The questions about Maple Tar Spot have started to come in to our local Extension offices, which is a standard occurrence this time of year.

Several different fungi in the genus Rhytisma infect the leaves of maples and cause raised, black spots to form on upper leaf surfaces. The diseases are called "tar spots" because their appearance so closely resembles droplets of tar on leaf surfaces, making this disease very easy to identify.

The good news about maple tar spot is that control is not usually warranted. The University of Illinois Plant Clinic notes that this disease may cause some early defoliation if severe, but even then, it is not believed to significantly impact tree growth and development. Since the causal fungus overwinters on fallen leaves, inoculum can be reduced by raking and removing fallen leaves. Infection occurs in the spring, so rake and remove the leaves in the fall.

Learn more here: http://hyg.ipm.illinois.edu/article.php?id=99

Photo by the University of Illinois Plant Clinic