United Kingdom 
My Rosa Mundi is looking rather sad and many of its leaves are lightly covered in what looks like flour. It has been in the ground for about three or four months and is in clay soil which has been severely dug over - by me.
7 Sep, 2010
Answers
Or buy a fungicide spray which treats for mildew.
8 Sep, 2010
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It sounds like a bad case of powdery mildew, a fungus disease that flares up as nights become cooler. I spray with a light solution of organic dish soap and baking soda, just enough to make the water slippery. Potassium bicarbonate is even better than baking soda, but is harder to find.
7 Sep, 2010