It would depend on a few things as to exactly what you are talking about. What part of the country are you from? Maples that grow in the North will not usually grow in the deep Southern States. A few like Red and Silver Maple do alright but suffer in the heat and drought at times and throw off leaves to conserve moisture. Japanese Maples have became popular but need to be planted in the shade or where they just get morning sun. First off, I dont think its powdery mildew. Mildew usually appears during wet weather but you can tell mildew easly, its just like powder and wipes off easly. Usually doesnt kill the tree but does effect the general health. If your in the South I would say it might be just from the heat, sun scald. During periods of drought the tree cant take up water fast enough causing the leaves to sun burn. Spots are usually irregular and get bigger with time. Turns a real dark black. Another thing it might be is bacterial leaf spot. Quite common on some plants but not usually on maples. Smaller spots and usually with a redish tinge on the outer part of the ring. Could also be fungal leaf spot. Doesnt harm the tree but makes it unsightly. If I were really worried I would spray it with triforine. Ortho Rose Pride Disease Control should be easy to find.
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