Notice Spotty Maple Leaves in Your Yard?

Symptoms of tar spot on a maple leaf. Photo by Annie Deutsch.

By Annie Deutsch, Door County UW-Extension Agriculture Agent

If you notice that a tree in your yard has black, circular spots on the leaves, don’t panic! The black spots are caused by several very common fungi in the genus Rhytisma. Due to the distinctive black circular lesions, the disease is commonly called tar spot.

While tar spot may not catch your attention until later in the fall, the initial infection actually took place in the spring. The fungal pathogen overwinters in leaf litter below the tree and in the spring it produces spores that infect the leaves. At that time, the only symptom of tar spot is small yellowish spots on the leaves, typically around 1/8 of an inch. The spots may stay small or start to slowly enlarge throughout the summer. Come fall, the infected area turns black with lesions ranging from ½ to 1½-inches. If there are enough spots on a leaf, the spots may start to merge, causing large sections of the leaf to turn brown.

Tar spot is primarily a cosmetic leaf disease that is also one of the least damaging diseases on its host. Silver maple is the most susceptible to the disease, but many other types of maple trees can be infected including big leaf, mountain, red, Rocky Mountain, sugar and sycamore maple. Boxelder, willow and tulip-tree can also be infected by tar spot.

Although tar spot may look bad and cause the tree to be a bit ragged, for most trees it will not kill the tree or even cause serious defoliation.

Commercial tree nurseries may need to use a fungicide to control tar spot, but for the homeowner, unless the infection is extremely severe, fungicide treatments are almost never worth the money or effort.

If you are concerned that your tree may warrant a fungicide treatment, contact the Door County UW-Extension office or a horticulture professional to have the tree assessed. Likewise, if your tree is showing signs of wilting, early defoliation, branch dieback or other atypical symptoms, there likely are other problems going on beyond tar spot. The problem should first be properly identified and then treated accordingly.

The best control for tar spot is sanitation: removing all the fallen leaves and burning, burying or composting them. Because the fungal pathogen overwinters in leaf litter, if you remove the leaves, you remove the pathogen. Mulching is not recommended because the fungal pathogen may be able to survive the winter in the mulch and thus produce spores in the spring.

In recent years incidence rates have been high, especially in the City of Sturgeon Bay, so even with thorough sanitation it may be difficult to prevent tar spot from infecting your tree. But if it does get spots, rest assured that things are probably okay; just think of it as the tree displaying a different array of fall colors.