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.R. acerinum is an Ascomycete fungus that locally infects the leaves of trees and is a biotrophic parasite. The disease is cosmetic and is therefore usually controlled only with sanitation methods.