Why Does My Maple Tree Have Spots?

If you've noticed black, tar-like spots appearing on the leaves of your maple tree, you're not alone. Every summer we get calls from concerned homeowners across Waterloo Region asking the same question: is my tree dying?

The good news: probably not. Those spots are most likely tar spot, a common fungal disease caused by fungi in the genus Rhytisma. The spots start as small yellow-green patches in early summer and darken into raised, shiny black blotches by late summer — hence the name.

Is tar spot harmful to my tree?

Tar spot is largely a cosmetic issue. It rarely causes serious harm to a healthy, established maple. In heavy infections it can cause leaves to drop a little earlier in the fall, but it does not kill trees. Think of it as a blemish, not a disease sentence.

What can I do about it?

Because the fungus overwinters on fallen leaves, the single most effective thing you can do is rake up and dispose of infected leaves in the fall. Don't compost them at home — the spores can survive and reinfect your tree (and your neighbours') next spring.

Fungicide sprays are generally not recommended for tar spot. By the time spots appear, it's too late for treatment to work that season, and preventive spraying of a mature tree is rarely practical or worthwhile for a cosmetic disease.

When should I call an arborist?

If your maple is showing other symptoms — dieback in the crown, peeling bark, mushrooms at the base, or leaves wilting in mid-summer — something more serious than tar spot may be going on. Our certified arborists can assess your tree and recommend the right course of action.

Want to learn more? Landscape Ontario has a helpful overview of tar spot of maple, and the University of Maine Extension covers the tar spot fungus in more detail.