I have several peonies. Several, but not all, are covered with white mildew. Will this weaken/kill the plant.
What can I do to prevent this. I have cut off the deceased stalks and cleaned up the area from any debris.
Why are some affected and not others. Please advise.
Thank you for contacting the Toronto Master Gardeners with your question about your peony mildew problem. Powdery mildew is a fungus that thrives in hot humid conditions which we saw quite a lot of this summer, and plants that are under stress due to drought are also susceptible. The good news is that while powdery mildew affects the appearance of peonies, it is generally considered harmless and doesn’t cause significant damage.
Good cultural practices are most important to prevent powdery mildew in peonies, including : planting in full sun, providing enough water and nutrients (low nitrogen, don’t over-fertilize), avoiding getting the leaves wet when watering, providing good air circulation / avoiding crowding plants, cleaning gardening tools well after each use to prevent the spread of disease. The fall is when you do the major protection from powdery mildew : cut the plant to the ground, and completely remove all traces of stalks and leaves (as you have done). Spores can overwinter on plant debris and splash onto the plant in the spring. This plant debris should be put in the garbage, not in your compost, since spores will remain viable.
It is best to wait until the fall to remove infected leaves from the plant, because removing leaves means less green growth for photosynthesis to create and store plant nutrients for the winter.
There could be several reasons why all of your peonies are not affected by powdery mildew. There are thousands of species of this fungus, each of which affects only a narrow range of plants. Some peony species are more resistant than others to powdery mildew. Depending on where your peonies are situated in your garden, perhaps the conditions differ : more / less sun, more / less crowding and air circulation, which could affect plant susceptibility to powdery mildew. This fungus is spread by spores, typically in wind or water, so perhaps your unaffected plants are not accessible enough to your affected plants for this spread.
Powdery mildew doesn’t necessarily occur on the same plants every year. Maintaining a healthy growing environment and using good cultural practices are the best ways to prevent this fungus. However fungicides can be applied preventatively if you wish, for high-value plants with a history of disease. Fungicides will not cure or remove existing powdery mildew infections.
Here are a couple of websites with information about powdery mildew :
https://extension.umn.edu/plant-diseases/powdery-mildew-flower-garden
Hopefully your peonies will be back in good health in the spring !