Black dots on maple: Tar spot
Tar spot detected on maples is a cosmetic problem and a yearly occurrence in Michigan.
Samples of maple leaves infected with tar spot have been recently reported by Michigan State University Diagnostic Services. Tar spot is a foliar disease of maple caused by two species of fungus in the genus Rhytisma, which results in tarry black lesions up to an inch in diameter on the leaves. Tar spot occurs frequently in Michigan, although the level of severity may vary substantially year to year.
What does tar spot do to maple trees?
Tar spot on maple is most commonly caused by either R. acerinum, which produces large spots between 0.5 and 1.5 inches, or R. punctatum, which produces pinpoint-sized lesions. While tar spot mostly reduces the aesthetics of a tree, severe fungal infections can result in premature defoliation. R. acerinum is much more common in Michigan than R. punctatum. Although the most noticeable symptoms are present in late summer, infection actually occurs in spring as leaves are developing.
Commercial nursery management
Nurseries growing the susceptible species of maples, including bigleaf, mountain, Norway, red, Rocky Mountain, silver, sugar and sycamore maples, should practice excellent sanitation in the fall to prevent outbreaks in the nursery. If detected, rake up and destroy all infected leaves. If tar spot is a persistent problem in the nursery, Michigan State University Extension recommends an application of a labeled fungicide containing the active ingredients triadimefon or mancozeb at bud break. The spray should be repeated twice at seven- to 14-day intervals, according to label directions.
Spraying mature maple trees can be costly and ineffective. It is difficult to achieve good fungicide coverage using application equipment typically available to homeowners, and several properly timed applications will be needed. MSU Extension recommends that homeowners rake up and destroy all infected leaves in the fall. Mulching leaves is often not sufficient. The fungus can overwinter on fallen leaves and provide a source of inoculum to re-infect the trees for the next growing season.
More information on tar spot disease can be found at: