While admiring the colors of some of the remaining fall leaves on the trees around my neighborhood, I noticed that all the Norway maples (Acer platanoides) had leaves with conspicuous black spots on them. By late fall, its not uncommon for leaves to have a number of marks and blemishes, but these seemed unique to me in the regularity of their shape (the spots are almost completely round) and the darkness of the marks (almost black).

Norway maples are not native to North America, but have become fairly common after having been introduced as a landscape species in the 1700s, prized for its shade producing canopy and its hardiness and adaptability to adverse conditions. The Norway Maple has since been classified as an invasive species in many parts of the eastern U.S., as it readily outcompetes native trees through the development of a large dense canopy that shades out the understory vegetation below it and hampers the growth of new seedlings.

The black spots are evidence of a tar spot fungus that attacks certain trees species. The fungus can belong to one of several fungi in the genus Rhytisma and infects maples, including Silver, Sugar, Red and Norway maples, as well as the related Box Elder. Although worrisome to many homeowners and disappointing to leaf peepers who believe it tarnishes the vibrancy of autumn colors, the tar spot fungus does relatively little damage to the host tree. The tar spot usually first appears in the early summer as small yellowish-green spots. As the summer progresses, these spots enlarge and color darkens. This trend continues until the spots are, in some cases, more than an inch in diameter, and almost completely black, resulting in what looks like tar splashed on the leaves. The result is, however, mostly aesthetic in nature, and although it can result in premature leaf drop, the tar spot fungus rarely results in any long-term impacts to the health of the tree. The fungi that cause the spots are able to survive the winter on fallen leaves, from which fungal spores can spread to nearby maple trees in the spring.