Powdery mildew is a common fungal leaf disease of many tree species. The powdery mildew appears as a white growth on the upper surface of the leaves. The powdery appearance comes from a mass of spores of the fungus covering the leaf surface.

Norway maple is the most common host of powdery mildew in Toronto's urban forest, however the disease can affect almost any deciduous tree species. Some Norway maple varieties, such as Crimson King and Schweidler are highly susceptible to the disease. The disease is most severe under dry climatic conditions, however high relative humidity is necessary for establishing new infections.

Although unsightly, powdery mildew is considered to be a cosmetic problem, since the infection begins late in the summer, usually too late to cause significant damage to trees. The fungus overwinters on fallen infected leaves and on buds of trees.

Clean up and dispose of infected leaves in the fall to prevent the overwintering fungus from producing spores next spring. Leaves may be composted by city composting programs.

For more information about Powdery Mildew, visit the section on the City of Toronto website. Visit toronto.ca/parks, select Trees & Ravines, select Forest Health Care & Pests, select All Fact Sheets and then Powdery Mildew.