Spotting black patches on your leaves? It may be anthracnose.

My maple has leaves that are curling with black edges. Some of the leaves are falling, which makes me concerned about the health of the tree. What is this and what can I do for my tree?

— Phil Litchfield, Franklin Park

Your description of the problem suggests that you are having an issue with anthracnose. Oaks, sycamores, birches, dogwoods and elms are other commonly infected trees. Anthracnose refers to a group of fungal diseases that cause discolored spots on leaves and occasionally on other parts of the tree. These spots usually are tan, purplish, dark brown, or black, and merge into dead areas that sometimes follow the leaf veins. The infected areas often expand out toward the leaf margin, which causes irregular brown patches and distorted leaves. Some leaves may prematurely drop to the ground. Numerous cankers (localized, injured areas) may occur on stems and branches and are of greater concern than the symptoms that show up on the foliage. The disease often infects branches on sycamores, causing girdling cankers and dieback. There are new cultivars of sycamores that show good resistance to anthracnose.

Anthracnose rarely kills a tree or seriously affects its health, though in some more severe cases, a tree may defoliate. A healthy tree will typically recover and grow a new set of leaves. Any trees that are affected year after year or develop cankers will be weakened. The disease, which becomes active in spring, can become severe when cool, wet spring weather persists. Fungal spores overwinter on cankers on infected stems and fallen leaves. Dispersed by wind and rain in the spring, they land on young foliage and cause infections. With sufficient moisture, the spores germinate, penetrate the leaf surface and cause lesions. Additional spores are produced from the recently infected leaf tissue, which causes the further spread of the disease during the growing season.

There is not much to do now to help your tree, as fungicide treatments are generally not warranted and rarely provide complete control. Once you see the damage, it is too late to spray. Clean up and dispose of any fallen diseased leaves to help prevent a cycle of reinfection. Good sanitation practices are important to help control garden diseases such as anthracnose. Your tree may benefit from being pruned to improve air circulation for the tree’s foliage as the disease needs a moist leaf surface to develop. Any trees that are severely infected and defoliated will benefit from supplemental water during any extended dry periods.

For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.