Anthracnose fungi overwinter in leaf debris on the ground or in dead areas of the bark on the tree called cankers.
In early spring, spores of the fungus are produced in fruiting structures and are dispersed by splashing rain. These spores can infect expanding leaf buds, shoots or in some cases young leaves.
The infection process is favored by relatively cool temperatures and prolonged periods of leaf wetness. Therefore, the disease tends to be more severe during wet, cool springs. After infection, the anthracnose fungus colonizes leaf tissue and begins to produce new fruiting structures and spores capable of re-infecting expanding leaf tissue. Disease development may continue throughout the spring into early summer if favorable weather persists. These diseases tend to be less of a problem during hot, dry summer weather.
Anthracnose rarely causes significant damage to shade trees; consequently specific control measures generally are not required. Nevertheless, the disease may be unacceptable in certain high visibility landscape settings. The disease also can increase susceptibility to other disease or insect problems in areas where trees are attacked year after year.
Several cultural practices can reduce the severity of anthracnose.
• Removing dead leaves in the fall will help limit the amount of fungal inoculum present for infection of new leaves the following spring. However, this practice rarely eliminates the problem, especially for those anthracnose fungi that also may survive in blighted twigs on the tree.
• Proper tree spacing and placement to promote good air circulation reduces the number of hours leaf surfaces remain wet, and decreases the likelihood of fungal infection.
• Stressed trees are more susceptible to the disease. Therefore, trees should be watered, fertilized, and pruned appropriately. Avoid soil compaction around the tree.
• Use less susceptible trees. There is considerable variation in the susceptibility of various tree species or cultivars to anthracnose. For example, London plane is more resistant to anthracnose than sycamore; red oaks tend to have fewer problems with the disease than the white oak group; and there appears to be variation in individual elms and black walnuts to their respective anthracnose diseases. Avoid planting highly susceptible trees in areas with poor air circulation.