Anthracnose is one of the most widespread and severe fungal diseases affecting avocado production globally, often leading to substantial post-harvest losses. This fungal disease affects the avocado fruit, foliage, and twigs, posing a considerable challenge for both commercial growers and home gardeners. While infection begins when the fruit is still on the tree, the most damaging symptoms typically become apparent only after harvest, as the fruit begins to ripen.
Identifying Anthracnose Symptoms
The appearance of anthracnose differs depending on the infected part of the avocado plant, though fruit symptoms are the most commercially significant. The fungus establishes a latent infection on young, green fruit, remaining dormant and undetectable until the fruit matures and begins to ripen off the tree. This latency occurs because the unripe fruit skin contains anti-fungal compounds that inhibit the pathogen’s growth.
Fruit Symptoms
The most characteristic sign of the disease on fruit appears as dark, circular, and increasingly sunken lesions that enlarge rapidly during ripening. These spots, usually less than 5 millimeters in diameter before harvest, become blacker and spread across the fruit surface after picking. When the decay extends into the pulp, the rot often exhibits a distinctive hemispherical or bowl-shaped pattern in the flesh. In humid conditions, the center of these lesions may develop masses of slimy, pinkish-colored spores.
Leaf and Twig Symptoms
Infection of the leaves usually results in irregularly shaped brown spots, often concentrated near the tips or margins, which can coalesce into larger necrotic areas. Severe leaf infection can cause premature leaf drop, weakening the tree over time. On the twigs and flowers, the fungus can cause dieback, resulting in brown or purplish lesions on new shoots. Flower blight can also occur, leading to the failure of the flower to produce fruit or causing young fruit to drop prematurely.
How the Fungus Spreads and Thrives
The primary cause of avocado anthracnose is the fungal species Colletotrichum gloeosporioides, though other species within the Colletotrichum genus can also be involved. This fungus survives and overwinters in infected plant debris, dead leaves, and mummified fruit left within the tree canopy or on the orchard floor. These materials serve as sources for new infections when conditions become favorable.
The fungal spores, known as conidia, are dispersed mainly by splashing water, such as rain, dew, or overhead irrigation. Spores can be moved from infected branches and leaves to healthy fruit surfaces. Once a spore lands on an avocado fruit, it can germinate and establish a latent infection within approximately seven hours if the fruit surface remains wet.
The disease thrives in warm, humid, and wet environments, with optimal infection temperatures often around 82°F (28°C). Conditions that prolong moisture on the plant surface, like poor air circulation within a dense canopy, significantly increase the risk of infection. The fungus can also gain easier access to the fruit flesh through entry points like insect feeding sites or mechanical abrasions.
Prevention and Treatment Strategies
Controlling anthracnose requires a season-long integrated approach combining cultural practices with targeted chemical treatments. The goal of pre-harvest management is to prevent the initial infection and establishment of the latent fungus on the fruit.
Cultural Control
Effective cultural control centers on reducing humidity within the tree canopy and eliminating sources of fungal inoculum. Growers should prune trees to improve air circulation, often by keeping the lowest limbs at least two feet off the ground and removing dead or diseased wood and twigs. All pruned material and infected fruit should be removed from the immediate area to prevent the fungus from sporulating and reinfecting the tree.
Ensuring proper drainage in the growing area is also helpful, as wet conditions encourage fungal growth. Harvesting should occur only during dry periods, as picking wet fruit increases contamination and spore spread. Utilizing windbreaks minimizes fruit scarring caused by abrasion, which serves as an entry point for the pathogen.
Chemical Control
Since the infection begins in the field, chemical control relies on protective fungicides applied during the fruit development stage. Copper-based fungicides, such as copper oxychloride or copper hydroxide, are commonly recommended for pre-harvest application. These protective sprays should begin during the flowering period and continue every two to four weeks until harvest. This schedule maintains a protective coating on the developing fruit.
Post-Harvest Handling
Once the fruit is picked, the development of latent infection must be slowed through careful handling and temperature management. Rapid cooling of the fruit immediately after harvest is a highly effective non-chemical intervention. Fruit pulp temperature should be cooled quickly, ideally to around 41°F (5°C). Delays in cooling accelerate the spread of the disease during ripening. Post-harvest dips with registered fungicides may also be used in commercial settings to reduce disease incidence before the fruit enters the supply chain.