Big black spots on maple leaves look like something is seriously wrong — they're dark, raised, and impossible to miss by late summer. The reassuring news is that maple tar spot, while common all over southern Ontario, is mostly a cosmetic problem.
What it is
Tar spot is a fungal disease that produces those raised, tar-black blotches on the leaves of maples — Norway and silver maple especially. It looks dramatic, but it affects the leaves, not the structure or long-term health of an established tree. The tree photosynthesizes fine and leafs out normally the next spring.
Why it shows up
- Wet springs — the spores spread from last year's fallen leaves onto new growth in damp weather.
- Fallen leaves left in place — the fungus overwinters on leaf litter and re-infects the next year.
- Crowded, humid conditions that keep foliage damp.
What to do (and not do)
For most homeowners, the best response is simple: rake and remove the fallen leaves in autumn to break the cycle, rather than reaching for fungicide. On a large shade tree, spraying is impractical and unnecessary — the disease doesn't threaten the tree. We mainly raise it because it worries people, and that worry is usually misplaced.
Maple tar spot looks like a disaster and behaves like a nuisance. Rake the leaves, and you've done most of what's needed.
When to look closer
If a maple has black spots and real problems — heavy early leaf drop, dieback, thinning canopy — then something beyond tar spot may be going on, and it's worth a look. Book an assessment if your maple's troubling you beyond the cosmetic spots.