Thank you for sending the excellent and helpful photos. This is powdery mildew, a fungus that typically appears as a white or grayish powder on the leaves and stems of affected plants. Powdery mildew is one of the most commonly found fungi affecting plants. It affects a wide variety of deciduous plants. Lilac, beebalm, garden phlox, and squash are commonly affected plants, as well as peonies. The fact is, the leaves of peonies normally look a bit bedraggled at this time of year. Their unsightliness is magnified if the plants were struck by powdery mildew. The good news is that powdery mildew on peonies is more of an eyesore than a health problem. It is not a life-threatening condition for the plant.
Powdery mildew is best controlled through preventive measures. At the close of the growing season, peonies should be cut completely to the ground. It’s best to remove the foliage now. Dried or diseased leaves carry the spores (microscopic, seed-like structures) of the fungus, which could trigger an outbreak again next year. Removing old infected vegetation before it falls to the ground helps to mitigate the risk of it reappearing next season. Don't put the diseased leaves in your home compost. Put them in the City of Denver green compost bin or in the trash.
Perhaps, if one has been experiencing fungal outbreaks in peonies year after year, you could apply a fungicide in the spring when the new growth of young peony tips breaks through the ground. Follow two weeks later with another application and every fourteen days thereafter until mid-June. Keep in mind, fungicides work as preventives. One needs to spray before the leaves become
infected. Spraying after will help keep the mildew from spreading to other parts of the plant but will not kill what is already there.
Outbreaks of powdery mildew may be attributed to the peony not being planted in the best location. Peonies planted in shaded, poorly-drained areas are much more prone to fungal infection. If this is the case, autumn is the ideal time to move peonies to a more suitable location. All types of peonies need adequate sunlight, well-drained soil and good air circulation to ensure healthy growth.
Here is a link to an article aobut powdery mildew by our own Colorado State University Extension Office:
Best of luck with your peonies. Every year is different and it is entirely possible that they will not get the powdery mildew next year.