Title: Florida plant disease management guide
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Permanent Link: http://ufdc.ufl.edu/UF00053871/00029
 Material Information
Title: Florida plant disease management guide
Alternate Title: Ornamentals and turf
Fruit and vegetables
General plant pathology, field crops and pasture grasses, fungicides, adjuvants and application techniques
Physical Description: v. : ; 28 cm.
Language: English
Creator: University of Florida -- Dept. of Plant Pathology
Florida Cooperative Extension Service
Publisher: The Extension
Place of Publication: Gainesville Fla
Frequency: annual
regular
 Subjects
Subject: Plant diseases -- Periodicals -- Florida   ( lcsh )
Pesticides -- Periodicals   ( lcsh )
 Notes
Statement of Responsibility: Plant Pathology Dept., University of Florida and Institute of Food and Agricultural Sciences, Florida Cooperative Extension, University of Florida.
Numbering Peculiarities: Issued in three volumes: v. 1, General plant pathology, field crops and pasture grasses, fungicides, adjuvants and application techniques; v. 2, Ornamentals and turf; v. 3, Fruit and vegetables.
General Note: Description based on: 1999-2000.
General Note: "SP-52"
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Bibliographic ID: UF00053871
Volume ID: VID00029
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 44549741
lccn - 00229071
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Preceded by: Florida plant disease control guide

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PP-233
U UNIVERSITY of
UF FLORIDA
IFAS Extension



2006 Florida Plant Disease Management Guide: Avocado

(Persea americana) 1


Aaron J. Palmateer, Randy C. Ploetz, and Philip F. Harmon2


Avocado Diseases Caused by
Fungi and Stramenopiles

Phytophthora root rot

Phytophthora root rot is the most important
disease of avocado and the limiting factor for
production in many regions throughout the world.

Symptoms and Causal Organism

Phytophthora cinnamomi, the causal agent,
forms several different spore stages that are involved
in disease development or survival: sporangia (which
produce motile zoospores), chlamydospores, and
oospores. Sporangia are broadly ellipsoid to ovoid,
do not have a rounded protuberance (are
nonpapillate) and are persistent (noncaducous).
Sporangia are not produced in sterile media, but are
produced in non-sterile soil extract or in sterile salt
solution.

Chlamydospores are common when growing P.
cinnamomi on agar media and are abundant on V8


juice agar or in V8 juice broth cultures.
Chlamydospores are thin-walled, globose to
pyriform, and range from 31-50 pm in diameter.
They are borne terminally or on short, lateral
branches, frequently in grape-like clusters of 3 to 10
spores.

Phytophthora cinnamomi is heterothallic having
Al and A2 mating types, but the A2 type may form
oospores in the presence of root extract, some species
of the soil borne fungus Trichoderma viride, and in
the presence ofoleic acid.

The pathogen primarily causes a rot of the fine
feeder roots that blacken, become brittle and
eventually die (Figure 1). As the disease progresses,
feeder roots are reduced resulting in reduced; soil
beneath the trees remains wet. Severely infected
trees show sparse foliage with pale green, often
wilted leaves and dieback occurs in advanced stages
(Figure 2).


1. This document is PP-233, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural
Sciences, University of Florida. Published October 2006. Visit the EDIS Web Site at http://edis.ifas.ufl.edu.
2. A. J. Palmateer, assistant professor, Plant Pathology Department, Tropical Research and Education Center--Homestead, FL; R. C. Ploetz, professor, Plant
Pathology Department, Tropical Research and Education Center--Homestead, FL; P.F. Harmon, assistant professor, Plant Pathology Department; Florida
Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.
The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the
products named, and references to them in this publication does not signify our approval to the exclusion of other products of suitable composition.
All chemicals should be used in accordance with directions on the manufacturer's label.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and
other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex,
sexual orientation, marital status, national origin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service,
University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry
Arrington, Dean







2006 Florida Plant Disease Management Guide: Avocado (Persea americana) 2


Figure 1. Blackened and brittle feeder rots infected with P.
cinnamomi.


Figure 2. Severely infected trees with sparse foliage, wilted
leaves and dieback caused by Phytophthora cinnamomi

Disease Cycle and Epidemiology

An avocado planting in poorly drained soil will
develop Phytophthora root rot if the pathogen is
present. Areas wit over irrigation and excessive
rainfall can be problematic.

The pathogen can be disseminated by several
means including soil movement, nursery stock,
infested seeds, equipment and especially water.
Infection by P. cinnamomi is optimum at soil
temperatures of 70-860F (21-300C), and increases
as soil moisture rises due to favorable conditions for


sporangia formation and zoospore release, motility,
and infection.

Management

Only disease-free nursery stock should be used.
Do not plant in areas that are subject to flooding.
Avoid the introduction of soil or water from infested
areas to clean fields on equipment or plant stock. For
chemical control use one of the following labeled
pesticides according to the manufacturer (Table 1).

Anthracnose

Anthracnose is a serious disease of avocado fruit,
causing drop and rot and reducing shelf life during
storage and transport. Anthracnose is the most
common rot of mature fruit, but also affects leaves,
twigs and young fruit under favorable conditions.

Symptoms and Causal Organism

The causal fungus, Colletotrichum
gloeosporioides, occurs on a large number of hosts.
Colonies on potato dextrose agar are grayish white to
dark gray. Production of aerial mycelia by isolates
varies, ranging from a dense white mat to a few tufts
associated with fruiting bodies (Figure 3).


Figure 3. Culture of C. gloeosporioides growing on potato
dextrose agar.

Conidia (7-20 X 2.5 Gim) are hyaline,
unicellular, and either cylindrical with obtuse ends or
ellipsoidal with a rounded apex and a narrow, truncate
base. They form on hyaline to lightly brown
conidiophores in acervuli that are irregular in shape
and approximately 500 pim in diameter.
Characteristic setae (4-8 X 200 pm) are one to four






2006 Florida Plant Disease Management Guide: Avocado (Persea americana) 3


septate, brown, slightly swollen at the base, and
tapered at the apex (Figure 4). The teleomorph,
Glomerella cingulata, is not known to play a role in
the disease cycle.

When the foliage remains damp, especially over
night, the fungus enters weakened and wounded
tissues, causing small, irregular, black spots that
eventually coalesce to cover a large areas (Figure 5).
Often, one can observe the pink to salmon-colored
spores oozing from the lesions.

Fruit lesions start as circular, slightly sunken,
brown to black spots. These lesions enlarge rapidly,
under favorable conditions, very often becoming
conspicuously sunken, and very often develop cracks
radiating from the lesion center (Figure 6). The most
serious aspect of this disease can be observed on
maturing fruit. The fungus can progress into the flesh
of the avocado fruit, producing a greenish-black
decay, which eventually may involve a large portion
of the fruit.


Figure 5. Anthracnose symptoms on foliage.


Figure 6. Anthracnose symptoms on fruit.


Figure 4. Characteristic setae of C. gloeosporioides.

Disease Cycle and Epidemiology

This fungus is considered to be a "weak"
pathogen of avocado; i.e., its damage is enhanced by
wounds created by wind, insects, and other pathogens
that assist penetration subsequent disease
development. Scab and Pseudocercospora spot
(blotch) lesions are common entry sites for the
anthracnose fungus. Since all varieties of avocado
are susceptible, good anthracnose control depends on
adequate control of other diseases and avoidance of
cuts and bruises to the fruit in handling. Fruits


showing any sign of anthracnose should not be
packed in cartons with healthy fruit. Harvesting fruits
in an immature condition may substantially
contribute to anthracnose appearance at the market
place, because the fungus may be carried on the
immature fruit and will subsequently invade the flesh
through small cracks made during post-harvest
handling procedures.

Management

It is recommended that wind and insect damaged
trees be treated with a fungicide such as azoxystrobin
or copper to prevent infection by the fungus. On late
maturing varieties in south Florida such as Nabal,
Taylor and Choquette, a fall application of fungicide
is recommended. Refer to Table 1 for a list of
fungicides currently labeled for use on avocado.






2006 Florida Plant Disease Management Guide: Avocado (Persea americana) 4


Pseudocercospora Spot (Blotch)

Pseudocercospora (Cercospora) spot or blotch is
one of the most common diseases of avocado in
Florida. This pathogen thrives when warm, humid,
and rainy conditions persist.

Symptoms

Symptoms may occur on leaves, stems and fruit.
Small lesions (2 5 mm) appear first on the abaxial
leaf surface and appear brown to purplish brown in
color. Spots eventually develop on both leaf surfaces
as irregular, dark smokey patches on the lower leaf.
Under high humidity, sporulation of the causal
fungus may be seen in lesion centers as gray, felty
growths. Individual lesions may coalesce to form
large areas of necrotic tissue. The fungus can infect
fruit and cause lesions and fruit cracking which may
lead to secondary anthracnose infection (Figure 7).


Figure 7. Symptoms of Pseudocercospora spot on
avocado fruit.

Causal Organism

Pseudocercospora purpurea causes
Psuedocercospora spot. Conidia (2-4.5 X 20-100
Gm) are obclavate cylindrical, pale olive, indistinctly
one to nine septate, and straight to curved.
Conidiophores (3-4.5 X 20-200 Gsm) are pale to
medium olive brown, multiseptate, straight or rarely
branched, and slightly bent at a sharp angle
geniculatee). The pathogen can be difficult to isolate,
but grows readily on nutrient agar. In culture, the
fungus produces tufts of gray mycelium that turn
brown to blackish brown with age.


Disease Cycle and Epidemiology

Infected leaves are sources of inoculum. The
pathogen may penetrate host tissue directly or via
wounds. Once infected, the fungus reproduces
abundantly from the lower leaf surfaces.

Sporulation is greatest during warm, rainy
weather and most abundant from May to September.
Spores are disseminated via wind, rain splash, insects
and irrigation.

Very small fruit and those at or near maturity are
less susceptible than those fruit which are one-fourth
to three-fourths of full size.

Management

Due to the favorable environment for disease
development in south Florida, strategic chemical
control is deemed necessary for successful avocado
production. The disease can be controlled readily by
timely applications of copper or azoxystrobin sprays
(Table 1). Be careful to cover both foliage and fruit.
Application in early May and June gives effective
control on varieties maturing in summer and fall. A
third application is recommended in mid July for
winter maturing varieties.

Stem-End Rot

Stem-end rot is a post harvest disease that
occasionally causes losses under suboptimal storage
conditions.

Symptoms

Fruit rot starts on the stem end and generally
initiates as the fruit ripens or after harvest. Small,
purplish-brown spots appear and can enlarge to
involve the entire fruit surface. The pathogen invades
the flesh causing tissue discoloration, degradation,
and offensive odors, thus compromising the quality
of the fruit.

Causal Organism

In the United States stem-end rot is caused by
Botryosphaeria spp. and their anamorphs, but
Colletotrichum gloeosporioides is also capable of
causing stem-end anthracnose. Literature indicates






2006 Florida Plant Disease Management Guide: Avocado (Persea americana) 5


that the anamorphs of Botryosphaeria spp. are
Fusicoccum luteum and Dothiorella aromatic.

Colonies ofD. aromatica on potato-dextrose agar
(PDA) produce aerial mycelia with dark, dendritic
growth patterns on the undersides and radially
aligned, immersed conidiomata. Conidia (17-25 X
4-6 pm) are narrowly fusiform to clavate, hyaline,
and granular. Other fungi have been reported to cause
stem-end rot either alone, or as a complex.

Disease Cycle and Epidemiology

The anamorph stages of Botryosphaeria spp.
occur as endophytes in avocado stems. They can
infect fruit from endophytically colonized
inflorescence and stem-end tissue pediclee and
adjacent peduncles). Symptoms of endophytic
infections are not apparent in the stem-end tissue until
fruit development is well advanced.

Spores of Fusicoccum spp., are produced on dead
leaves, twigs, and branches in the canopy and spread
by wind and water. In addition to endophytic
colonization, preharvest infections of the stem end
and associated fruit tissue can occur through wounds
and direct penetration by some fungi. Preharvest
infections remain quiescent until the fruit ripen after
harvest.

Environmental conditions influence which
pathogens are present and subsequently cause
stem-end rot. Water stress is reported to promote
endophytic infection. Cool storage conditions
promote infection of C. gloeosporioides and
Phomopsis perseae over L. theobromae.

Management

The causal fungi survive on dead host debris.
Avoid buildup of this material in groves. Use low
sprinklers to avoid movement of the fungi up to the
fruit. Avoid water stress to reduce endophytic
infection. Mulching under the trees to promote tree
litter decomposition should be practiced.

Further, do not harvest fruit when it is raining.
Copper fungicide sprays and other management
practices used for anthracnose control will reduce
inocula of stem-end rot pathogens.


Scab

Scab is a serious problem in humid areas such as
the tropics and subtropics.

Symptoms and causal agent

Avocado scab is caused by the fungus
Sphacelomaperseae. Acervuli are formed, and
conidia (2-30 X 2-5 pm) are hyaline, one-celled, and
ovoid. They may be produced acrogenously or
pleurogenously on conidiophores and conidia are
continuous to several septate. Growth of the fungus
appears as olive to brownish olive in color.

The disease is most prominent and most easily
diagnosed on the fruit of very susceptible varieties.
Spots are first oval, slightly raised, and brown to
purplish-brown (Figure 8). As the fruit mature, spots
coalesce and the centers of these spots become
sunken and a large portion of the fruit may become
rough in appearance.

The lesions on the leaves are less well known
and less readily observed, because they most often
occur in the upper part of the tree canopy. Scab starts
on leaves as discrete, small spots less than 1/8 inch
(3.5 mm) in diameter.

The spots are especially common on veins on the
underside of leaves. As leaf spots develop, they very
often take on a star-like pattern, with the center
eventually dropping out to give a "shot-hole"
effect.

Symptoms on petioles and twigs include oval to
elongate spots that may, on initial inspection, be
confused with scale insects. Eating quality is not
impacted, but severely affected fruit are very
unattractive. Varieties vary in susceptibility to scab.
Lula is the most susceptible commercial variety.
Fuschsia, Pollock, Booth 1 and Waldin are quite
resistant.

Disease cycle and Epidemiology

Sphacelomaperseae, is carried over one season
to the next on leaf and stem lesions. With abundant
moisture and cool temperature, the fungus readily
infects young succulent tissues of avocado leaves,
twigs, and fruits, forming the characteristic scab






2006 Florida Plant Disease Management Guide: Avocado (Persea americana)

undersides. Under
", leaves are general
white powdery gro
S I ., have a yellowish-g


sides of infected areas on mature
y purplish brown and covered with
wth. The upper sides of leaves
reen discoloration (Figure 9).


Figure 8. Symptoms of scab on avocado fruit.


lesions in which spores are produced. These spores
are readily spread through the grove by wind, rain,
dew, and most likely insects. Young leaves are
susceptible, but become resistant once they mature
and the tissues become more hardened. Fruits are
extremely susceptible just after the petals fall and
become more resistant as they develop. One of the
most important economic aspects of scab fruit
infection is the creation of portals for entry by many
fruit-rotting organisms.

Management

Efficacy trials indicate that copper fungicides
seem to give good control and Folpet is also labeled
for controlling avocado scab (Table 1). The
Pseudocercospora spray program works for scab, but
if you're growing Lula it requires 3 additional sprays.
Timed as follows: just as the bloom bud opens (late
January), near the end of the main bloom period (mid
February-March), and 3-4 weeks after all the fruit
have set in a normal season.

Powdery Mildew

Powdery mildew is usually a minor disease
problem, but may become serious enough to warrant
chemical sprays.

Symptoms and Causal Agent

Oidium sp. frequently occur on avocado foliage.
Powdery mildew can become serious on untreated
trees if favorable conditions persist. Infections on
young leaves appear as dark green areas with a
powdery, white, spore-bearing growth on the


Figure 9. Symptoms of powdery mildew on the upper and
lower leaf surface.

The disease is caused by an Oidium sp.
Barrel-shaped conidia are formed in chains on the
underside of the leaves.

Disease Cycle and Epidemiology

Spores of the fungus are dispersed mostly by
wind and rain to young leaves in the spring. Spores
from these primary infections attack subsequent
flushes. Powdery mildew is most severe in damp,
shaded areas, especially in nurseries.

Management

Be certain to scout trees for symptoms when
conditions are cool and humid especially during early
spring in south Florida. Adequate pruning to reduce
humidity and increase transmission of sunlight may
reduce the disease. For chemical control apply sulfur
fungicides according to the manufacturer's label
(Table 1).

Avocado Diseases Caused by
Nonfungal Agents

Alga Leaf Spot

This is a relatively minor disease that occurs
most frequently in tropical avocado production areas;
it is fairly common on avocado in south Florida.






2006 Florida Plant Disease Management Guide: Avocado (Persea americana) 7


Symptoms and Causal Agent

Alga leaf spot is caused by Cephaleuros
virescens in the Americas. The alga thallus is orange
to rust colored and develops below the host cuticle. It
produces 32 X 25 pm sporangia on the terminals of
erect stalks (Figure 10). Biflagellate zoospores are
produced in the sporangia.


Figure 10. Erect stalks of Cephaleuros virescens.

Leaf spots appear round, orange to red, raised,
velvety and are produced predominately on leaves
(Figure 11). Numerous infections can cause some
leaf drop. The spots turn greenish gray with age and
become quite prominent. Spots turn red to bright red
when the alga is reproducing. A yellow halo may
surround the spots.


Figure 11. Symptom and sign of alga leaf spot.

Disease Cycle and Epidemiology


The pathogens spores are spread from old leaves
to new leaves by wind and rain. Spores are produced
continuously during periods of high rainfall and


prolonged humidity, which contribute to disease
development.

Management

Humidity in the canopy should be reduced by
judicious pruning. Use of copper-based fungicides in
the spring to control other diseases of avocado will
aid in the control of alga spot.

Sunblotch

Sunblotch was originally described as a
physiological disorder in California in 1928, but with
the description ofviroids in 1970s, sunblotch was
established as a viroid-induced disease. It is the only
known disease of avocado caused by a viroid.
Sunblotch can occur anywhere avocados are grown.
It is considered to be a minor problem, although there
have been reports of serious outbreaks in the past.

Symptoms and Causal Agent

Symptoms on twigs include a yellow, red, or
necrotic streaking that often is associated with a
shallow indentation along the length of the twig.
Fruit appear white or with yellow blotches or streaks
that may or may not be depressed (Figure 12).
Leaves may have white or yellow variegated areas
and they may be deformed. Trees are often stunted
and display a sprawling growth. Symptomatic trees
may have reduced yield.


Figure 12. Symptoms of avocado sunblotch viroid on fruit.

Sunblotch is caused by the avocado sunblotch
viroid (ASBVD). ASBVD is a single-stranded,






2006 Florida Plant Disease Management Guide: Avocado (Persea americana) 8


circular RNA molecule of 247 nucleotides with a
molecular weight of 0.8 X 10.

Disease Cycle and Epidemiology

The viroid is carried within the host tissue.
Cultivar, environmental conditions, and possibly the
viroid strain all impact symptom expression. Some
trees are symptomless carriers in that the symptoms
are absent even on vegetative tissue that have high
titers of the viroid. A drastic reduction in yield of a
healthy appearing avocado tree may indicate
symptomless infection of ASBVD.

The viroid is transmitted through the seed of
infected trees with great frequency. Although
symptoms may not be present when infected
seedlings are used as rootstocks, the disease will
often appear on scions grafted to them. Trees with
symptoms transmit the viroid to seed infrequently,
and the resultant infected seedlings often show
symptoms. The most frequent method for
transmitting the viroid is through the use of infected
budwood or rootstock seedlings when grafting. The
viroid can be transmitted by the use of contaminated
pruning tools, by root-to-root grafting, and in seed
resulting from fertilization of a non-infected tree with
pollen from an infected tree. Other than root-to-root
grafting of infected trees, sunblotch has not been
observed to spread in the field.

Management

Trees with symptoms can be removed from the
grove and the remaining stumps killed. Pruning tools
and harvesting clippers should be disinfested between
trees. Practice careful selection of disease-free scions
and seed sources.







2006 Florida Plant Disease Management Guide: Avocado (Persea americana) 9


Table 1. Fungicides registered for use on Avocado in Florida.


Chemical Fungicide Maximum Rate/ Acre Minimum Disease or Remarks2
Group1 Application Season Days to Pathogen
Harvest

Abound, Heritage 11 15.4 oz. 1.5 lbs. 0 Anthracnose Abound applications
(azoxystrobin) Cercospora & should begin prior to
Pseudocercospora disease development.
Leaf Spot

Aliette 33 5 Ibs. 20 Ibs. 12 hrs Phytophthora root Do not mix with copper.
(Fosetyl-AI) ___ rot
Armicarb 2.5-5.0 0 General contact Refer to label.
(potassium lbs./100 gal biocide, numerous
bicarbonate) water pathogens
Fosphite 1-2 qts. Scab Do not mix with copper.
Fungicide,
Helena Prophyt,
and others
(potassium
phosphite)
Kocide and M1 4.5 Ibs. 1 Anthracnose Make initial applications
others (copper Algal Leaf Spot just before flowering and
hydroxide) Pseudocercospora repeat on a weekly
Leaf Spot schedule until just before
harvest. Apply in
sufficient water for
thorough coverage. Use
the higher rates for
severe disease.
Oxidate, 40-128 fl.oz. General contact Refer to label.
Oxyfresh biocide, numerous
(hydrogen pathogens
dioxide)
Phostrol 33 4.5 pints in 18 pints Phytophthora root Apply at transplant or
(phosphorous up to 500 gal rot beginning of growing
acid) water season.
Ridomil Gold EC, 4 1/8-14.5 fl.oz 1.5 gal/A 28 Phytophthora root Refer to label.
Ridomil Gold GR rot
(mefenoxam)

Rootshield 5.5-12 Ibs. Root system Soil temp must be greater
(Trichoderma disease protectant than 50F
harzianum)
Serenade Max 1-3 Ibs. 0 Anthracnose
(Bacillus subtilis)
Sulfur 6L, Sulfur Powdery Mildew See label.
Flowable,
Thiolux (Sulfur)
Trilogy (Neem oil) 32 fl.oz/25 Anthracnose, Scab See label.
gal water







2006 Florida Plant Disease Management Guide: Avocado (Persea americana) 10


Table 1. Fungicides registered for use on Avocado in Florida.


Chemical Fungicide Maximum Rate/ Acre Minimum Disease or Remarks2
Group' Application Season Days to Pathogen
Harvest

Fungicide group (FRAC Code): Numbers (1-37) and letters (M, U, P) are used to distinguish the fungicide mode of action
groups. All fungicides within the same group (with same number or letter) indicate same active ingredient or similar mode of
action. This information must be considered for the fungicide resistance management decisions. M= Multi -site inhibitors,
fungicide resistance risk is low; U= Recent molecules with unknown mode of action; P= host plant defense inducers. Source:
http://www.frac.info/ (FRAC = Fungicide Resistance Action Committee). Be sure to read a current product label before applying
any chemicals.

information provided in this table applies only to Florida. Be sure to read a current product label before applying any chemical.
The use of brand names and any mention or listing of commercial products or services in the publication does not imply
endorsement by the University of Florida Cooperative Extension Service nor discrimination against similar products or services
not mentioned.




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