Damage, symptoms and biology
Oval or irregular-shaped yellowish-green spots on the upper surface of leaves are the first sign of the disease. These spots spread and become raised, then blacken over time. During severe infections, which are extremely rare, the infected leaves may drop prematurely. The fungus over winters in the black tarry spots on the dead leaves that fall in autumn. In spring, the fungus produces spores that spread to young maple leaves causing new infections.
This disease affects the appearance of ornamental trees, but rarely poses a serious threat to their health. Since the disease is spread from the leaf litter in autumn, gathering and destroying the leaves is sufficient to stop its spread.
Canadian Forest Service Publications
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