How it’s spread: The disease cycle is similar for both species of the fungus that cause tar spot. In early spring, sticky spores are released from fruiting bodies on diseased maple leaves lying on the ground, and travel in the air to developing maple leaves. Within a month or two, light green spots develop on infected leaves. The tarlike spots don’t appear until late summer or fall. After overwintering, the tarlike lesions on fallen leaves produce sexual spores that infect young maple leaves and continue the cycle of infection for another season.

Management: In recent years, tar spot caused by R. acerinum has been increasing in frequency and severity in Michigan. The fungus overwinters on fallen, diseased maple leaves. Rake up and destroy maple leaves in autumn to reduce the amount of inoculum for the following spring. In the home landscape, raking up fallen leaves may be sufficient to manage the disease. In nursery settings, protective fungicide applications may be warranted as leaves develop in spring; consult your county Extension office for currently recommended products.