Tar spot is caused by the fungus Rhytisma acerinum and most commonly affects the leaves of Silver and Norway maples.
The fungus survives through the winter on fallen leaves. In the spring the spores from the diseased leaf debris are transferred by way of air currents to young maple leaves. Other maples susceptible to tar spot are: red, and sugar. Tar spots are most often more damaging to the visual appearance of the maple than to the tree’s health. There is not believed to be a detriment to the host tree’s health by the presence of tar spots.
Tar spots appear on maple leaves in late spring and early summer after full maturation of the leaf. The spots first appear as light green or yellowish. During mid- to late summer, raised black tar like spots 1/8 to 1/2 of an inch in diameter develop within the yellowish spots.
Tar spots are rarely injurious enough to require fungicide treatment. Fallen leaves should be raked up and disposed of in the autumn to destroy the fungi’s overwintering site.