pembroke, a Google search reveals that this question has come up before--on this forum. Try that search; good luck!

Sounds like powdery mildew and in your zone your solution should be a good one. Her peonies may be too close together and not enough air circulation could be the cause. After cutting them back I think I would spray the soil well with a fungicide such as Bordeaux mix to neutralize mildew spores in the soil. In our climate they can live over. Al

Powdery mildew. Cut them down, dispose of the debris.

You can spray peonies, like roses, but if they're in the sun and don't have a problem till cool Fall hits, it's usually not necessary.

Here is a link that might be useful: Garden Musings