WHAT IS IT?
Tar spot of maple is a raised boac spot that occurs on the surface of maple leaves. These spots are caused by the fungus Rhytisma acerinum. The name comes from the tar-like appearance of the spots that occur.
Tar spot of maple can affect several species of maple trees, however the most susceptible species is the Norway maple. Silver maple and sycamore maple are common hats for tar spot of maple as well.
The first symptoms of infection for tar spot of maple are pale yellow spots that appear on the leaves typically around mid-june. As the disease progresses the yellow color of these spots intensifies. The black "tar spots" manifest differently depending on the species of the infected tree. In Norway maples the black spots behind as small dots and they do not enlarge. Overtime however these spots will fuse together to create one large mass. In Red and Silver maples the black spots develop within the original yellow spot and then expand to a fully formed "tar spot".
The fungus that cause tar spots spend the winter in the infected leaves that have fallen to the ground. In the spring, this fungal tissue ripens and release spores into the air. These spores are carried by the wind until they land on the leaves of a susceptible host. The spores penetrate the leaf tissue of their new host and the cycle begins again.
IF LEFT UNTREATED:
Tar spot of maple will not cause any longterm damage to the host tree, however the tar spots can be very unsightly and undesirable. Some research has shown that horses can be made sick by consuming leaves that are infected with tar spot.
No treatment is necessary. Best case scenario would be approximately 60% control, but regardless the disease will not kill the tree. The best method of prevention is a thorough fall clean up of fallen leaves.