Janesville52.4°

Notice black spots on maple leaves?

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Greg Peck
November 5, 2012

The city of Janesville started its two-week leaf collection today. Did you get your leaves raked out to the curb line over the weekend?

If you did, perhaps you noticed black spots on some maple leaves. I was walking our dog a week ago on Saturday morning when I first noticed it on otherwise yellow maple leaves a block or two from our home.

Right away, the thought crossed my mind of an invasive insect I’d read about earlier this year, when the city announced the emerald ash borer had been found in Janesville. I arrived home to tell my wife about the spots, and Cheryl was listening to WCLO’s “Flower Hour” featuring Phyllis Williams of K&W Greenery. Phyllis was just finishing up explaining to a caller what those spots were all about, and I didn’t catch all the details. Cheryl pointed out that our maples had them, too.

Sure enough; our two maples out front have leaves with dark fall colors, so I hadn’t noticed the spots. But they indeed had these black spots, often larger than a quarter.

I called Phyllis on Friday to get an explanation. She said the spots come from anthracnose, a fungal disorder that occurs when weather conditions are just right.

She said the fungus doesn’t threaten the trees in any way; rather, the spots are cosmetic.

“Think of it as a big pimple on the leaves," she told me. "It’s not something you should worry about. Sometimes the things that look really bad aren’t so bad, but the things you don’t see—such as tiny emerald ash borers—will get you.”

If you prefer to compost your leaves rather than push them to the curb for the city pickup, there’s no harm in doing so with these black-spotted leaves, Phyllis said.

The only way to eliminate the fungus is with a fungicide, but you have to do it preemptively, so it’s a waste of time, money and energy, Phyllis suggested.

The insect I was thinking about, by the way, is the Asian longhorn beetle. It invaded Chicago and can kill maples. Phyllis, however, said the Windy City seems to have it under control for now.

Unfortunately, she said, if the Asian longhorn does arrive here, it’s bigger and uglier than the emerald ash borer and not only likes maples but virtually every tree.

As Cheryl and I nurture our newly planted hackberry tree, that was disappointing to hear.

Greg Peck can be reached at (608) 755-8278 or gpeck@gazettextra.com. Or follow him on Twitter or Facebook



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