Anthracnose is a leaf blight disease caused by a collection of fungal and bacterial spores. Each species of plant or tree is susceptible to its own collection of fungi or bacterium, which causes the disease. In oak trees, the anthracnose infection is caused by the Apiognomonia quercina fungus, or oak anthracnose.
Oak anthracnose is transported by fungal spores. These fungal spores germinate and multiply on the dead leaves and stems that lie around your oak tree. The spores lie dormant throughout the winter months, only to become active again in the early spring. The rain and wind sweep these fungal spores onto the newly developing foliage and twigs of your oak tree. There, the infection spreads throughout the tender tree as the leaves develop.
Leaf spot is usually the first symptom of oak anthracnose infections. The spots appear on the young leaves and are centralized near the primary veins of each leaf. These tan-colored spots are accompanied by dark-colored fungal bodies, which appear on the underside of the leaf. The severely infected oak tree may also experience leaf curling and shriveling, twig and branch die-back and cankering.
The effects of oak anthracnose are dependent upon the severity of the infection, as well as the vigor of your oak tree. Most oak anthracnose infections result in premature defoliation of some foliage. Severely infected trees, however, will be overwhelmed with necrotic foliage, which will result in heavy, premature defoliation. This extreme loss of leaves results in the tree’s loss of vigor and reduction of acorn production. This loss can causes lasting effects into the following season, with reduced leaf production and continued growth stunt.
Oak anthracnose infections are greatly reduced when your oak tree’s area is kept free of defoliated debris. Removing the debris from the area provides the fungal spores with fewer areas in which to germinate. The disease can also be controlled with pruning; however, the pruning should be completed during the oak’s dormant season. Remove the infected areas from the tree with sharp, sterile pruning shears that are sterilized between each cutting to prevent further infection throughout the tree. The interior branches should also be thinned to promote increased air circulation throughout the tree.
Fungicidal treatments are helpful in controlling the spread of the disease but are ineffective on severely infected trees. These treatments are quite effective, however, in preventing oak anthracnose infections when combined with proper pruning and care. For best results, these fungicidal treatments should be thoroughly applied to every area of the tree, including its highest points. Apply the treatments in the early spring as your tree enters the growing season. Use multiple applications for the best results.