My peony bushes developed a white powdery substance, which spread like wildfire. I cut them back to the ground, which I usually do in the fall anyway, hoping that I would stop whatever it was. A little Googling reveals that they have powdery mildew. Will cutting them back for the year be enough or do I need to treat the remaining stumps? I live in New Jersey, the plants are in full sun all day.
Powdery Mildew on Peonies
Certified GKH Gardening Expert
Treating the pruned plants for the Powdery Mildew is a good idea.
Dispose of the trimmings, do not put in your compost.
Here are some links to help you.