Host Trees; Norway, Silver, Red, Mountain, Manitoba, Bigleaf and Sugar maples,
Life Cycle; Emerged leaves are infected in spring during cool, wet weather. This fungus is found mostly on Norway Maple trees and causes large spots, up to nearly an inch in diameter. As maple leaves develop to full size, light to yellowish green spots develop in the infected areas of the leaves. The area becomes yellow, with numerous small, raised black spots forming within the yellow area. As late summer and early fall approaches, the black spots coalesce to form a large, irregular, shiny raised spot with the appearance of wet tar, called a stroma. Severely infected leaves may fall prematurely.
How it’s Spread; In early Spring, sticky spores are released from fruiting bodies on disease 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 of 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; There are no registered fungicides in Ontario to control this disease. In recent years, tar spot caused by Rhytisma acerinum has been increasing in frequency and severity in Ontario. 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. Removing fallen leaves from all infected neighbourhood trees may not be practical but may help reduce disease incidence the following year. Pruning to improve air flow and reduce leaf wetness periods and fertilizing trees that have been severely impacted will help.
Call Shady Lane Expert Tree Care to make recommendations for your trees.
Reference; A Pocket IPM Scouting Guide for Woody Landscape Plants compiled and edited by Diane Brown-Rytlewski and Nursery & Landscape Plant Production and IPM; OMAFRA