What are Those Black Spots on My Maple Tree?

By Doug Marrin, STN Reporter

If you see large, black spots on your maple leaves, don’t worry. The blight looks worse than it actually is.

According to the Michigan State University Extension, the condition is a symptom of a fungal disease known as “tar spot.” The disease can infect silver, sugar, red, and Norway maple trees as well as box elders.

The spots typically show up in early summer as small, yellowish-green spots. Over the course of summer, rain and heat allow the fungus to grow into black spots which increase in diameter as the fungus thrives. Late summer into fall the tree can have the appearance of being splashed with tar, hence the name.

According to MSU, the good news is that the fungus is usually a cosmetic concern and does not affect the overall health of the tree. One of the results can be an early leaf drop.

Tar spot fungi survive the winter in fallen leaves to reproduce in the spring. The best practice for combating the disease is to rake and destroy the leaves in the fall. Mulching the infected leaves can destroy many of the spots before maturity, but the mulch should be covered or turned prior to when new leaves bud in the spring.

Control of tar spot is difficult even when a fungicide is used. Mature maples are difficult to adequately cover, and if neighbors are not managing the disease, spraying can be a waste of time and money.

For additional information, call the MSU Extension Lawn and Garden Hotline at 1-888-MSUE-4MI (1-888-678-3464). You can also visit www.migarden.msu.edu.

Photo: STN stock photo.