Anthracnose is a common fungal disease that affects avocado trees and their fruit. It poses a challenge for both commercial producers and home gardeners. This disease diminishes the quality of the fruit, often becoming most apparent after the avocados have been picked. Understanding its signs, causes, and management is important for maintaining tree health.
Identifying Anthracnose on Avocados
The most recognizable signs of anthracnose appear on the avocado fruit, especially as it begins to ripen post-harvest. Initially, small, light brown to black, circular spots form on the skin. These spots are often sunken and can expand rapidly, sometimes merging into larger, irregular blotches that damage the skin and flesh. In humid conditions, the center of these spots may develop pink or orange-colored masses of fungal spores.
While fruit symptoms are the most damaging, the fungus can also affect other parts of the tree. On leaves, the disease manifests as yellowish or brown spots that can grow into larger dead patches, often starting at the leaf tip or margins. In severe cases, this can lead to significant leaf drop. Young twigs and flowers can also show signs of infection, appearing as dark, necrotic lesions.
Causes and Spread of the Fungus
The primary cause of anthracnose is the fungus Colletotrichum gloeosporioides. This pathogen causes latent infections, infecting young fruit on the tree but remaining dormant and invisible. Symptoms do not show until the fruit is harvested and the ripening process begins. As the avocado softens, its natural resistance to the fungus decreases, allowing the pathogen to become active and cause decay.
The spread of this fungus is heavily dependent on environmental conditions. It thrives in warm, wet weather, with high humidity and frequent rainfall creating the ideal environment for spores to develop and disperse. The spores are primarily spread by water, such as splashes from rain or overhead irrigation, which carry them from infected dead leaves and twigs to healthy fruit and foliage. Harvesting during wet weather can increase the likelihood of post-harvest disease.
Management and Prevention Strategies
Controlling anthracnose relies on consistent cultural practices. A primary step is pruning to improve air circulation throughout the tree’s canopy. Removing lower limbs so the canopy is at least two feet off the ground and thinning out dense areas helps leaves and fruit dry more quickly. This practice is best done during dry periods to avoid spreading spores.
Sanitation is another important component of disease management. Regularly removing and disposing of dead branches, fallen leaves, and old fruit from around the trees helps to reduce the amount of fungal inoculum present. Since the fungus survives on dead plant material, this cleanup minimizes the sources from which spores can spread. When irrigating, it is best to avoid wetting the foliage and fruit, directing water to the base of the tree.
For more persistent issues, preventative fungicide applications can be used. Copper-based fungicides are often used and are typically applied at intervals after the flowering period, ensuring thorough coverage of the developing fruit. These treatments should be applied before the onset of warm, rainy seasons to protect the fruit. Careful handling of fruit after harvest to avoid bruising or injury also helps prevent the fungus from entering and causing decay.
Post-Harvest Impact and Consumption
The reason anthracnose symptoms often appear after picking is linked to the ripening process. While on the tree, immature avocados have natural defenses that keep latent fungal infections in check. Once harvested, these defenses break down as the fruit ripens and softens, allowing the fungus to multiply. Promptly cooling the fruit to about 41°F (5°C) after harvesting can significantly slow this process.
Despite the unappealing appearance of the dark, sunken spots, the fungus itself is not harmful to humans. The unaffected flesh of an avocado with anthracnose is perfectly safe to eat. To consume the fruit, you can simply cut away and discard the spoiled, discolored portions. The remaining parts of the avocado will have their normal flavor and texture, though severely affected areas might taste sour.