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Tar spot

By: Garden Gate staff
There’s nothing more frustrating than having the beauty of your maple tree foliage marred by big black spots.

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Tar spot Rhytisma spp.

IDENTIFICATION — There’s nothing more frustrating than having the beauty of maple tree foliage marred by big black spots. If your tree looks like it’s been splattered with black paint, it probably has tar spot, a fungal disease. There are several different species of this fungus, but the most common are Rhytisma acerinum and R. punctatum.

DAMAGE — Tar spot starts out as 1?8-in.-diameter pale yellow spots in late spring to early summer. By mid- to late summer, you’ll notice black spots inside the yellow ones. The “tar” is raised above the leaf surface, giving it a three-dimensional feel. Spots caused by R. acerinum grow to about an inch while spots caused by R. punctatum stay much smaller. Tar spot doesn’t kill the tree, but it just doesn’t look good, and heavy infestations can cause premature leaf drop.

CONTROL — The fungus that causes tar spot overwinters on leaf debris, and spores are spread by the wind in spring. There are fungicides available but they’re difficult to use and don’t work very well. Your best bet for controlling this fungus is to rake up the fallen leaves in the fall and send them away in the yard waste.

Published: May 18, 2010
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