Black spots on maple leaves?

Tar Spot on Maples

Have you noticed round or irregular black spots on your maple leaves in late summer? These markings often look like drops of tar—hence the name tar spot.

What Is Maple Tar Spot?

Maple tar spot is a common fungal disease caused by fungi in the genus Rhytisma. In Oakville, the most noticeable cases occur on Norway maples - non-native fast-growing trees planted on streets and parks throughout the city.

Will Maple Tar Spot Kill My Tree?

No. Tar spot is primarily a cosmetic issue. It rarely harms the overall health of the tree, though it can cause some early leaf drop due to stress. Even when infected, most trees continue to function normally through the season, storing energy in their roots and forming buds for next spring.

How Can We Prevent Tar Spot?

Good sanitation is key. Rake up fallen leaves and send them to your Town’s or Region’s composting facility, where high temperatures will destroy the pathogen. Managing tar spot requires leaf cleanup both in the fall and during summer. If you compost leaves at home, be aware that most backyard compost piles don’t reach temperatures high enough to kill the fungal pathogen.

Do We Spray to Cure Maple Tar Spot?

Generally, no. Because tar spot is mostly cosmetic and already widespread in our area, fungicides are rarely recommended and often ineffective. They also end up in soil and waterways, so unnecessary use should be avoided. If your tree appears to be in poor overall health, consult a certified arborist. In severe cases, and only with proper assessment, targeted fungicide treatments may be applied at specific times of the year.

Sources:



Hsiang, T., Tian, L. X., & Sopher, C. (2008). Tar spot of maple: Where did it come from and is it getting worse? Horticulture Review, 26(1), 35–37. University of Guelph; Canadian Tree Fund. Retrieved January 5, 2026, from https://canadiantreefund.org/wp-content/uploads/2018/08/2008-Maple-Tar-Spot.pdf

Independent Tree. (2022, July 20). Maple tar spot: Identification, prevention & treatment. Independent Tree. Retrieved January 5, 2026, from https://www.independenttree.com/maple-tar-spot/

Toronto Master Gardeners. (n.d.). Tar spot of maples. Toronto Master Gardeners. Retrieved Month Day, Year, from https://www.torontomastergardeners.ca/gardeningguides/tar-spot-of-maples/