We got a lot of questions from homeowners this year concerning spots on their maple trees and hostas.

The spots on their maple tree leaves are due to a fungal disease known as tar spot, which is a disease caused by several fungi in the genus Rhytisma. Members of this genus also infect silver, sugar, red and Norway maple, and box elder. Although unsightly, the disease is not harmful to the host tree. According to Michigan State University plant pathology specialists, the first tar spot symptoms usually show up in early summer as small (less than 1/8 inch diameter), light-green to yellowish-green spots. The spots enlarge and color intensifies as summer progresses. Small, black, tar-like raised structures form on the upper surface within these yellow spots. The black spots continue to grow in diameter and thickness to the point where it looks like someone splashed tar on the leaves. (This is the time when homeowners become rather alarmed.) Symptoms tend to be more common on trees growing in moist, sheltered locations.

Spraying most trees with a fungicide is usually impractical. The tar spot fungi overwinter in fallen leaves, therefore the best way to manage the disease is to rake and destroy affected leaves in the fall to reduce the number of overwintering organisms that can infest newly emerging leaves the next spring. Neighbors should also rake and destroy their infected leaves in order for your management efforts to be effective. Mulching leaves will destroy many of the spots before they mature if the mulch pile is covered or turned before new leaves begin to emerge in the spring…Read more