My peonies appear to be covered with a white, powdery substance. What is it?

FAQ
Question: 

My peonies appear to be covered with a white, powdery substance. What is it?

Answer: 

The white, powdery material on the peony foliage is powdery mildew.  Powdery mildew is a fungal disease.  Powdery mildew occurs on a large number of plants (peonies, lilacs, viburnums, roses, garden phlox, bee balm, turfgrass, and many others).  Fortunately, powdery mildew seldom causes serious harm to plants.  The damage is mainly aesthetic.  

Powdery mildew tends to be more common on peonies growing in partial shade.  Moving peonies growing in partial shade to a sunny location often helps reduce the incidence of powdery mildew.  Late September is an excellent time to move peonies in Iowa.