Peony garden in full bloom
Peony fungus

Above, left to right: With some fall prep, this is what your peonies will look in the spring. This is a view of the Nichols Arboretum Peony Garden, which will celebrate its 100th anniversary in 2022. Center: powdery mildew on peony leaves. Right: botrytis fungus on peony leaves.

By Alyssa Abaloz
What can we do to prevent and treat diseases in our peonies in the fall? We talked to our Peony Garden Fellow Sarah Gizzi to find out.
Peonies can develop botrytis, a soil-based fungus that causes blackening of the leaves, stems, and flower buds. Removal of infected parts is crucial to ensure your peony’s survival. Place these diseased parts in the trash (not compost), or burn them if your local ordinance allows. Botrytis can live on in your soil or compost and spread to other areas of the garden.
“Make sure there is good drainage. Wet feet and soil with a lot of clay are breeding grounds for disease. That and planting in full sun are super important.” Gizzi said. “If it’s a serious problem you can amend the soil. Because botrytis is a soil-based fungus, you can take the peony out of the ground and amend the soil with non-infected soil–that can also help.”
Treat your peony for botrytis in the spring. Spray emerging shoots with modern Bordeaux mix, a copper-sulphur organic fungicide. Spray again when leaves emerge, then weekly or bi-weekly as the buds begin to form.
Powdery mildew is another common affliction of peonies. There are many species of this fungus–most thrive in humid conditions that cause excess moisture. “This disease isn’t detrimental to the peony’s health, but it is not aesthetically pleasing. If there are any infected leaves or stems, remove them and make sure they aren’t composted–throw them away.” Gizzi advised.
Treatments for powdery mildew can include horticultural oil or neem oil. “Commercial fungicides work too,” Gizzi said.
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