Algal leaf spot

Title: Diseases of avocado
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00066885/00001
 Material Information
Title: Diseases of avocado
Series Title: Plant Pathology Fact Sheet PP-21
Physical Description: Book
Language: English
Creator: Pernezny, Ken
Marlatt, R. B.
Affiliation: University of Florida -- Florida Cooperative Extension Service -- Department of Plant Pathology -- Institute of Food and Agricultural Sciences
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Publication Date: 1988
Spatial Coverage: North America -- United States of America -- Florida
 Record Information
Bibliographic ID: UF00066885
Volume ID: VID00001
Source Institution: Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location: Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management: All rights reserved by the source institution and holding location.

Table of Contents
        Page 1
        Page 2 (MULTIPLE)
    Algal leaf spot
        Page 3
        Page 4
        Page 5
Full Text

Plant Pathology Fact Sheet

Diseases of Avocado in Florida

Ken Pernezny, Professor, EREC, Belle Glade,and R. B. Marlatt, Professor (De-
ceased) TREC, Homestead, FL., November 1988, Revised December 2000.

Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean

Commercial and backyard plantings of
avocadoes have increased all over south Florida
in recent years. In the subtropical environment,
where this crop is grown, diseases, especially
those incited by plant parasitic fungi, com-
monly cause important reductions in yield and
quality of avocado fruit. This fact sheet de-
scribes some of the commonly encountered dis-
eases of avocado in Florida, and the weather
conditions conducive to outbreaks of each dis-
ease. Due to frequent changes in the availabil-
ity and use restrictions for specific agricultural
chemicals, consult the University of Florida
Extension Service for specific, current fungicide


Avocado scab is caused by the fungus
Sphaceloma perseae. The disease is most promi-
nent and most easily diagnosed on the fruit of
very susceptible varieties. On the fruit, spots
are first oval, slightly raised, and brown to pur-
plish-brown. As the fruit mature, spots coalesce
and the centers of these spots become sunken
(Fig. 1). A large portion of the fruit may become

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 (Fig.
2), 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 hurried inspection, be confused with
scale insects.

Scab infection is favored by cool, moist
conditions. This fungus is a pathogen of young
tissue. Leaves become quite resistant after 1
month. The fruit of all varieties become resis-
tant once they reach about half size. One of the
most important economic aspects of scab fruit
infection is the creation of portals for the entry
of fruit-rotting organisms (see anthracnose be-

Chemical sprays play an important role
in the adequate control of this disease. There is
considerable variability among varieties in their
susceptibility to scab. Lula, Hall, and most avo-
cado seedlings are very susceptible. The vari-
eties Booth 3, 5, 7, 8; Monroe; Choquette; and
Trapp are moderately susceptible. Waldin,
Pollock, Booth 1, and Collins are only slightly

Cercospora Spot

Cercospora spot is caused by the fungus
Cercospora purpurea. Individual spots on leaves
are very small, less than 1/10 inch (2.5 mm) in


diameter, and brown to purple in color. The
angular appearance of the leaf spots is highly
diagnostic (Fig. 3). Many of these leaf spots are
surrounded by yellow haloes. During the rainy
season, grayish spore masses may be seen on
the surface of the spots with a hand lens. Indi-
vidual leaf spots may coalesce to form irregu-
lar areas of brown tissue.

On the fruit, damage begins as small, ir-
regular, brown spots that enlarge and coalesce
(Fig. 4). Fissures often appear in these spots and
are very commonly entry points for the anthra-
cnose fungus.

Wind and rain play an important role in
dissemination of C purpurea spores. Insects
may also spread the pathogen. The most favor-
able time of year for fruit infection appears to
be during the rainy season from May through

The disease can be controlled to a great
degree by timely fungicide applications. How-
ever, control is more difficult and requires more
fungicide use for late-maturing varieties, such
as Lula and Choquette.


Anthracnose, caused by the fungus
Colletotrichum gleosporioides, is a serious disease
of avocado fruit. As far as is known, no other
plant parts are infected. 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 radiat-
ing from the lesion center (Fig. 5). One observes
the most serious aspect of this disease on ma-
turing fruit. The fungus can progress into the
flesh of the avocado fruit, producing a green-
ish-black decay, which eventually may involve
a large portion of the fruit.

This fungus is considered to be a "weak"
pathogen of avocado fruit; i. e., it requires some

type of wound created by some other means,
in order to penetrate the avocado and subse-
quently cause disease. Mechanical damage,
scab, and especially Cercospora spot lesions are
known entry sites for the anthracnose fungus.
Insects may also provide wound-infection sites.

Since all varieties are susceptible, good anthra-
cnose control depends on good control of other
diseases (especially Cercospora spot) and
avoidance of cuts and bruises to the fruit in
handling. Fruits showing any sign of anthra-
cnose should not be packed in cartons with
healthy fruit. Harvesting fruits in an immature
condition may substantially contribute to an-
thracnose appearance at the market place, be-
cause the fungus may be carried on the imma-
ture fruit and will subsequently invade the flesh
through small cracks made during postharvest
handling procedures.

Powdery Mildew

Powdery mildew, caused by the fungus
Oidium, is a sporadic disease affecting only avo-
cado foliage. To diagnose this problem the ob-
server usually needs to examine the
undersurface of leaves. On young leaves the
powdery mildew spots are dark green and cov-
ered with a dry powdery layer of the causal
fungus. On mature leaves, these spots turn
purplish brown (Fig. 6), with a whitish fungal
growth. These spots eventually lose their
undersurface fungal coating, and leave distinc-
tive net-like, brown blotches on the
undersurface (Fig. 7). Yellowish areas may later
appear on the upper leaf surface, opposite the
net-like lesions.

Powdery mildew can occur in all sea-
sons, but can be particularly bad during cool,
dry times of the year. If the trees are being
sprayed for the other diseases mentioned
above, the disease severity is not thought to be
above a damage threshold for the crop.

Algal Leaf Spot

During a period from late summer to late
winter an alga named Cephaleuros produces leaf
spots on avocado, barely visible at first, but at-
taining 1/4 inch (6.2 mm) in diameter within a
few months. The slightly raised roughly circu-
lar spots are green, yellowish-green, or rust
colored, with rather smooth or fringed margins.

The alga eventually produces rust-col-
ored, microscopic "spores" on the raised sur-
faces of spots (Fig. 8). For this reason, the dis-
ease is sometimes called red alga spot.

Young spots are visible on the upper leaf
surface. Later, as they enlarge and become gray
to white, they discolor the opposite, lower side
of the leaf darkgreen, olive-green, or brown. A
yellowish halo may surround these old spots.

Wind and rain carry the "spores" from
diseased to healthy leaves. Copper fungicides
used for other avocado diseases have provided
control in the past.


The authors gratefully acknowledge
the assistance of Seymour Goldweber and C.
W. Campbell in preparation of this fact sheet.

Figure 1. Severe scab infection with much Figure 2. Scab infection on the underside of
coalescing of lesions on 'Lula' avocado fruit. avocado leaf.

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Figure 3. Cercospora leaf spot on leaf of
'Booth 8' avocado.

Figure 4. Advanced state of Cercospora
infection on fruit of 'Ruehle' avocado.

. *^*"

Figure 5. Avocado fruit with anthracnose.
The anthracnose is specifically the very
large, dark lesion, in the middle of the fruit,
with the characteristic splits in the center.

Figure 6. Powdery mildew on underside of
mature avocado leaf.

Figure 7. Star-like pattern characteristic of
old powdery mildew lesions on underside of
avocado leaf.

Figure 8. Agal Leaf spot (Cephaleuros) on
avocado leaf.

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