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PP24
FloridaPlantDiseaseManagementGuide:'Tahiti'Lime
(Citrus latifolia)1
Tara Tarnowski, Aaron J. Palmateer, Ian Maguire, and Jonathan H. Crane2
DISEASESCAUSEDBYFUNGIAND
STRAMENOPILES
(Figure 1). Symptoms on young, green twigs are
quite similar to those on leaves except that they tend
to be distinctly raised.
MELANOSE
Melanose (caused by the fungus Diaporthe citri)
does not usually cause yield loss, but can at times
cause significant financial loss, especially if there is
substantial fruit blemishing, resulting in down
grading of unsightly fruit to juice production. The
fruit blemishes are superficial and do not affect fruit
quality. The disease is most prevalent during the
rainy season and generally causes more severe
damage on flushes in the summer than the spring.
Generally, the control program for greasy spot
(Mycosphaerella citri) also controls this disease.
Symptoms
Distinct melanose symptoms occur on leaves,
green wood, and fruit. On leaves, melanose lesions
begin as small, dark brown to black sunken spots. As
the leaf tissue ages, the spots may become raised
pustules, with chlorosis that disappears as lesions age
Figure1.
On fruit, lesions begin as light brown or red
circular spots that later become brown to black raised
pustules (Figure 2), imparting a sandpaper-like feel to
the fruit. If infections become numerous, symptoms
appear as large areas of dark, rough scar tissue on the
rind that can crack and is referred to as "mudcake".
Pustules may occur in a tear-streaked pattern due to
1. This document is PP24, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural
Sciences, University of Florida. Original publication date December 1993. Reviewed April 2003. Revised October 2009. Visit the EDIS Web Site at
http://edis.ifas.ufl.edu.
2. Tara Tarnowski, graduate research assistant, Department of Plant Pathology, Tropical Research and Education Center (TREC)--Homestead FL; Aaron J.
Plamateer, assistant professor, Department of Plant Pathology, TREC--Homestead FL; Ian Maguire, biological scientist, TREC--Homestead FL; Jonathan
H. Crane, professor, Department of Horticultural Sciences; TREC--Homestead FL; Florida Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida, Gainesville, FL 32611.
TheInstituteofFoodandAgriculturalSciences(IFAS)isanEqualOpportunityInstitutionauthorizedtoprovideresearch,educationalinformationand
otherservicesonlytoindividualsandinstitutionsthatfunctionwithnon-discriminationwithrespecttorace,creed,color,religion,age,disability,sex,
sexualorientation,maritalstatus,nationalorigin,politicalopinionsoraffiliations.U.S.DepartmentofAgriculture,CooperativeExtensionService,
UniversityofFlorida,IFAS,FloridaA.&M.UniversityCooperativeExtensionProgram,andBoardsofCountyCommissionersCooperating.Millie
Ferrer-Chancy,InterimDean.
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Florida Plant Disease Management Guide: 'Tahiti' Lime (Citrus latifolia)
the dispersal of spores in water running down the
fruit surface.
Figure2.
Causalorganism
Diaporthe citri (anamorph: Phomopsis citri)
produces pycnidia (flask-shaped black structures) in
dead twigs. Two types of conidia (asexual spores) are
produced: small elliptical, hyaline alpha conidia
containing an oil droplet at each end, and long
filiform beta conidia. The fungus is easily cultured on
potato dextrose agar, where it produces colonies with
dense, white mycelia. Flask-shaped sexual structures
called perithecia can be recovered from dead twigs
subsequent to pycnidia production. They contain
two-celled, hyaline asocspores with an oil droplet at
each end.
Diseasecycleandepidemiology
The fungus completes its life cycle in dead wood
tissue. Spores of the melanose fungus are produced in
pycnidia that develop only in dead wood. The spores
are embedded in a sticky matrix and, for all practical
purposes, are released only by splashing rainfall, after
which they can infect young leaves, green twigs, and
fruit. Subsequent lesions on living tissue do not serve
as inoculum sources because no fruiting structures
are formed in them. Fruit remain susceptible for
approximately 3 months after bloom in Florida. High
temperature and prolonged periods of wetness are
required for infection. Melanose tends to be more
severe in groves with lots of dead wood, such as older
groves, groves that are not well maintained or that
have trees with frost damage.
2
Management
Since dead wood is the primary source of
melanose spores, removal of dead wood is a
significant control factor. It is important to note that
serious out breaks of melanose usually occur after
freezes that result in a lot of dead wood in groves.
While pruning may not be feasible on older trees,
removal of frost-damaged limbs in younger groves
can decrease disease incidence. Copper fungicides
are important in controlling melanose on fruit,
especially those applied during the rainy season. It is
not effective in controlling the disease on foliage;
fungicides should be applied frequently as the fruit
are expanding to ensure proper coverage of the fruit
surface.
GREASYSPOT
Greasy spot, caused by the fungus
Mycosphaerella citri, is a major disease of ''Tahiti''
lime. Severe defoliation, resulting in substantial yield
reductions and subsequent long-term loss of tree
vigor can result if it is not adequately controlled. For
more information, see PP154: Greasy Spot
http://edis.ifas.ufl.edu/CH015.
Symptoms
Fungal invasion occurs through stomata on the
underside of the leaves. The first symptoms begin as
small, localized waterlogged areas on the underside
of leaves. Yellow mottling appears on the upper leaf
surface, accompanied by orange to yellow blisters
opposite to the mottling on the underside surface
(Figure 3). With time, infections turn dark brown to
black, with a definite greasy appearance (Figure 4),
and chlorosis disappears. Infected leaves very often
drop prematurely, leading to large-scale defoliation.
The fungus also causes a greasy spot rind blotch,
which is characterized by necrotic specks on the fruit
surface, but this disease is not commonly seen on
lime.
CausalAgent
Mycosphaerella citri (anamorph: Stenella
citri-grisea) forms sexual fruiting structures
(pseudothecia) only after diseased leaves have fallen
and begin to decompose. Dark pseudothecia appear in
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Florida Plant Disease Management Guide: 'Tahiti' Lime (Citrus latifolia)
3
Management
Effective control of greasy spot can be obtained
with well-timed sprays of copper fungicides or
horticultural oil. A single spray in May-June is
usually effective in controlling the disease, although
in groves where greasy spot is severe a second spray
in August can be added.
Figure3.
Because leaf litter provides inoculum for the
disease, rapid decomposition of fallen leaves is
desirable. Mowing leaf litter can speed up
decomposition. Application of urea fertilizer has been
shown to reduce ascospore production. However, the
efficacy of these measures is questionable because
even low levels of inoculum can cause disease
epidemics.
SCAB
Figure4.
tight clumps, and produce hyaline, fusiform, single
septate ascospores.
Conidia are produced on mycelia colonizing the
leaf surface, and are olive brown, septate, and
verricose (long and thin). The fungus can be cultured
on most media, and forms a slow growing, dark grey
to green colony.
DiseasecycleandEpidemiology
Infection of new leaves occurs from invasion by
ascospores released during periods of wetness in
diseased and decomposing lime leaves on the grove
floor. Maximum ascospore production occurs in
Florida from April-May, although peak infection
likely occurs later (June-September), when humidity
and temperatures are higher. The fungus infects
mature leaves through the stomata, and symptoms
take 45-60 days to develop. There is evidence that the
disease is exacerbated when there are high population
levels of rust mites or honey dew-producing insects
(aphids, whiteflies, mealy bugs). Fruit infection can
occur on ''Tahiti'' lime, but does not seem to be that
common.
The fungus Elsinoe fawcetti causes scab. 'Tahiti'
lime is not as susceptible to scab as some other citrus.
However, sporadic outbreaks can cause economic
loss. The disease is most common in groves where
'Tahiti' lime has been grafted onto rough lemon
rootstock. Rough lemon is extremely susceptible to
this fungus and inoculum can build up on suckers
arising at the base of the tree.
Symptoms
Infections on lime leaves appear as light-colored,
raised areas. Older lesions can become warty.
Severely infected leaves are distorted. Infections of
fruit appear as prominent, light-colored, raised areas
(Figure 5) that throw fruit out of grade. Pustules can
be differentiated from mechanical damage in that
they are circular and that they often occur in a pattern
that follows water flow over the fruit surface.
Figure5.
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Florida Plant Disease Management Guide: 'Tahiti' Lime (Citrus latifolia)
Causalorganism
Elsinoe fawcetti produces two types of conidia in
acervuli (asexual structures) on the lesion surface.
These are hyaline and aseptate, or colored and
spindle-shaped. In culture the fungus produces a
slow-growing tan or beige colony.
DiseasecycleandEpidemiology
Conidia spread by rain splash to susceptible
tissue. Only newly emerging leaves are susceptible;
fruit are susceptible until 6-8 weeks after petal fall.
Optimum temperature for infection is 75-80°F
(24-27°C) and only short periods of wetness are
required (several hours), which means dew periods
may be sufficient for infection and disease
development. Long distance disease dispersal occurs
on equipment and especially infested nursery stock.
the vascular tissue. Severe root rot can cause death of
small trees.
Foot rot, also known as gummosis, occurs most
often at the point where the scion joins the rootstock,
causing a canker that can extend down to susceptible
rootstocks and up the trunk. The bark initially softens
and the wood underneath is discolored. Eventually
the bark cracks and citrus gum is secreted
(gummosis). Lesions can spread around tree trunks,
eventually girdling and killing the tree (Figure 6).
Affected trees may display pale green leaves with
yellow veins, defoliation, and dieback.
Management
Cultural controls are very important in
management of scab. If rough lemon has been used
as the rootstock, emerged lemon sprouts must be
promptly pruned out. Use of air layers will eliminate
the rough lemon problem. Overhead irrigation can
increase disease severity. Pruning summer flushes
can decrease the inoculum densities. There are
several fungicides labeled for scab control (see
Fungicide Table). A spray at petal drop and several
weeks later are usually effective in controlling scab.
In heavily infested groves, a third spray can be added
after the spring leaf flush.
PHYTOPHTHORAROT
Phytophthora spp. cause several important
diseases on citrus roots (root rot), trunks (foot rot or
gummosis), and on fruit (brown rot). Root rot and
foot rot can lead to trees that lack vigor, decreased
yields, and even tree death. Brown rot can affect high
percentages of fruits.
Symptoms
Aboveground symptoms of root rot include a thin
canopy, lack of new growth, and reduced yields.
Fibrous roots are rotted and often have a softened and
discolored cortex, which may slough off leaving only
Figure6.
Brown rot lesions on fruit are tan and leathery.
Fruit near the ground are affected first, and then the
disease can spread throughout the canopy.
Causalorganism
Both P. palmivora and P. nicotianae are
prevalent in Florida; with P. palmivora causing
severe brown rot epidemics and P. nicotianae only
causing root rot and foot rot. Both species produce
pear-shaped sporangia that can release motile,
biflagellate zoospores under wet conditions,
thick-walled, circular chlamydospores, and
thick-walled, sexual spores called oospores. Isolates
can be recovered from soil using fruit piece baits or
PARPH media.
DiseasecycleandEpidemiology
Root rot occurs during periods when soil is wet
or flooded, and pathogen populations are maintained
on fibrous roots. During wet periods, sporangia
release zoospores that are attracted to wounded roots
or zones of root elongations. The pathogen can
survive for long periods of time in root debris as
oospores or chlamydospores. Root rot is more severe
4
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Florida Plant Disease Management Guide: 'Tahiti' Lime (Citrus latifolia)
when P. palmivora is present in conjunction with the
weevil Diaprepes abbreviatus. Trees on Swingle
citrumelo rootstock are more susceptible to the
Phytophthora-Diaprepes (PD) complex. For more
information on the PD complex see extension bulletin
2009 Florida Citrus Pest Management Guide: Citrus
Root Weevils http://edis.ifas.ufl.edu/CG006
Foot rot occurs when trunks are infected through
wounds or cracks in the bark. The lesions do not
produce new inoculum, so are not able to spread the
disease from the original trunk lesion.
5
microorganism. Instead, it occurs in conjunction with
an attack by pest insects such as scales and the citrus
whitefly. When whiteflies and related insects feed on
lime leaves and fruit, they secrete sweet, sticky
honeydew that is promptly invaded by the sooty mold
fungus. The result is a black discoloration (due to the
color of the fungus) over leaf (Figure 7) and fruit
surfaces. Sooty mold can be identified by the fact that
it can usually be rubbed off of the plant surface.
Brown rot occurs in the rainy season when
sporangia/zoospores are splashed onto low-hanging
fruit by rain. These infections eventually sporulate
and infect new fruit, and eventually the entire canopy
can be affected.
Management
Figure7.
Control of foot rot is primarily cultural.
Nurseries should use pathogen-free seed and potting
mix. It is important to prevent disease entry into
groves by strict sanitation in the propagation nursery.
Irrigation should be timed to avoid overwatering.
Rootstocks have varying degrees of tolerance to foot
rot. Rough lemon and sweet orange are susceptible
and tangerines, tangerine hybrids, citranges, and
trifoliate orange are more tolerant.
It is thought that sooty mold has little, if any
effect on the general health of the lime tree, and it is
considered of economic importance only when the
black growth builds up to a point where it cannot be
easily cleaned off fruit at the packinghouse. Heavy
colonization on leaf surfaces could lead to decreased
photosynthetic capacity, which could results in
lowered yields.
To avoid foot rot, the bud union should be
planted well above the soil line and weeds should be
adequately controlled to prevent moisture from
building up around tree trunks. Mechanical injury to
tree trunks should be avoided, because they can be
sites for pathogen entry.
Fungicides can be affective in certain
circumstance, depending on pathogen density, grove
history, and susceptibility of rootstocks. For more
information on factors influencing spray regimes, see
PP156: 2009 Florida Citrus Pest Management Guide:
Phytophthora Foot Rot and Root Rot
http://edis.ifas.ufl.edu/CG009.
SOOTYMOLD
Sooty mold is not, strictly speaking, a "disease"
of lime, since it does not involve the invasion and
colonization of the host by a pathogenic
Diseasescausedbybacteria
Canker
Canker is one of the most serious diseases of
citrus, causing yield losses that can lead to
abandonment of citrus cultivation in some areas.
'Tahiti' lime is less susceptible than other types of
citrus, and younger trees are at the most risk from the
disease. With proper management practices, the crop
can be grown successfully in areas where the disease
is present. The bacterium attacks leaves, twigs and
fruit.
Symptoms
Lesions become visible 7-10 days after infection,
and are first seen on the underside of leaves. Lesions
begin on leaves as circular, pinpoint spots and enlarge
to lesions of varying size, depending on
environmental conditions. Lesions become raised,
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Florida Plant Disease Management Guide: 'Tahiti' Lime (Citrus latifolia)
corky pustules, especially on the under-side of leaves,
and eventually the center of the lesion sinks so that
pustules are crater shaped (Figure 8). These
depressed areas occasionally drop from the leaf,
leaving lesions with a shot hole appearance. A yellow
halo can often be seen on younger lesions, but fades
as the lesion ages (Figure 9). Water soaking around
lesion margins is also typical of canker. Lesions on
limes tend to be darker in color than on other citrus
crops. Lesions are often aggregated along leaf
margins or tips.
Canker severity is exacerbated by the presence
of the Asian leaf miner (Phyllocnistis citrella)
because lesions found along feeding galleries often
coalesce to form large diseased areas (Figure 10).
Figure8.
CausalAgent
Canker is caused by the bacterium Xanthomonas
axonopodis pv. citri. It is an obligate aerobe with
yellow-pigmented colonies growing on most culture
media. In media containing sugars, colonies produce
extra-polysaccharides that make them mucoid (slimy).
DiseasecycleandEpidemiology
Bacterial cells survive in lesions and to some
extent in plant debris. When free moisture is present,
bacteria cells ooze out of lesions and are dispersed
short distances by wind-driven rain. Only young host
tissue is susceptible to infection. Bacteria infect the
plant through open stomata or wounds caused by
wind-blown thorns and sand, leaf miners, or pruning.
Optimal infection occurs at 68-86°F (20-30°C).
Bacteria on plant or soil surface do not survive
very long do to sensitivity to UV light, but bacteria
associated with plant debris can survive for several
months, and cells in lesions can survive several years.
X. axonopodis pv. citri has been associated with
several weed species, but the role of weeds as an
inoculum source is not known. High populations of
the Asian leaf miner increase inoculum and disease
levels.
Trees on less vigorous roots stocks and older
trees often have less serious canker problems due to
fewer and less frequent leaf flushes.
Figure9.
Long distance inoculum dispersal occurs mostly
with infected propagative materials (budwood,
rootstock seedlings), infested clothing and
equipment, wooden harvest boxes, and the dumping
of infected cull fruit close to orchards. Major weather
events such as hurricanes and tornados can also
transport inoculum long distances.
Management
Until recently, Florida had an eradication
program in place for the disease. After the 2004-2005
hurricanes spread the disease beyond a controllable
area of south Florida, the program was abandoned,
and growers must now try managing the disease
using several methods.
Figure10.
6
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Florida Plant Disease Management Guide: 'Tahiti' Lime (Citrus latifolia)
7
One of the most effective methods of control is
the installation of windbreaks, which not only
decrease short distance dispersal of inoculum, but
also decrease plant wounding due to wind-blown sand
and debris. Planting inter-rows in a cover crop can
also decrease plant wounding by decreasing the
amount of sand or soil particles that can blow into the
canopy. Planting in sites with high wind should be
minimized, and work in groves should not be done
under wet conditions. Pruning should be done in the
dry season. All worker clothes and equipment should
be disinfested before moving from a diseased to a
healthy orchard.
Leaf miner control is important for minimizing
canker levels. This can be achieved through
insecticide applications. Copper sprays on young fruit
should provide adequate protection from infections.
Two to three treatments will be necessary during the
first 90 days after petal drop, but 6 or more annual
sprays may be necessary in wet years.
Figure11.
CitrusGreening(Huanglongbing)
Citrus greening is a serious disease widespread
in Asia that was recently identified in Florida.
Lemons and limes are more tolerant than other citrus
types.
Symptoms
Figure12.
The earliest symptom of greening is the
appearance of a yellowed shoot (Figure 11)3.
Individual leaves often have a mottled appearance
and can have a deformed shape. On 'Tahiti' lime,
mottled areas are larger than those on other citrus
(Figure 12). Eventually the entire canopy yellows and
trees die. What happens to fruit quality on 'Tahiti'
lime has not been documented at this time.
Adults remain infective for several months.
Symptoms take 4-6 months to develop and symptoms
can be hard to diagnose (they often resemble zinc
deficiency). Therefore, symptomless but infected
plants can be unknowingly distributed from nurseries,
causing long distance spread of the disease).
CausalAgent
Recent research has shown the disease can also
be graft transmitted.
Greening is caused by the bacterium Candidatus
Liberobacter spp. It is a phloem limited bacterium
that has not been successfully cultured.
DiseaseCycleandEpidemiology
The bacterium is transmitted primarily by the
citrus psyllid, Diaphorina citri (Figure 13). Adults
vector the disease, which can propagate in the insect.
Management
The primary management tactic is chemical
control of the citrus psyllid. In addition, biocontrol of
the psyllid using parasitic wasps has been effective in
some regions of the world.
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Florida Plant Disease Management Guide: 'Tahiti' Lime (Citrus latifolia)
8
Figure15.
Figure13.
Infected trees should be promptly removed from
orchards, and only disease-free propagative material
should be planted.
ADDITIONALDISEASES
ALGALDISEASE(ALGASPOT,REDALGAL
DISEASE)
Figure16.
This disease is caused by a parasitic alga,
Cephaleuros virescens. This organism has a wide
host range among tropical and subtropical trees, but
is a particularly serious problem on 'Tahiti' lime.
Lesions on leaves are roughly circular, raised, and
greenish-gray in color. The alga will eventually
produce rust-colored microscopic "spores" on the
surface of the leaf spots, giving the spots a reddish
appearance (Figure 14). Severe leaf symptoms can
cause defoliation.
The alga may also attack branches, and bark
splitting may occur. If wood infections become
severe, girdling and death of branches may result. The
appearance of the masses of red "spores" on the wood
(Figure 15) is highly diagnostic.
Algal disease can be prevented and controlled
with copper sprays during the rainy season (June
through August).
MECHANICALDAMAGE
Leaves, twigs, and thorns that rub against the
rind of the fruit, especially under very windy
conditions, can easily damage young fruit. The
rubbing causes the rupture of oil glands that cause
phytotoxic damage to the rind. These scars are
usually grey in color and appear on the upper side of
fruit (Figure 16). Installation of windbreaks can
decrease mechanical damage. Growers occasionally
report feeding by birds on mature fruit.
References
Gottwald, T.R., da Graça, J.V., and Bassanezi,
R.B. 2007. Citrus huanglongbing: the pathogen and
its impact. Plant Health Progress.
doi:10.1094/PHP-2007-0906-01-RS
Gottwald, T.R.. Graham, J.H., and Schubert,
T.S. 2002. Citrus canker: the pathogen and its impact.
Plant Health Progress
doi:10.1094/PHP-2002-0812-01-RV
Graham, J.H., and Timmer, L.W. 2009. 2009
Florida Citrus Pest Management Guide: Phytophthora
Foot Rot and Root Rot. PP-156, Florida Cooperative
Extension Service, IFAS, University of Florida.
Figure14.
Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office.
Florida Plant Disease Management Guide: 'Tahiti' Lime (Citrus latifolia)
McCoy, C.W., Rogers, M.E., Futch, S.H.,
Graham, J.H., Duncan, L.W., and Nigg, H.N. 2009.
2009 Florida Citrus Pest Management Guide: Citrus
Root Weevils. ENY-611, Florida Cooperative
Extension Service, IFAS, University of Florida.
Mondal, S.N., and Timmer, L.W. 2006. Greasy
spot, a serious endemic problem for citrus production
in the Caribbean Basin. Plant Disease 95:532-538.
Timmer, L.W., Garnsey, S.M., and Broadbent, P.
2003. Diseases of Citrus. pp. 163-196 in Diseases of
Tropical Fruit Crops, ed. R.C. Ploetz. CAB
International; Cambridge, MA.
Timmer, L.W., Garnsey, S.M., and Graham, J.H.
2000. Compendium of Citrus Diseases, second
edition. APS Press; Minneapolis, MN.
3
Figures 11-13 used with permission from the
United States Department of Agriculture's Animal
and Plant Health Inspection Service (APHIS/USDA).
9
Jonathan H Crane