic losses. They affect vascular tissue
as well as storage and fibrous roots, thus causing vine
wilting and rots. Fungal pathogens are classified according
to the type of disease they cause, such as foliar, stem,
storage root, and postharvest diseases. In general, foliar
and stem diseases are mild and cause little damage, except
for scab, which is a very important disease in Southeast
Asia. These diseases contribute to lower yields by reducing
photosynthetic area and transport of nutrients and products
to the storage roots. In some countries, storage rots do
not cause much damage because sweetpotatoes are
consumed shortly after harvest. Tuber-rot pathogens,
however, are present in the field and can cause significant
losses. Plant parasitic nematodes are included as the cause
of serious damage to storage roots both in the field and
during storage.

Although the symptoms of virus diseases appear in
the foliage, these have been accorded a separate section
because of their importance. Of all the sweetpotato patho-
gens, viruses appear to contribute the most to yield losses.

65

Viral Diseases
Sweetpotato Feathery Mottle Virus
(SPFMV)
Aphid-transmitted potyvirus

Symptoms. Symptoms of SPFMV on the foliage
of sweetpotato are generally slight or absent. If present,
they appear as faint, irregular chlorotic spots occasionally
bordered by purplish pigment. Chlorosis (feathering) along
midribs (Fig. 57) and faint-to-distinct chlorotic spots with
or without purple margins occur in some cultivars (Fig.
58). Symptom visibility on foliage is influenced by cultivar sus- 57
ceptibility, degree of stress, growth stage, and strain viru-
lence. Increased stress can lead to symptom expression,
whereas rapid growth may result in symptom remission.
Symptoms on storage roots depend on the strain of SPFMV
and the sweetpotato variety. The common strain causes
no symptom on any variety, but the "russet crack" strain
causes external necrotic lesions or internal corking on
certain varieties (Fig. 59). SPFMV can be latent in vines.

Biology. SPFMV is transmitted by a wide range of
aphid species in the nonpersistent manner through brief
feeds of only 20–30 seconds. Both colonizing species
of aphids and winged aphids of noncolonizing species
58
may transmit the disease. It is also perpetuated between
cropping cycles in infected cuttings, but the lack of symp-
toms in the foliage makes it difficult for farmers to select
SPFMV-free cuttings. In Uganda, symptom-free cuttings
were mostly virus-free. SPFMV is found with SPSVV (see
next section) in some countries; the combination results in a
severe disease known as sweetpotato virus disease (SPVD).

Distribution and importance. Occurs worldwide.

Control. Aphid control is not economically feasible.
The main controls are avoidance of use of diseased plants
for cutting material, sanitation, and use of resistant varieties.
59

66 67

Sweetpotato Sunken Vein Virus Sweetpotato Virus Disease (SPVD)
(SPSVV)
Whitefly-transmitted closterovirus Symptoms. Diseased plants become severely
stunted and the leaves become small and narrow (straplike),
Symptoms. The symptoms reported for SPSVV vary often with a distorted edge. Puckering, vein-clearing, and
geographically; in East Africa, the disease may cause stunt- mottling may occur. The mottling is often pale so that
ing and color change in leaves (reddening or yellowing the whole plant may appear chlorotic.
usually) depending on the variety. Elsewhere, symptoms
include mild vein yellowing, some sunken seconday veins Biology. This disease seems to be caused by a
on adaxial leaf surfaces, and swollen veins on abaxial synergistic combination of SPFMV and SPSVV; it is unclear
surfaces. The disease may also cause no symptoms. whether other virus combinations are involved.

Biology. SPSVV is transmitted by the whitefly B. Distribution and importance. SPVD is common
tabaci in the semipersistent manner, needing feeds of in Africa, where it is the main virus disease of sweetpotato
several hours to acquire or transmit efficiently. It may in Nigeria, Cameroon, Ghana, and Uganda. It causes virtually
also be perpetuated through cropping cycles via infected total yield loss in affected plants. It may be identical to
cut-tings. SPSVV is generally identified in combination with a severe disease reported in the Americas. SPVD has
SPFMV, causing the severe disease SPVD (see next section). also been reported in Argentina, Brazil, Peru, Kenya, the
United States, and Taiwan.
Distribution and importance. By itself, SPSVV
may cause only small yield losses, but combined infection Control. The main controls are avoidance of diseased
with SPFMV causes SPVD, a severe disease associated plants as sources of planting material and use of resistant
with almost complete loss of yield. SPSVV occurs in Kenya, varieties. Farmers usually avoid diseased planting material
Uganda, and Nigeria, and has been reported in Asia, Ar- because symptoms are so severe.
gentina, Brazil, Peru, and the United States.

Control. The main controls are avoidance of diseased
plants as sources of planting material and use of resistant
varieties.

68 69

Other Viral Diseases
Sweetpotato Mild Mottle Virus
Other viruses have been identified through serological
(SPMMV) identification techniques, such as sweetpotato latent virus
Whitefly-transmitted potyvirus (SPLV), reported in Taiwan, Japan, Kenya, China, and Israel;
sweetpotato chlorotic fleck virus (SPCFV), present in south-
Symptoms. The predominant symptoms associated east Africa, Indonesia, the Philippines, China, Japan, and
with SPMMV are leaf mottling and stunting (Fig. 60). Vein Central and South America; sweetpotato caulimo-like virus
clearing and distortion may also occur. None of these (SPCV), found in Puerto Rico, Madeira, New Zealand, Papua
symptoms is easily diagnosed in the field and the virus New Guinea, the Solomon Islands, and Kenya; sweetpotato
can be latent. ring spot virus (SPRSV), reported in Papua New Guinea
and Kenya; and cucumber mosaic virus (CMV), found only
Biology. SPMMV is transmitted nonpersistently by in Israel, Kenya, and the United States. Sweetpotato chlorotic
the whitefly B. tabaci. It is also carried in infected cuttings. stunt virus (SPCSV) (Fig. 61) is found in Kenya and the
There is some evidence that SPMMV forms a complex Caribbean.
with SPFMV, but this is unclear.

Distribution. It has been identified in Kenya, Uganda,
Tanzania, and Indonesia, but yield effects are unknown.

Control. Some sweetpotato varieties appear to be
immune and others are tolerant. Sanitation and selection by
farmers of symptomless planting material also help achieve
control.

61

60

70 71

Bacterial Diseases

Bacterial Stem and Root Rot
Erwinia chrysanthemi

Symptoms. Aerial symptoms are water-soaked
brown to black lesions on stems and petioles. One or
two branches may wilt, and eventually the entire plant
collapses (Fig. 62). Localized lesions on fibrous roots may
also be present. On fleshy roots, localized lesions
with black margins can be observed on the surface,
but more frequently the rotting is internal, with no evidence
outside (Fig. 63).
62
Biology. The pathogen has several other hosts in
warm, humid areas of the world, where it remains in
the soil on plant debris and weeds. Infection occurs through
wounds.

Distribution and importance. This disease is
found worldwide. Losses can be economically important.

Control. Cuttings for transplanting should be taken
above the soil line. Using less-susceptible cultivars and
taking care to avoid wounding can reduce disease in-
cidence.

63

72 73

Bacterial Wilt
Pseudomonas solanacearum

Symptoms. Infected stands usually contain some
wilted plants (Fig. 64A). The disease starts at the base
of the stem as yellowish water-soaked lesions that soon
turn brown. The vascular bundles of affected stems and
sprouts are discolored (Fig. 64B). In storage roots, vascular
discoloration is also present, but mainly longitudinal brown
streaks appear as well as brown water-soaked lesions
on the surface (Fig. 64C). Slightly affected fleshy roots,
when stored, can rot completely and develop a distinctive
odor (Fig. 64D).

Biology. The bacterium is soil-borne, but it is usually
carried with the propagative material. Once the soil is
infested, the bacterium can persist from one to three
years. Dissemination in the field can also occur via irrigation
water.

Distribution and importance. The disease is
important in some areas of southern China when sus-
ceptible varieties are grown.
64
Control. The use of less-susceptible varieties and
disease-free planting material reduces disease incidence.
When the bacterium is already present in the soil, flooding
and crop rotation with graminaceous hosts are recom-
mended.

74 75

Soil Rot
Streptomyces ipomoea
Symptoms. The first indication of the disease is
an extensive chlorosis and bronzing of the foliage as a
consequence of the destruction of fibrous roots (Fig. 65).
On storage roots, besides dark brown necrotic lesions,
we frequently find cracks radiating from the center and
malformations such as dumbbell-shaped roots (Fig. 66).

Biology. Soil rot causes more damage in dry alkaline
soils. The pathogen can survive in soil for long periods.

Distribution and importance. This disease re-
duces yield and can be destructive in some areas of the 65
United States and Japan.

Control. Planting material should come from areas
where the disease is not present. Maintaining soil moisture
helps reduce disease incidence. Using sulfur to reduce
soil pH is another alternative, but large amounts of this
element are required.

66

76 77

Foliar and Stem Diseases
Caused by Fungi

Leaf and Stem Scab
Elsinoe batatas, Sphaceloma batatas
Symptoms. Brown to tan raised corky lesions, with
purple to brown centers, appear along the stems (Fig.
67). Coalescing tiny lesions cover the leaf veins, thus making
them shrink and causing the leaves to curl (Fig. 68).

important in Southeast Asia and the South Pacific Islands,
where the pathogen causes serious losses from poor for-
mation of fleshy roots. The disease is also present in
Brazil. 67

Little is known about the biology of the pathogen.
Humid weather, however, favors the disease.

Control. Good levels of varietal resistance are avail-
able. Pathogen-free planting material of the most resistant
varieties and good sanitation practices should be used.
The resistance of native and introduced material is being
evaluated in Southeast Asia and the Pacific.

68

78 79

Alternariosis, Anthracnose, Blight
Alternaria bataticola

Symptoms. Brown lesions with a typical bull’s-eye
appearance of concentric rings occur on leaves, especially
older leaves. Black lesions appear on petioles and stems
(Fig. 69). Bases and middle sections are more affected
than the vine terminals. Death of vines can occur. The
ground under affected vines is often carpeted with black-
ened leaf debris (Fig. 70).

Biology. Disease and lesion size increase with alti-
tude. High relative humidity or free water is necessary
for infection and sporulation. The fungus survives in debris,
and the spores are spread through infected planting material,
wind, splashing rain, and water.
69
Although Alternaria spp. can be found infecting
sweetpotato in all agroecological zones, the form known
as alternariosis or anthracnose occurs at mid to high el-
evations.

Distribution and importance. Published infor-
mation and experience point to Alternaria blight as the
most important fungal disease in East Africa and Brazil.

Control. Susceptibility to the pathogen varies among
varieties. Pathogen-free planting material of the more re-
sistant varieties and good sanitation practices will help
control the disease.

70

80 81

Phomopsis Leaf Spot
(Phyllosticta Leaf Spot)
Phomopsis ipomoea-batatas
(Phyllosticta batatas)

Symptoms. Whitish to tan to brown lesions, usually
less than 10 mm in diameter, form on the upper and
lower surfaces of leaves. The lesions usually have a dark
brown or purple margin (Fig. 71). Pycnidia are visible in
the center of the lesions (Fig. 72).

Biology. The fungus survives in debris and is not
known to have other hosts. Spores spread through in-
fected planting material, wind, splashing water, and possibly
71
insects.

widespread and occurs in all agroecological zones. It is
not known to depress yield, but it can reduce the quality
of vines for planting material and fodder.

Control. No control measures are known. Control
is not normally necessary.

72

82 83

Minor Leaf Spot Fungi

Other fungi cause leaf spots and can be identified
by inspecting spores with a microscope. These fungi
are Alternaria spp., Cercospora sp. (Fig. 73), Septoria
sp., Ascochyta sp., Curvularia sp., Colletotrichum sp.,
and Pestalotia batatae.

Control. No control measures are known. Control
is not usually needed.

73

84 85

Chlorotic Leaf Distortion
Fusarium lateritium
Symptoms. The first noticeable sign or symptom
is a white, waxy (crusty) mucilaginous layer, which contains
mycelium and sporodochia, that covers newly expanded
leaves (Fig. 74). Microscopic observation reveals it on apical
meristems and axillary buds. As the leaves age, the waxy
covering spreads along the leaf margin and eventually
disappears. In some cultivars and environments, leaves
become chlorotic. Occasionally, leaves become distorted
and plants are stunted (Fig. 75).

Biology. The pathogen may be present on the entire
surface of the aerial part of the plant and it can be transmitted
through true seed. It cannot be eliminated by surface 74
disinfection of the seed. Symptoms are more severe in
hot, dry weather. There is a long latent period (3–6 weeks)
from infection to symptom expression.

Distribution and importance. The disease has
been found in Peru, a few areas of East and Central Africa—
primarily at low altitudes where it is hot and dry—and
in the United States. It is not known to cause economic
damage in its less-virulent form. When stunting and distortion
occur, losses should be expected.

Control. Pathogen-free planting material is essential.
Varietal differences in susceptibility are observed. Use
vegetative planting material from plants free of symptoms.
Do not harvest true seed from diseased plants, especially
if the seed is to be shipped to areas where chlorotic
leaf distortion is not present. No chemical control is known.
75

86 87

Fusarium Wilt
Fusarium oxysporum f. sp. batatas

Symptoms. The first indication of this disease
is a dullness and yellowing of the leaves, followed by
wilting and death of the vine. Affected vines show the
vascular discoloration typical of this disease (Fig. 76).

Biology. The fungus is soil-borne and specific to
sweetpotato and a few close relatives, and barley and
flue-cured tobacco. It can survive in the soil and in debris
for several years. Though tip cuttings are usually pathogen-
free, roots and cuttings from the base of the vine can
be infected. Movement of infested soil on tools and by
animals can lead to outbreaks in new areas. The disease
occurs under a variety of environmental conditions. Yield
reduction depends on the stage of plant growth when
disease occurs.

found in most areas where sweetpotato is grown and
is more important in temperate areas than in the tropics.

Control. Good sanitation will help reduce the impact
of the disease and limit its spread. Some varietal resistance 76
has been observed, and breeding programs in some coun-
tries have released resistant varieties.

88 89

Violet Root Rot
Helicobasidium mompa

Symptoms. Affected plants become chlorotic and
may defoliate. Fibrous roots rot and become covered with
thick whitish threads of mycelium that soon become pink
and finally violet (Fig. 77). Storage roots start rotting apically
and then they completely decay and are covered by the
same mycelial mat as the fibrous roots. At the same time,
flat black sclerotia are formed. This violet mat of coarse
mycelium and sclerotia may be found on the ground in
places where plants have rotted. Rotted storage roots
have a characteristic smell of alcohol.

Biology. The fungus has a wide host range besides
sweetpotato. It can survive in the soil for at least 4 years
as mycelium or as sclerotia. Infected transplants and irrigation
water can disseminate the fungus. Temperature is not
a limiting factor for disease development, but considerable
moisture in the soil favors the disease.

present in several areas of Asia and the Americas. It can
cause serious losses in Asia.

Control. Planting material should come from healthy
plants. Early maturing varieties can escape the disease. 77
Rotation with cereals can also help prevent the disease.

90 91

Sclerotial Blight and Circular Spot
Sclerotium rolfsii

Symptoms. Sclerotial blight and circular spot are
two diseases caused by the same pathogen.

Blight symptoms start in both seedbeds and newly
planted stands. Shoots emerging from the mother root
suddenly collapse and die. Affected shoots are easily
pulled and separated from the rest of the plant. A mat
of white mycelium and numerous round brown sclerotia
resembling rapeseed are found at the base of affected
plants (Fig. 78). Circular spots are observed only in fleshy
roots. Symmetric brown sunken lesions that sometimes
show crackings are present (Fig. 79). 78

Biology. The fungus attacks several plant species.
It is soil-borne and survives for long periods as sclerotia.
Moisture and organic matter in the soil favor attack.

common in tropical and subtropical regions of the world.
Losses are not usually serious.

Control. Disease incidence can be reduced by avoid-
ing growing sweetpotato in infected soils and using dis-
ease-free planting material. The use of good sanitation
and less-susceptible cultivars also helps to reduce the
disease.
79

92 93

Black Rot
Ceratocystis fimbriata

Symptoms. Dark to black sunken cankers in the
lower part of the stem are the most distinctive symptom.
In severe infections, yellowing, wilting, and plant death
can occur. Affected storage roots develop black to gray
sunken areas (Fig. 80) on which black spine-like structures
of the fungus can be seen protruding from the surface
of roots. A smell of alcohol resembling that of fermenting
sugar is frequent.

Biology. The use of infected cuttings for planting
perpetuates the disease. Transmission occurs through
wounds made by sweetpotato weevils (such as Cylas spp.), 80
wireworms, crickets, and mice. The fungus is a soil in-
habitant that can remain 1–2 years in crop debris. Moisture
does not affect disease development.

particularly important in Southeast Asia and Oceania where
it reduces yield and quality of fleshy roots.

Control. Cuttings for transplants should come from
pathogen-free planting material. In places where it is difficult
to find healthy mother plants, cuttings should be made
2 cm above the soil line to avoid infected portions of
the plant. Rotate with nonhost plants for at least 2 years
and use good sanitation practices. Cure during the 5 days
following harvest at 30–35°C and 85–90% relative humidity.

94 95

Storage Root and
Postharvest Diseases

Foot Rot
Plenodomus destruens

Symptoms. Brown lesions form on the stem at
or below the soil line. Wilting and death occur in severe
cases. Black pycnidia can be seen (Fig. 81). A canker ex-
tends down the stem and affects the proximal end of
the storage root (Fig. 82). This decay is dark brown, firm,
and dry.
81
Biology. The fungus does not survive well in the
soil except in infected roots and stems. It is spread by
infected cuttings, especially those from the base of the
vine, and by contact with spores from infected roots in
storage. Other hosts include members of the
Convolvulaceae. Diseased roots should not be stored.

found in Peru, Brazil, and Argentina. Storage damage can
reduce marketability.

Control. Sanitation and the use of healthy vine tips
for planting are the best means of control in the field.

82

96 97

Java Black Rot
Lasiodiplodia theobromae
(Diplodia gossypina)

Symptoms. This rot is firm and moist initially,
but storage roots soon become totally blackened and
mummified. Rot starts at either or both ends of the storage
root and is initially brown, before turning black. Eruptive
black stromatic masses that bear pycnidia are a diagnostic
feature (Fig. 83).

Biology. Java black rot is spread by infested soil,
infected storage roots, and contaminated storage boxes,
baskets, or tools. Infection occurs via wounds, especially 83
the cut stem end. Though the pathogen can infect stems,
it grows very little and is seldom a problem. Yields can
be reduced in the field or through storage losses.

distributed worldwide. It is one of the most important
storage diseases of sweetpotato.

Control. Timely harvesting can reduce losses.
Good sanitation and care in handling to reduce
wounding are important.

98 99

Charcoal Rot
Macrophomina phaseolina

Symptoms. This disease is found only on fleshy
roots during storage. The fungus does not attack other
plant parts. Infection starts on the surface of the root
and progresses through the vascular ring toward the pith.

Three distinct zones are found in a cross section of
an infected root: an unblemished periderm, an inner zone
about 6 mm wide of reddish brown tissue where a crusty
layer of sclerotia is found, and the inner part of the root
with light tan tissue. Sometimes the center of the pith
splits and the entire root becomes mummified (Fig. 84).
84
Biology. The fungus is distributed worldwide and
attacks several plant species. It is soil-borne and can survive
saprophytically on plant debris or freely as sclerotia.

found in tropical and subtropical areas of the world. Losses
are seldom serious.

Control. No control measures are known.

100 101

Soft Rot
(Rhizopus stolonifer, Mucor sp.)

Symptoms. Soft rotting occurs after harvest. Storage
roots become soft, wet, and stringy, often starting at one
end. A strong alcohol-like odor is produced. These fungi
are commonly seen sporulating on the surface of rotting
storage roots (Fig. 85).

Biology. The disease is spread by infested soil or
air-borne spores that enter wounds. Optimum relative hu-
midity and temperature for progress of infection and dis-
ease vary by variety, but are usually high. Soft rot can
destroy harvested roots in 48 hours if they are left 85
unprotected under sunlight.

found worldwide in sweetpotato and other crops. It attacks
the fleshy organs of plants that are rich in sugar or starch.

Control. Washing storage roots is especially con-
ducive to rot. Care in handling and proper curing can reduce
disease incidence. So far, no resistance has been found,
but some varieties rot faster than others because they
are more susceptible. Curing is accomplished by storing
after harvest at 29–32°C and 95–100% relative humidity
for 5–7 days with adequate ventilation (at least 8 cubic
feet of air per ton per day). Subsequent storage is best
at around 13°C and 95% relative humidity.

102 103

Diseases Caused by
Nematodes

Root-Knot Nematode
Meloidogyne spp.

Symptoms. Affected plants become stunted, foliage
turns yellow and flagging, and flower production is ab-
normal. On fibrous roots, round to spindle-shaped
swellings (galls) are produced together with egg masses
on the surface (Fig. 86). Large portions of the root system
can become necrotic. The storage roots of some varieties
react with longitudinal cracking (Fig. 87), whereas in others,
blister-like protuberances emerge through the epidermis 86
(Fig. 88).

Biology. Meloidogyne spp. are distributed worldwide
on several hosts, such as potato and tomato. These nema-
todes survive in the soil as egg masses and in plant debris
as infective juveniles. They can be transported by irrigation
water and disseminated through infested propagating
material.

Distribution and importance. This root-knot
nematode is one of the most destructive on sweetpotato
because of its wide distribution and damage caused to
storage roots.

Control. Resistance, crop rotation (such as with rice
in Asia), and selected nematode-free planting material 87 88
can help to control this disease. In East Africa, nematodes
are rarely associated with sweetpotato and no control
measures are needed.

104 105

Brown Ring
Ditylenchus destructor, D. dipsaci

Symptoms. Fleshy roots, some time after they are
stored, show symptoms as depressed areas (Fig. 89). In
cross sections, initial infections appear as necrotic isles
of brown tissue scattered throughout the flesh. In
advanced stages, the pulp becomes completely blackened,
slightly soft, and corky (Fig. 90). These nematodes affect
fleshy roots only during storage. No symptoms have been
found in the field.

Biology. The two species of Ditylenchus are dis-
tributed worldwide and have a vast host range. They are 89
migratory endoparasites.

Distribution and importance. On some occa-
sions, storage losses can be serious.

Control. No control measures are known.

90

106 107

Sweetpotato guide to major pests, diseases and disorders

Sweetpotato guide to major pests, diseases and disorders

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