Less serious is the fungus " Rhytisma acerinum " which often forms the disease known as tar spot, in which black spots with yellow margins form on the foliage.
2.
Maple leaves in late summer and autumn are commonly disfigured by " tar spot " caused by " Rhytisma " species and mildew caused by " Uncinula " species, though these diseases do not usually have an adverse effect on the trees'long-term health.
3.
Norway maple is generally free of serious diseases, though can be attacked by the powdery mildew " Uncinula bicornis ", and verticillium wilt disease caused by " Verticillium " spp . " Tar spots " caused by " Rhytisma acerinum " infection are common but largely harmless.
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