As you might guess, the disease is characterized by raised, black spots on leaves. Tar spot occurs primarily on silver maple.
Early infections appear as yellowish spots on the upper leaf surface.
These black spots have a ridged appearance if examined closely. The undersides of the leaves appear cupped directly beneath the tar spots. Tar spot is caused by the fungus Rhytisma acerinum . The tar-like spot is a fruiting structure of the fungus that survives the winter on fallen leaves. In the spring, mature spores of the fungus are released and blown by wind to newly emerging leaves.