My peony leaves seem to be getting powdery mildew. Can I just cut them back or do I need to treat the mildew? How low would I cut them back? or what is the best way to treat the mildew?
Montgomery County Maryland
Powdery mildew has become a bigger problem on peonies the past few years. Make sure there is adequate sunlight and good air circulation to reduce humidity levels. Allow proper plant spacing for the same reasons. Pruning for better air circulation also may help. Practice good sanitation. Since the powdery mildew does not show up until late in the season it may be best to tolerate it.
Cut down all infected foliage this fall as soon as frost turns it brown (or before if the mildew has killed it.) Get rid of this infectious material. Next spring, monitor your peonies, and as soon as you see one infected leaf, pull it off. You will have to decide it you want to spray a fungicide or a horticultural oil labeled for powdery mildew or tolerate it. Fungicides are preventative--not curative. They will halt further infection, but cannot restore infected leaves to health. Here is our website info on powdery mildew:http://extension.umd.edu/hgic/powdery-mildew-annuals-bulbs-groundcover-perennials-and-vines
If the problem becomes chronic on one particular peony, you may want to dig it up and replace it with a more mildew resistant variety.