Hi Carole,

First, make sure that irrigation doesn't hit the leaves of the peony. Second, after it blooms, consider pruning some of the leaves to open the canopy up and help prevent disease pressure. Powdery mildew thrives in areas with heavy foliage and poor circulation.

While there are preventative sprays you can use, cultural practices are as important (and sometimes more important) to discourage disease pressure. 

For plant health, powdery mildew rarely, if ever, causes any harm to the peony and almost always happens later in the summer. 

If you wish to try fungicides, follow these recommendations: Chlorothalonil, commonly called Daconil could be applied 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 has already grown.