Plant Pathology Fact Sheet PP-21

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

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.

Commercial and backyard plantings ofavocadoes have increased all over south Floridain recent years. In the subtropical environment,where this crop is grown, diseases, especiallythose incited by plant parasitic fungi, com-monly cause important reductions in yield andquality of avocado fruit. This fact sheet de-scribes some of the commonly encountered dis-eases of avocado in Florida, and the weatherconditions conducive to outbreaks of each dis-ease. Due to frequent changes in the availabil-ity and use restrictions for specific agriculturalchemicals, consult the University of FloridaExtension Service for specific, current fungiciderecommendations.

Scab

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

The lesions on the leaves are less well-known and less readily observed, because theymost often occur in the upper part of the treecanopy. Scab starts on leaves as discrete, smallspots less than 1/8 inch (3.5 mm) in diameter.

The spots are especially common on veins onthe underside of leaves. As leaf spots develop,they very often take on a star-like pattern (Fig.2), with the center eventually dropping out togive a “shot-hole” effect. Symptoms on petiolesand twigs include oval to elongate spots thatmay, on hurried inspection, be confused withscale insects.

Scab infection is favored by cool, moistconditions. This fungus is a pathogen of youngtissue. Leaves become quite resistant after 1month. The fruit of all varieties become resis-tant once they reach about half size. One of themost important economic aspects of scab fruitinfection is the creation of portals for the entryof fruit-rotting organisms (see anthracnose be-low).

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

Cercospora Spot

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

diameter, and brown to purple in color. Theangular appearance of the leaf spots is highlydiagnostic (Fig. 3). Many of these leaf spots aresurrounded by yellow haloes. During the rainyseason, grayish spore masses may be seen onthe 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 andare very commonly entry points for the anthra-cnose fungus.

Wind and rain play an important role indissemination of C purpurea spores. Insectsmay also spread the pathogen. The most favor-able time of year for fruit infection appears tobe during the rainy season from May throughSeptember.

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

Anthracnose

Anthracnose, caused by the fungusColletotrichum gleosporioides, is a serious diseaseof avocado fruit. As far as is known, no otherplant parts are infected. Fruit lesions start ascircular, slightly sunken, brown to black spots.These lesions enlarge rapidly, under favorableconditions, very often becoming conspicuouslysunken, and very often develop cracks radiat-ing from the lesion center (Fig. 5). One observesthe most serious aspect of this disease on ma-turing fruit. The fungus can progress into theflesh of the avocado fruit, producing a green-ish-black decay, which eventually may involvea 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 areknown 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 otherdiseases (especially Cercospora spot) andavoidance of cuts and bruises to the fruit inhandling. Fruits showing any sign of anthra-cnose should not be packed in cartons withhealthy fruit. Harvesting fruits in an immaturecondition 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 fleshthrough small cracks made during postharvesthandling procedures.

Powdery Mildew

Powdery mildew, caused by the fungusOidium, is a sporadic disease affecting only avo-cado foliage. To diagnose this problem the ob-server usually needs to examine theundersurface of leaves. On young leaves thepowdery mildew spots are dark green and cov-ered with a dry powdery layer of the causalfungus. On mature leaves, these spots turnpurplish brown (Fig. 6), with a whitish fungalgrowth. These spots eventually lose theirundersurface fungal coating, and leave distinc-tive net-like, brown blotches on theundersurface (Fig. 7). Yellowish areas may laterappear on the upper leaf surface, opposite thenet-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 beingsprayed for the other diseases mentionedabove, the disease severity is not thought to beabove a damage threshold for the crop.

Algal Leaf Spot

During a period from late summer to latewinter an alga named Cephaleuros produces leafspots on avocado, barely visible at first, but at-taining 1/4 inch (6.2 mm) in diameter within afew months. The slightly raised roughly circu-lar spots are green, yellowish-green, or rustcolored, 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 leafsurface. Later, as they enlarge and become grayto white, they discolor the opposite, lower sideof the leaf darkgreen, olive-green, or brown. Ayellowish halo may surround these old spots.

Wind and rain carry the “spores” fromdiseased to healthy leaves. Copper fungicidesused for other avocado diseases have providedcontrol in the past.

Acknowledgement

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

Figure 1. Severe scab infection with muchcoalescing of lesions on ‘Lula’ avocado fruit.

Figure 2. Scab infection on the underside ofavocado leaf.

Figure 3. Cercospora leaf spot on leaf of‘Booth 8’ avocado.

Figure 4. Advanced state of Cercosporainfection on fruit of ‘Ruehle’ avocado.

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

Figure 6. Powdery mildew on underside ofmature avocado leaf.

Figure 7. Star-like pattern characteristic ofold powdery mildew lesions on underside ofavocado leaf.

Figure 8. Agal Leaf spot ( Cephaleuros) onavocado leaf.