Tar Spot Fungus

Tar spot fungus 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.

In late spring, spots appear on tree leaves and develop into brown-black lesions, retaining the yellow border. The lesions continue to grow, and by the end of summer form a leaf spot that looks like tar. The spots can grow up to 1.5 inches (4 cm) in diameter.

Tar spot has been recognized since 1794, when it was discovered in Europe by Christiaan Hendrik Persoon, a prominent mycologist of that time.