It could be tar spot disease on your maple. The disease is caused by several fungi in the genus Rhytisma. It causes disease in silver, sugar, red and Norway maple as well as their relative, box elder. Normally the disease is very cosmetic and doesn't cause any harmful effect on trees.
The initial symptom appear as small, light-green to yellowish-green spots on the upper surface of leaves. As the disease progress the spots enlarge and become darker. Later the spots appear as black, tar-like raised structures within these yellow spots.
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As the disease progress the spots enlarge and become darker
Initial symptom
Black, tar-like raised structures on maple leaf
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