Thom Smith | Nature Watch: Tar spot fungus doesn't harm maple trees

 Q: While raking leaves, leaves from a neighbor's maple tree have blown into our yard. Most of these leaves have large black patches on them, usually one or two per leaf. We have no maples, so it isn't ours, but wonder nonetheless, what they can be.

— Edward, Williamstown

A: I, too, was surprised when I first noticed what we call tar spots, of which there are three species, I believe. Rhytisma acerinum is the larger black spot, although each attract attention in late summer and fall when the tar-like fruiting bodies are obvious. During the early stages of the fungus, it is well-camouflaged and resemble rounded green spots. Norway maple is commonly infected by these pathogens, that cause more of a cosmetic problem. Fungicides may be applied early in the season on valuable "show" specimens. To keep infected maples sound, fertilize, water during dry periods and mulch. Keep in mind the Norway maple, although a widely planted "street" tree, is now known to be invasive, self-seeding itself along roadsides and in forests. It is thought to out-compete our native and certainly more valuable sugar maple. Distinguish it by its wide leaves with milky sap (tear a fresh leaf or its stem, maybe too late this year).

From what I have read ( the giant tar spot does not threaten the health of vigorous trees. To limit the spread and intensity, collect and dispose of infected leaves.

"When leaves drop from the tree they tend to fall with the fruiting structure (tar spot) side up, so that they are then effectively oriented for spore release into the air in the spring." — Dan Gillman of the UMass Extension Landscape, Nursery and Urban Forestry Program.

Thom Smith welcome readers' questions and comments. Email him at or write him care of The Berkshire Eagle, 75 S. Church St., Pittsfield, MA 01201


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