Many silver maple leaves showing signs of tar spot have been arriving in the Plant Disease Clinic. Tar spot is caused by the fungus Rhytisma acerinum and related species. As the common name suggests, the fungus causes slightly raised, tar-like spots on leaves. These black spots are often one-half inch in diameter. By fall, spots appear ridged or wrinkled. The "tar" is actually stromata (a fungal structure which contains fruiting bodies).
Fungal spores develop in the stromata. In the spring spores mature and are released from the infected, fallen leaves. These spores may then infect newly emerging leaves, repeating the disease cycle.
Tar spot seldom causes serious harm to trees. Early leaf drop may occur, giving the appearance of thinning. Several successive years of early leaf loss may cause stress to the tree, causing it to be susceptible to other problems.
To control the disease, rake and remove fallen leaves to prevent overwintering of the fungus. Fungicide use is seldom warranted. If used, fungicides must be applied at 2-3 week intervals during leaf emergence in the spring.
This article originally appeared in the October 11, 1996 issue, p. 162.