Answer: Based on your description, this sounds like mildew or a fungal infection. It is not usually life threatening to the plant, just a little unattractive. It appears during certain weather conditions so it may be better or worse from year to year depending on the weather -- some years you may not see it at all. In general, good air circulation will help limit its occurrence; also avoid wetting the foliage if you water the plant. There is nothing you would need to do, especially this late in the season. Do try to clean up any fallen leaves from the plant this winter and dispose of them in the trash to reduce sources of infection for next year.
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