Q: My maple tree has black spots on most – if not all – of its leaves. Can you tell me what this is and if I should be concerned about it? I would hate to lose this tree since it is the only one in my front yard.
A: The writer enclosed a leaf sample that had large black spots on it, typical of a common fungal disease appropriately called maple tar spot. Despite affecting an alarming percentage of the leaves on a tree, maple tar spot is a cosmetic problem rather than anything life threatening. This disease has been prevalent this season, probably because the wet conditions in spring as maples were leafing out created very favorable conditions for maple tar spot to thrive.
The disease is caused by the fungus Rhytisma and overwinters on fallen leaves. Infection occurs in the spring as new growth starts and spores are blown from those old leaves onto the tender new foliage. Maple tar spot first appears as very small yellow spots that enlarge to an inch or so in diameter through the growing season. In late summer, those spots turn black as the fungus matures and produces reproductive structures.
Maple tar spot
Photo: Sandy Feather
Maple tar spot generally does not cause premature leaf drop, and it occurs so late in the growing season that it does not impact the overall health of affected trees. They have already formed next year’s buds, and those leaves will fall soon anyway.
Fungicide sprays are not recommended to control maple tar spot. The best recommendation is rake up and destroy fallen, infected leaves to reduce the amount of fungal spores around next spring.