Answer: Based on your description, I suspect the plant has some powdery mildew. This season has brought out many unusual foliage problems on normally healthy types of plants. 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 has grown and seems healthy otherwise.
This fall, pick up and destroy the spent foliage 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 (489-4315) for a more specific identification and suggested treatments, if any.
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