According to the Michigan State University Extension, a host of fungal diseases, including tar spot, anthracnose and maple leaf blister, are cosmetic problems that are unlikely to impact a tree permanently.

According to the Michigan State University Extension, a host of fungal diseases, including tar spot, anthracnose and maple leaf blister, are cosmetic problems that are unlikely to impact a tree permanently.

Photo by Brian Louwers


What’s wrong with my maple?

We asked the experts to explain leaf spots, early drop and lackluster color this year

By: Brian Louwers | Metro | Published November 9, 2021

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METRO DETROIT — Maybe the maple you anxiously wait to see bud every spring, doze in the shade under all summer and ogle as it explodes in fiery fall colors isn’t its old self this year.

Maybe it developed ugly spots on its leaves. Perhaps those leaves fell in early September, before they even had a chance to turn bright orange, yellow or red. Or maybe your tree still looked fine — too fine, actually, with its green leaves on Halloween.

The good news is that, in many cases, there’s nothing to fear about your beloved maple’s long-term health. Probably.

According to the Michigan State University Extension, a host of fungal diseases, including tar spot, anthracnose and maple leaf blister, are cosmetic problems that are unlikely to impact a tree permanently.

A query using the Michigan State University Extension’s “Ask Extension” tool at www.canr.msu.edu/outreach/ask-an-expert to ask for guidance about maple tree problems led to a reply that included a list of bulletins and articles, available diagnostic testing resources, and tips for finding a local arborist to perform an on-site tree health analysis.

In June 2020, Lori Imboden, consumer horticulture supervising educator for the MSU Extension office in Oakland County, updated an article originally written by Extension Educator Diane Brown in response to questions received each year about maple leaves.

“Both anthracnose and maple leaf blister can be the cause of late spring and early summer damage to the leaves of these popular landscape trees,” Imboden wrote.

In the article, she described anthracnose as “a common spring disease on maple trees” including red, silver, sugar and Japanese maples. The condition is caused by several different fungi that overwinter in fallen leaves or mulched material, and symptoms appear in years where cool, wet weather supports fungal growth.

Trees infected with anthracnose have “dark, irregularly-shaped damaged areas, or lesions, composed of dead tissue,” between the veins on leaf margins.

Maple leaf blister was described as having similar symptoms but with spots that are more rounded, with “small, raised blisters” and lesions that range in color from light brown to black. Again, the fungus is able to survive the winter freeze and can attack leaves again when they emerge in the springtime.

In an article published in 2014, Heidi Lindberg, an MSU Extension greenhouse and nursery extension educator based in Ottawa County, said tar spot is a yearly occurrence among maple trees in Michigan.

“Tar spot occurs frequently in Michigan, although the level of severity may vary substantially year to year,” she wrote.

Tar spot is caused by two different Rhytisma fungi that affect a variety of maple species in Michigan including Norway, red, silver and sugar.

“While tar spot mostly reduces the aesthetics of a tree, severe fungal infections can result in premature defoliation,” Lindberg wrote. “Although the most noticeable symptoms are present in late summer, infection actually occurs in spring as leaves are developing.”

All of the experts agreed: practicing thorough sanitation to gather and remove (not mulch) all infected leaves is the best management practice when dealing with fungal diseases.

“Some years, you get better control of diseases than others,” said Mike Zanoli, owner of Pure Turf & Tree in Auburn Hills. “Those spores have the ability to overwinter and reinfect in the spring, when they get wet. You’ve got to move those leaves off-site. That goes a long way.”

The experts at the MSU Extension said spraying with a labeled fungicide is an option at bud break and at intervals specified on the label instructions. However, spraying mature maple trees can be “costly and ineffective.”

Zanoli said it can be done effectively by professionals but that care must be taken when using a copper-based fungicide to avoid product drift that can damage nearby metal structures.

An MSU Extension bulletin going back to June 1982 discusses other problems affecting maples, particularly those declining in vigor. These environmental stresses include above-ground girdling roots that affect the tree’s ability to move nutrients and water through the trunk, leaf scorch, nutrient imbalance, salt injury and soil compaction.

Zanoli said a deep root feeding is one of the things homeowners can do to ensure their maple is living its best life.

“It’s a defense system. If you’ve got a strong tree, it has more strength to fight disease naturally,” Zanoli said.

For healthy-looking maples turning color late or holding onto their leaves as we head toward Thanksgiving, Zanoli said look no further than the weather as a contributing factor.

“We haven’t had a frost yet,” he said Nov. 1. “That is keeping the leaves on the trees a little longer, and all the water we’ve had is keeping them on a little longer. They have vigor. They’ll  hang on.”

Michigan State University also offers a plant and pest diagnostics service. They are not accepting walk-in consultations right now, but samples can be dropped off or sent directly to the lab. There is a $20-$25 fee for the service if a sample is required. Complete instructions for submitting samples and their accompanying forms can be found at www.canr.msu.edu/pestid/submit-samples.

To send images accompanying physical samples or to reach the lab, email pestid@msu.edu.

To find a local arborist near you to perform an on-site health evaluation of your maple tree, visit the International Society of Arboriculture’s website at www.treesaregood.org.

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