With the wet weather this spring, conditions have been favorable for the development of anthracnose on shade trees. Anthracnose is a fungal disease that occurs most commonly on ash, maple, sycamore, and white and bur oak. Anthracnose is caused by several closely related fungi. Each fungus is specific to the host tree it affects.
The most common symptom is brown blotches on leaves, which frequently develop along the leaf veins. As the infected area expands outward to the leaf margin there is frequently a distortion of the leaf. Heavily infected leaves fall to the ground. On sycamore, buds and twigs may be infected.
In most cases, trees are able to withstand loss of some leaves and push out a new crop of leaves. Infected leaves should be raked and removed in order to reduce the overwintering population of the fungus. Infected and dead branches should be pruned out to open up the canopy and improve air circulation. Maintain tree vigor with good cultural practices such as proper watering and mulching. Fungicide sprays are usually not warranted. However, young trees or trees that have had repeated severe infections can be sprayed with a labeled fungicide such as chlorothalonil (Daconil 2787) or copper-containing fungicides. The first spray should be applied when buds first start to open and two additional sprays should be made at 10-day intervals.
This article originally appeared in the May 19, 1993 issue, p. 74.