Tar Spot on Maple

The following question was sent to the P&PDL diagnosticians here at Purdue University:

Question: Almost every summer the leaves on my maple trees have black spots on them that look like tar. What is it and what should I do about it?

Answer: Tar Spot on Maple LeavesTar spot on maple is not actually "tar" on maple, but rather a fungal disease. Tar spots on maples are caused by fungi in the genus Rhytisma. The most common species are Rhytisma acerinum and R. punctatum.

Symptoms first appear in late spring or early summer as infected leaves develop light green or yellow-green spots. During mid to late summer, black tar-like raised structures are formed on the upper surface of leaves within the yellow spots. R. acerinum causes spots that are 0.5 to 2 cm in diameter; R. punctatum causes spots that are smaller (about 1mm in diameter). Spots caused by R. punctatum are sometimes called speckled tar spots.

Tar spot diseases seldom are detrimental to the overall health of infected trees. Tar spots may cause premature defoliation, but are not known to kill trees. Tar spot diseases are best managed by raking and destroying fallen leaves because the fungi overwinter on leaves.

-Peggy Sellers