Tar Spot disease infecting Norway Maples in Ottawa

Photo at left: Typical appearance of Tar Spot on Norway Maple leaves in late summer

Several different fungi infect the leaves of maples and, this year in Ottawa, the exotic and invasive Norway Maples are especially hard hit. Our native Sugar Maples seem not to be affected.

Symptoms
Early in the year, 5-15 small spots no larger than a pin head appear as light, yellowish-green areas on the upper surface of the leaf. By mid-to-late summer, many have matured into raised, shiny black spots. Tar Spot alone is rarely serious enough to threaten the health of trees, but sometimes there can be so many spots that the tree becomes unsightly. Heavy infections can also cause early leaf drop, as is occurring this year in Ottawa. As symptoms develop late in the growing season, the overall health of trees is usually not affected.

Management
The fungus overwinters on fallen infected leaves, from which it infects new leaves in the spring. Removal of fallen infected leaves in the fall or early spring is an effective way to control this disease. Tar Spot is seldom so injurious to justify spraying with a fungicide.