Avocado trees are highly susceptible to damaging fungal infections, especially in environments with high humidity or poorly draining soil. These pathogens can rapidly weaken a tree, reduce fruit yield, and, in severe cases, cause tree death. Recognizing specific symptoms and intervening promptly with targeted management strategies is necessary for maintaining a healthy and productive tree.
Recognizing Symptoms of Major Fungal Infections
Accurate identification of the specific fungal disease is the first step toward effective treatment. One of the most serious issues is Phytophthora Root Rot, caused by the water mold Phytophthora cinnamomi, which attacks the tree’s fine feeder roots. Above-ground signs include sparse, small, pale green foliage, and leaves that wilt and develop brown tips even when the soil is wet. The canopy gradually thins, small branches may die back, and the tree may set a heavy crop of small, undersized fruit just before decline.
Another common concern is Anthracnose, caused by Colletotrichum gloeosporioides, which primarily targets the fruit and foliage in humid conditions. On fruit, symptoms appear as small, circular, sunken lesions that are dark brown or black. These spots can enlarge, sometimes cracking, and may exude pinkish, moist masses of fungal spores in wet weather. On leaves, Anthracnose causes irregular brown or black spots, often resulting in premature leaf drop and dieback of young shoots.
Avocado Scab, caused by Sphaceloma perseae, is primarily a cosmetic disease that can affect marketability and allow secondary infections. This fungus causes small, raised, corky lesions that are brown to purplish-brown on the fruit surface. These lesions can merge to form large, rough, scabby patches. The fungus also causes young leaves to become stunted, crinkled, and distorted. Scab infections are most active on young, succulent tissue and are favored by cool, moist weather.
Immediate Non-Chemical Management Techniques
Managing the tree’s immediate environment through cultural practices offers the best long-term defense against fungal pathogens. Water Management and Drainage are particularly important for preventing Phytophthora root rot, which thrives in saturated conditions. Avocado trees require well-draining soil. Where drainage is naturally poor, planting trees on raised soil mounds or berms can significantly improve aeration. Irrigation should be deep and infrequent, allowing the topsoil to dry slightly between waterings, instead of shallow and frequent watering which keeps the root zone perpetually wet.
Sanitation and Pruning mitigate the spread of airborne and water-splashed fungi like Anthracnose and Scab. Removing all fallen, infected leaves, fruit, and dead wood from under the canopy reduces the sources of fungal spores (inoculum). Proper pruning should focus on thinning the canopy to improve air circulation and allow sunlight penetration, which rapidly dries leaf surfaces and inhibits fungal growth. This involves removing inward-growing, crossing, or excessively dense branches to create “windows” in the canopy.
Tools used for pruning must be sanitized between trees, especially when working on diseased material, to prevent carrying pathogens to healthy trees. Mulching is effective for maintaining soil moisture and temperature stability, but it must be applied correctly to avoid creating a fungal haven. A layer of coarse organic mulch, approximately four inches deep, should be applied around the tree. Leave a substantial gap, typically eight to twelve inches, between the mulch and the tree trunk to prevent moisture buildup at the collar, which encourages bark and crown rot.
Selecting and Applying Appropriate Fungicides
When cultural practices are insufficient or disease pressure is high, chemical treatments become necessary. Copper-based fungicides are commonly used as protective, contact treatments for foliar and fruit diseases like Anthracnose and Scab. Application timing is crucial, typically starting when flower buds begin to swell and continuing through the fruit-set period to protect the newly forming fruit. These products must be applied thoroughly to cover all susceptible surfaces, acting as a barrier against spore germination.
For the management of Phytophthora Root Rot, systemic treatments are required because the pathogen resides in the root zone. Products containing phosphite or phosphorous acid help trees tolerate the disease by moving throughout the plant and stimulating natural defenses. These systemic compounds are often applied as a foliar spray or, for severely infected trees, as a trunk injection. Trunk injections are the most direct and effective method for delivering phosphite to the roots of declining trees.
The optimal time for applying phosphite treatments is during periods of active root growth, which typically occurs when new leaf flushes have hardened, usually in late spring or summer. When applying a foliar spray, the product should be mixed precisely according to label instructions, often requiring a pH adjustment for maximum effectiveness. Following the manufacturer’s directions for mixing ratios and application frequency is necessary to ensure the treatment is effective and avoids damaging the tree.