what's wrong with my maple tree? - Knowledgebase Question

Owen, Wi
Question by codybear1
August 19, 2009
On certain area of leaves there are large brown spots and on other leaves in same area there are small raised black spots. What is causing this - don't see any bugs


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Answer from NGA
August 19, 2009

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The symptoms sound very much like a fungal disease called Tar Spot. Early infections appear as yellowish spots on the upper leaf surface. Later in the summer, black tar-like spots form and may be one-half inch in diameter. These black spots have a ridged appearance if examined closely. The undersides of the leaves appear cupped directly beneath the tar spots.

Tar spot is caused by the fungus Rhytisma acerinum. The tar-like spot is a fruiting structure of the fungus that survives the winter on fallen leaves. In the spring, mature spores of the fungus are released and blown by wind to newly emerging leaves.

Fortunately, tar spot does not cause serious harm to established trees. Some early leaf drop may occur. Raking and removing fallen leaves can help to destroy overwintering fungal inoculum. Although fungicides can be applied in the spring to protect newly emerging leaves, their use is seldom warranted.

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