The great maple leaf mystery

Black spots on maple leaves might not be a big deal, but look out for invasive species, salt, and drought.

December 3, 2012

As WBEZ special investigations editor, Cate Cahan has doggedly pursued some of Illinois' most intractable issues. But earlier this week Cahan ran up against a real head-scratcher that hit close to home. Literally – the case in question began under a tree in her backyard.

And so Cahan arrived at the office on Monday with what she thought might be a telling piece of evidence: a seemingly once-beautiful maple leaf covered in pitch-black spots the size of quarters.

“It looks like it got burned,” said one WBEZ reporter. “It looks sick,” said another.

It was a disturbing sight, indeed.

Between low water levels, extreme drought, and the fact that it reached nearly 70 degrees on December 3, it wasn’t a stretch to imagine that some environmental funny business might be behind the splotchy-leaf dragnet of 2012. Plus, Cahan recently experienced the loss of most of her garden to what she described as an oozing, yellow mold. So there was reason to be worried.

It turns out the coal-colored stain on our city’s autumnal gem is a) harmless, and b) not all that unusual.

The black spots on maple leaves, aptly named tar spots, are evidence of the fungus known as rhytisma.

“It looks awful, and it makes people concerned,” said Sharon Yiesla, plant clinic assistant at the Morton Arboretum. “But it’s more of a cosmetic problem than a health problem. It makes the leaves look ugly.”

She did say tar spots have been showing up more frequently in the region in the last few years, but did not recommend any treatment for the fungus. The best thing to do is thoroughly rake up and dispose of the afflicted leaves, Yiesla said. Otherwise, the fungal spores that stick around all winter may float up onto the fresh leaves come spring.

The emerald ash borer, on the other hand, “that’s a more serious problem,” Yiesla said. Ash borers have devastated the ash population in Michigan, and they’ve been digging away at Illinois ashes since 2006. The larvae of the ash borer get under the bark of ash trees and gnaw away at it, slowly cutting off ash trees from their water supply at the roots. In just two or three years, your ash can be grass.

Yiesla said ash borers can be stopped if you catch them early – but catching them isn’t easy. They make a small hole the shape of a capital D in the trees bark, but other than that, they’re invisible. A weak-looking ash, loss of leaves, or a sudden influx of hungry woodpeckers (who dig under the bark to eat the borers) can all be telltale signs.

Salt damage from ice melters used on roads and sidewalks is another concern this season.

“As cars are going by, you’ll get it spraying up onto the needles of an evergreen, and it can do physical damage to the needles,” Yiesla explained. “But then it also gets into the soil, and can do some harm at the root level.”

The salt in the ground makes it harder for trees to absorb water.

Water absorption is particularly pressing given this summer’s drought, which will likely affect next year’s plant growth.

“We might see reduced growth, reduced flowering, and weaker plants,” Yiesla said. “We’re not getting much additional rain this fall, and who knows what snow will come this winter.”

In other words, maple leaves with black spots are the least of her worries.