Answer: That's powdery mildew, a common fungal leaf disease of peonies. It can weaken and disfigure peonies but usually not enough to kill them or even have much of a detrimental effect on their spring blooms.
I'd suggest picking off and burning or bagging the mildewed leaves once they become ugly enough that they bother you. Getting infected leaves out of the garden will help cut down on future outbreaks. Outbreaks also will vary depending on weather, so just because you have a mildewy summer one year doesn't mean you'll have the problem every year.
Mildew is hard to stop once it gets going, so prevention is the prescribed action if you have zero tolerance for this disease. Serenade is a new organic fungicide labeled for mildew control. A homemade fungicide is 1 tablespoon baking soda, 1 tablespoon of horticultural or canola oil and 1 tablespoon of liquid dish soap mixed in 1 gallon of water. Several chemical fungicides also prevent mildew. The down side is that you have to start spraying in spring before trouble starts, then continue spraying every 10 to 14 days throughout the summer.
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