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 in what appears to be white mold, some leaves are even turning brown already. What is this problem and will it affect plants next year? Should I do something about it? “
– Sylvia Mendoza, Highwood
The problem with your peonies is probably powdery mildew, which is caused by a fungus. The onset usually appears between late June and September. It is easily identified by the powdery white appearance of the leaves.
This disease does not usually kill plants, but it weakens them, leaving them more susceptible to other pests and diseases in addition to looking poor.
A severely infected plant with brown foliage is not necessarily dead. The later the disease appears in the season, the less impact it will have on your plants. They should still perform well next year and flower in the spring.
Powdery mildew can affect many other plants, including lilacs, phlox, and beebalm. A patch of balm in my garden has lost all of its leaves due to powdery mildew. I still believe the Beebalm patch will come back next year.
Powdery mildew outbreaks vary from year to year and the severity varies depending on weather conditions. A fall season with a lot of powdery mildew doesn’t necessarily mean 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 sensitivity. If you have a plant that deteriorates year after year, it is best to replace it with another resistant plant or cultivar.
Conditions that favor the development of powdery mildew include poor air circulation, high humidity, moderate temperatures, shade, and humid conditions. Lots of heat and full sun as well as good air circulation hamper 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 a suitable location in full sun with good air circulation will help prevent future powdery mildew problems.
There are fungicides to use to prevent powdery mildew, but I generally do not recommend implementing a spray schedule. Spraying should be done early in the season before powdery mildew appears and should be repeated every 10-14 days.
Good sanitation can help prevent disease, so consider pruning off parts of infected plants as you see them. Consider what the plant will look like after you’ve done this work before you take action. It is best to remove the foliage of diseased plants from the garden at the end of the season.
Botrytis is a more serious disease of peonies in which the young stems wither and die. Peony buds turn black. The 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 portions of diseased plants when you see them, and be sure to disinfect your pruning shears between each cut to prevent the spread of this disease. Botrytis doesn’t seem to affect your peonies.
For more plant advice, contact the Chicago Botanic Garden’s Plant Information Service at [email protected]. Tim Johnson is Senior Director of Horticulture at the Chicago Botanic Garden.