Although peonies are rarely bothered by pests or diseases, powdery mildew has become a problem in some areas. We first wrote about it in 2012, and expert Don Hollingsworth recently offered his perspective in the APS Bulletin. Don has been growing peonies since the 1930s but he says “it was not until 2014 that I noticed the striking sight of white mildew” on a few of the hundreds of peonies at his farm in Missouri. He searched the web but found no explanation as to “why powdery mildew is only now taking hold on peonies, while it has long been known to affect other commonly grown ornamental plants.”
To control it, Don says “the first line of defense is to clean up and destroy all infected plant parts at season’s end” to prevent spores from overwintering – and earlier is better than later. Instead of waiting until late fall, cut infected plants to the ground and carefully bag and remove all foliage “before the leaves dry up, which is best accomplished well before frost.” Don also offers a recipe for a preventative spray by the Massachusetts Master Gardeners: “In a quart of water add a few drops of liquid dish soap and a teaspoon of baking soda.” That’s similar to the spray we recommended two years ago: Mix 1 tablespoon of baking soda and 1 tablespoon of horticultural oil (or vegetable oil) in a gallon of water. Spray weekly throughout the spring, using a new mix every time and avoiding overuse to prevent a build-up of salts in the soil.