Rhytisma acerinum is a plant pathogen that commonly affects sycamores and maples in late summer and autumn, causing tar spot. Tar spot does not usually have an adverse effect on the trees' long-term health.[1] R. acerinum is an Ascomycete fungus that locally infects the leaves of trees and is a biotrophic parasite.[2] The disease is cosmetic and is therefore usually controlled only with sanitation methods.[3]