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Tree fungus called 'a cosmetic disorder'

Photo

Photo by University of Illinois Extension photo

Anthracnose, also known as venturia leaf spot, is seen on this silver maple.

JANESVILLE — It's a creeping fungal disease with hideous black spots, causing ordinary people to turn their heads away in horror.

It's called—cue scary music—anthracnose.

Or Venturia leaf spot, but that doesn't sound as scary.

Either way, it's affecting local trees and driving homeowners crazy.

Those homeowners, in turn, are calling garden centers, arborists and UW Extension agents who are giving them the bad news: There's nothing they can do about it.

"It's a cosmetic disorder," said Mike Maddox, director of education at Rotary Botanical Gardens and UW Extension horticulture educator. "It's bothering you more than it's bothering the plants."

Chris Ranum, certified arborist and owner of LP Tree Service, described it as "a small stomach staple" to a tree. The tree won't get as many nutrients as usual.

"It does affect photosynthesis to a certain extent, but it's nothing fatal," Ranum said.

Anthracnose is the general name for several common fungal diseases that affect trees in Wisconsin, said Brian Hudelson, UW-Madison plant pathologist.

This year, the predominant fungal disease in our area is Venturia leaf spot.

What's the difference between anthracnose and Venturia leaf spot?

Not much.

"Just take the whole anthracnose spiel and insert the words Venturia leaf spot," Maddox said.

So here's the spiel:

Anthracnose—or Venturia— causes irregular spots, dead areas in the leaves and spots that are black or brown. Sometimes leaves fall off.

All deciduous trees are susceptible to the fungus, but it does have its favorites: maples, ash, white oak, sycamore and walnut.

Venturia—or anthracnose—is caused by a variety of fungi that live in the leaf litter.

For fungi to develop, they need cool and moist conditions, and we've had plenty of those this spring.

There's nothing you can do about it. Applying fungicide is not necessary and not recommended.

"Fungicides are prophylactic; they're preventives," Maddox said.

It is a good idea, however, to do good leaf cleanup in the fall so the fungi have no place to live, Maddox said.