A fungal infection called tar spot causes black spots on maple leaves. The disease usually appears during wet years. It causes little damage and is rarely a cause for serious concern.
The Rhytisma acerinum and Rhytisma punctatum fungi cause tar spots on maple leaves. The pathogens spend the winter on infected leaves and mature during the spring, releasing tiny spores that travel on the wind. The spores create new infections if they land on the leaves of a susceptible host.
Pale yellow or greenish-yellow spots appear on infected foliage during spring and early summer. The yellow areas grow and their color becomes more pronounced as the disease progresses. Raised, tar-like black splotches appear within the yellow spots. The Rhytisma punctuatum fungus creates numerous small spots, according to the University of Nebraska-Lincoln, while the Rhytisma acerinum fungus creates several large spots.
Tar spot, which peaks shortly before the leaves fall, does not harm the tree. It is primarily a cosmetic problem that does not require chemical treatment, according to the Michigan State University Extension. Removing fallen leaves from the ground reduces the number of spores that can infect new growth the following spring.
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