Susan Dynarski found these Norway Maple leaves, infected with the fungus known as "tar spots" (rhytisma acerium), on Norway Street in Burns Park in Ann Arbor.

Courtesy Susan Dynarski

Michigan State University Extension master gardeners are fielding calls from area citizens who are seeing unsightly black blotches on their maple trees. These tar spots are the fungus rhytisma acerium, and while they are unsightly they don't pose a threat to the health of the trees.

Experts

Tar spots were Tom Volk's Fungus of the Month for October 2007:

"Both of these Rhytisma species form black spots on maple leaves late in the season (September and October until leaf fall is a good time to observe tar spot in the northern latitudes). R. acerinum forms comparatively few, large spots on a given leaf, while R. punctatum forms clusters of many, small ("punctate") spots. While the aesthetic value of spotted leaves may be open to dispute, neither tar spot organism causes serious damage to the maple tree."

David L. Roberts of Michigan State University Extension Southeast in Novi writes about tar spots with a simple suggestion — do nothing:

"Tar spot is primarily a cosmetic, all-natural, nuisance disease that causes no harm to the tree, primarily because of its late appearance in the season, closely timed with senescence. Nothing can be done at this time except carry on with our normal day to day activity. Some arborists with certain clients who demand "zero tolerance" of insects and diseases, related that they have achieved excellent control of tar spot when they applied typical "anthracnose sprays" (for sycamore, oak, etc.) to maples in the spring. Normally, zero tolerance is not achieved and the disease is of such little significance that the best approach is to do nothing."

The Iowa State Extension recommendations say more about the life cycle:

"In all cases, these spots occur in summer, but spores are not produced until the following spring. The spores are forcibly discharged from the location where they are produced in the black spot (called a stroma because it is actually composed of fungal filaments that are tightly interwoven to form a raised, flat fruiting body), and are carried away by wind. If wind currents bring spores to developing maple leaves they can germinate under moist conditions and get into the young leaf by growing through the natural leaf openings where gas exchange takes place. Yellowish-green spots develop where the fungus colonizes the leaf, followed by production of the black stroma on the upper leaf surface. The stroma forms ridges that give it a wrinkled appearance. Meanwhile the lower leaf surface, opposite the stroma, turns brown."

The blog ">Thank You Very Mulch has a good perspective, and is generally good lawn and garden reading:

"Another fungi in the spotlight right now is Tar spot on Maple trees. Yellowish green spots on tree foliage become raised black spots which eventually become "wet tar" called a stroma. Released spores from fruiting bodies of the disease travel in the air to tender developing maple leaves in spring. Within a month or two light green spots develop on the foliage. Later the green spots become yellow to black with the "tar-spots" visible in late summer and into the fall. Again just like with the roses good cleanup in fall and spring makes a big difference. Some winter pruning to open up the canopy of the tree for better air movement is good idea too."

What to do with the leaves?

If you can't destroy the leaves by burning, then careful mulching or removal to a municipal compost facility is suitable.

A recommendation from Maine:

"Raking prematurely fallen, infected leaves is a sound sanitation practice, but leaves should be removed from the property or burned (where permissible) to prevent the fungus from re-infecting new leaves the following spring. The fungus will survive the winter in fallen leaves. Removing the leaves to a municipal composting facility is also recommended. If composting the leaves on-site, the leaf piles should be buried, or covered with a thin layer of soil or a dense layer of grass clippings or other compost, to prevent the fungus from spreading next spring. This sanitation practice will help to reduce infections the next spring, but will not eliminate the disease entirely."

Cornell University on leaf management:

"The most effective management practice in a home lawn situation is to rake and destroy leaves in the fall. This will reduce the number of overwintering "spots" (fungal reproductive structures) which can produce spores the following spring. However, where other infected trees are growing nearby, those leaves should also be raked and destroyed. Mulching leaves will suffice to destroy many of the spots before they mature, but the mulch pile should be covered or turned before new leaves begin to emerge in the spring."

Elsewhere

The West Island Gazette, a local newspaper based near Montreal, notes: "The beloved and ubiquitous maple tree is once gain under siege this season by an insidious fungus, rhytisma acerium, more commonly known as tar spots. It's worth a link through to the story to see the colorful costume of Dennis Chopping who is "fighting the blight".

Fun with headlines: "Fungus flattens foliage: Tar spot blunts most common local tree - Norway maple", in the Boston Globe, November 2, 2006. This story is not readily online.

Edward Vielmetti is the lead blogger for AnnArbor.com. Contact him at edwardvielmetti@annarbor.com.