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What are the dark spots on our maple trees?

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Several maples on our property have large, dark circles on their leaves. Should we do something about this?
—Wendy and Bob Norman, Rockport, Ont.

Eventually. But first: While the blotches—called tar spot, for obvious reasons—look alarming, the fungus that causes them, Rhytisma acerinum, “does not seriously affect a tree’s health or function,” says Ken Farr, a dendrologist with the Canadian Forest Service. “It’s largely a cosmetic problem.”

Infected trees first show signs in early summer, when yellow or light-green specks appear on fully grown leaves. By August or September, these spots have become black. On a Norway maple (the species in the photo), the spots can merge.

“You end up with noticeable, up-to-three-centimetre spots,” explains Sylvia Greifenhagen, a forest health researcher with the MNR. Sometimes, a heavily infected tree may drop its leaves a little early. This is upsetting for you, but not for the tree: “By the time the leaves are shed, the tree has finished much of its growing for the year,” she says.

Once the leaves fall, rake up and remove them, as the fungus can send out spores in the spring and infect new trees.