Tar Spots

You'll see small dark tar-like spots (stroma) slightly raised on maple leaves (Acer pensylvanicum), with yellowing beneath on the lower surface. Rhytisma punctatum overwinters on fallen leaves. Leaf drop occurs if infection is severe. The disease rarely kills trees but is common on bigleaf maple along the coast. Rake and burn fallen leaves in fall to reduce spring inoculum.

Leaf spot and blotch diseases are primarily cosmetic on established plants, though repeated years of heavy infection can weaken them. Rake up and remove fallen leaves to reduce the spore load going into the next season. Avoid overhead watering that splashes spores from the ground onto lower leaves. If the problem is severe, protective fungicide sprays during spring leaf emergence can help, but sanitation does more than chemistry for most leaf spots.