I've been studying about tar spot, a fungus on maples, this evening, after reading an article in the Detroit Free Press. It seems that tar spot is a bigger problem recently in Michigan and throughout the Midwest. The article includes a really ugly photo of maple leaves with big black blotches on them. Tar spot was also a problem in heavily-mapled Massachusetts a couple of years ago.
Tar spot is caused by Rhytisma fungi. (There are a couple of types.) The fungus actually starts growing on the leaf early in the growing season, but it's a pale yellow then and not too noticable. In late summer, the fungus darkens to the tar-like color for which it is named and becomes a raised-up spot on the leaf.
The affected leaves usually fall from the trees early. After overwintering on the leaf, the fungus finally releases its spores in the spring -- just at the right time for the wind to carry the spores to an emerging tree leaf.
To interrupt the cycle, rake and remove the infected leaves as they fall in the late summer. Cornell University's fact sheet on tar spot says that composting the leaves will kill most of the fungus, but to be safe, cover or turn over the compost before the trees begin getting their leaves in spring.
We have quite a few maples on our little rural property, so I hope the fungus doesn't get started here. However, it's more of a disfiguration and inconvenience than a lethal threat. Healthy unstressed trees can withstand an infection of tar spot.
Images of tar spot