In the Fall of 2005, Chittenden county residents raking leaves were expressing concern over the black spots appearing on maple leaves. "It looks like it was hit by an oil spill," said one resident. The splotches are a case of "giant tar spot" and Rhytisma sp., is the fungus that causes this giant tar spot on Norway maple. It is related to the species R. acerinum which is found on red and silver maples, but is thought to be a different species.
Fungi are able to establish themselves on plants during rain periods. This particular fungus only infects during the spring. At the time of infection, there is no noticable symptom on the foliage. There may be a slight chlorotic area developing through the summer. This area will then begin to turn black. First small dots will appear and eventually these dots will coalesce forming the large tar-like spot. The spots develop over the course of the summer and if spotting is severe enough, defoliation can result. The fungus overwinters on infected foliage.
To reduce infections next year, foliage should be raked up and removed. Although the disease is unsightly, it does not affect the overall health of the tree. The leaves have photosynthesized and produced enough energy reserves for the tree before defoliation begins. Fungicide applications can also reduce infection if started early in the spring.
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. Tar spot is most often more damaging to the visual appearance of the maple than to the tree's health.
And here's a fact sheet from Cornell, complete with pictures:
http://plantclinic.cornell.edu/FactSheets/tarspot/tarspots.htm