Tar Spot - Maple

Tar spot is a fungal disease that doesn't require a fancy microscope for diagnosis. As you might guess, the disease is characterized by raised, black spots on leaves. Tar spot occurs primarily on silver maple.

Early infections appear as yellowish spots on the upper leaf surface. Later in the summer, black tar-like spots form and may be one-half inch in diameter. These black spots have a ridged appearance if examined closely. The undersides of the leaves appear cupped directly beneath the tar spots.

Tar spot is caused by the fungus Rhytisma acerinum. The tar-like spot is a fruiting structure of the fungus that survives the winter on fallen leaves. In the spring, mature spores of the fungus are released and blown by wind to newly emerging leaves.

Fortunately, tar spot does not cause serious harm to established trees. Some early leaf drop may occur. Raking and removing fallen leaves can help to destroy overwintering fungal inoculum. Although fungicides can be applied in the spring to protect newly emerging leaves, their use is seldom warranted.



This article originally appeared in the August 21, 1998 issue, p. 111.

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on August 21, 1998. The information contained within may not be the most current and accurate depending on when it is accessed.