Anthracnose

(View photos to identify causes of fruit damage)

Anthracnose symptoms can develop on flowers, fruit, leaves, or twigs. Infected fruit is the most serious concern, but most fruit damage does not develop until after harvest. External symptoms are difficult to see on ripe 'Hass' fruit because of its dark skin color. Unhealthy or dead leaves are the most obvious symptom in groves. Spots form on leaves, beginning as yellow, then brown discolorations that coalesce into large dead areas. Necrosis occurs across or between leaf veins, on leaf margins, and most often at leaf tips. If disease is severe, trees drop many leaves prematurely. New shoots can develop brown or purplish lesions, and shoots may dieback. Infected flower heads can turn dark and die without producing fruit, or young fruit may form and then drop.

Before harvest, brown to black lesions less than 0.2 inch (5 mm) in diameter develop around lenticels on infected fruit. These small discolorations can be overlooked while fruit are still on the tree, and lesions usually do not enlarge until fruit ripens after harvest. Large lesions sometimes occur on avocados on the tree, usually after infected fruit is injured by insects or mechanical wind rubbing.

After harvest, lesions become blacker, larger, and increasingly sunken. Lesions eventually spread over the entire fruit surface and throughout pulp. When the fruit is cut in half through one of the lesions, rot extending into the flesh often exhibits a hemispherical pattern. Decayed pulp initially is firm, but becomes soft and putrid as decay advances. Pink spore masses may form on the fruit surface and, under wet conditions, a slimy mass of pink spores erupts through the fruit skin.

Colletotrichum gloeosporioides is widespread in avocado and citrus groves. It normally is of little importance because unusually large numbers of spores are required to produce damaging infections. Low humidity and no rain during much of the growing season limit disease development in California. With extended foggy or rainy conditions and mild winter temperatures, and where many dead leaves and twigs and mummified fruit accumulate in trees, the fungus can produce enough spores to cause a disease problem. Spores spread in splashing water and can cause infection anytime from fruit set to harvest. Once infected fruit starts to ripen, temperatures of 75°F and above will accelerate anthracnose development, while temperatures below 59°F retard disease development.

Fuerte, Rincon, and Wurtz scion cultivars are more susceptible to anthracnose than Hass. Healthy trees often recover from foliar infections and defoliation once conditions become dry. Anthracnose becomes a postharvest problem after the grove has been excessively wet for extended periods. Poor growing practices and mishandling of fruit during or after harvest greatly increase the potential for significant fruit loss.

Control anthracnose primarily with good cultural practices in the grove and proper preharvest and postharvest fruit handling.

• Prune out dead limbs and twigs where fungi sporulate. If many dead leaves are entwined in the canopy, knock them out of the tree.
• Prune low limbs to at least 2 feet off the ground to reduce humidity within canopies by improving air circulation.
• Prune and harvest only during dry conditions and minimize fruit contamination and injury.
• Dispose of dead wood and old fruit away from avocado trees before bloom.

Postharvest treatments should not be needed if fruit is properly handled. Keep fruit dry and cool until sold. Postharvest temperature is especially critical to anthracnose development. Cool fruit to 41°F as soon as possible after harvest. Delays of longer than 6 hours before cooling and higher pulp (air) temperatures during these delays will result in increased postharvest fruit decay. Cooling fruit promptly is of increasing importance as the season progresses because fruit ripens faster as it increases in maturity. Avoid storage temperatures below 41°F because chilling injury may occur. Market fruit rapidly.

Anthracnose is rarely significant enough in California avocado groves to warrant fungicide application. Copper or other fungicides thoroughly sprayed on healthy tissue can prevent infection.

• Photos of causes of damage to fruit