Tar spot is a fungal disease that causes black tar-like spots on the leaves of Red, Silver, Norway, Sugar and Manitoba maples (but it doesn’t seem to affect Japanese maples). Tar spot is a foliar disease: the spores do not affect other parts of maple trees.
While tar spots are unattractive, the good news is that this fungal disease does not injure the tree itself. Tar spots develop late enough in the growing season that they do not usually affect the health of the tree. While the fungal spores infect young leaves early in the season, they do not continue to cause new infections throughout the summer. The infections first appear a yellow or light green spots on the leaves in early summer. By late summer, the infections take on a black, tar-like appearance.
To avoid the spread of fungal spores it is best to rake the affected leaves this fall. Destroy the leaves or remove them from your yard by bagging them for municipal collection. If you ignore tar spot and allow the fallen leaves to remain on the ground through the winter, your maples will develop tar spot again next year. If, however, you remove the infected leaves from the area you reduce the chances of the tree being infected the following year