Your soil around these plants is full of spores ready to infect new growth. Cut off all the mildew, in your case it sounds like all the above ground plant. Treat it like the disease it is and put it in your garbage, out of the garden. I would apply a good fungicide to the soil. I like the Bordeaux mix which is long lasting, in an attempt to prevent a reinfecting next year. Your plants will return with strength next year from the roots. While I am not gardening in your climate, I doubt you would need to cover your peonies for winter protection. Al

Calistoga is right - cut them down very close to the ground and clean up all cuttings as well any fallen leaves and stems. Here in zone 4 (considerably colder than your zone 6) peonies do not need any winter protection. They come up every year bigger and better. I've never had powdery mildew or botrytis on peonies so I can't speak to fungicides.

Can someone ID the type of peony I have from the photo?
I've read that bad air flow contributes to mildew problem. My flower bed is a bit thick at this point. Once I cut the plant back to ground, is now a good time to dig and split up the roots to improve air flow around the new plants in the spring ? Thank you!

Thank you, Calistoga. I've cut them all back down to ground. White powder was pervasive throughout. No visual symptoms of mildew on other neighboring plants such as hydrangeas and pachysandras. Turns out that three peony plants are within1-2 ft from each other. Next priority is to figure out and apply "Bordeaux mixture" to the perimeter.
They are currently in partial sun -to the west of house structure. Can they thrive in full sun - in south-facing flower bed?

Peonies here in the sunny Napa valley, better known for grapes, takes the full sun as long as they get adequate summer water. Apply your fungicide directly over the bare soil including the place where you removed or cut back the plants. The object is to KILL the remaining mildew spores remaining dormant just waiting for the right conditions to reinfect your susceptible plants. Al