Powdery mildew is a fungal disease common on peonies, other plants. Here’s what to do about it.
“The leaves of my peonies are covered with what appears to be a white mold, with some of the leaves even turning brown already. What is this problem, and will it affect the plants next year? Should I be doing anything about it?”
— Sylvia Mendoza, Highwood
The problem with your peonies is most likely powdery mildew, which is caused by a fungus. Onset generally appears between late June and September. It is easily identified by the white powdery appearance of the leaves.
This disease does not usually kill the plants, but it does weaken them, leaving them more susceptible to other pests and diseases in addition to looking bad.
A severely infected plant with brown foliage is not necessarily dead. The later in the season the disease appears, the less impact there will be on your plants. They should still perform well next year and bloom in spring.
Powdery mildew can affect many other plants including lilacs, phlox and beebalm. A beebalm patch in my home garden has lost all its leaves due to powdery mildew. I still believe the beebalm patch will return next year.
Outbreaks of powdery mildew vary from year to year and the severity varies depending on weather. One fall season with lots of powdery mildew does not necessarily mean the next year will be the same, so you may or may not have a problem next year.
Peony varieties, like other plants prone to powdery mildew, differ in their susceptibility. If you have a plant that is damaged year after year, it is best to replace it with a different plant or a resistant cultivar.
Conditions that favor development of powdery mildew include poor air circulation, high humidity, moderate temperatures, shade and wet conditions. Lots of heat and full sun along with good air circulation hinder the development of powdery mildew.
Once your peonies are infected, there is no spray to cure them, so prevention is important. Selecting resistant cultivars along with proper siting in full sun with good air circulation will help prevent future problems with powdery mildew.
There are fungicides available to use for preventing powdery mildew, but I generally do not recommend implementing a spray program. Spraying must be done early in the season before powdery mildew is present, and it must be repeated every 10 to 14 days.
Good sanitation can help prevent diseases, so consider pruning out infected plant parts as you see them. Consider how the plant will look once you do this work before taking action. It is best to remove diseased plant foliage from the garden at the end of the season.
Botrytis is a more serious disease of peonies in which young stalks wilt and die. Peony buds turn black. Onset is in early spring when temperatures are below 70 degrees Fahrenheit.
This disease will kill peonies, so treatment is important if you have this problem. Cut off diseased portions of plants when you see them, and make sure to disinfect your pruners between each cutting to prevent the spread of this disease. It does not appear botrytis is affecting your peonies.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.
