Answer: Based on your description, I suspect the plant has some powdery mildew. Warm daytime temperatures and cold nights, along with poor air circulation around the tree can contribute to the development of powdery mildew. This particular type of mildew can usually be washed off with a strong spray of water from the hose, but it will then likely reappear once it has had a start. It does not usually kill plants, it just looks ugly. At this late stage in the season I would not worry about it too much as long as the plant seems healthy otherwise. This fall, pick up and destroy the fallen leaves to limit the chance of reinfection next year. If you see it beginning again next summer, you might want to take a sample to your county extension for a more specific identification and suggested treatments, if any.
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