White mold can reduce your peony plant's vigor and cosmetic appeal. The disease is nearly impossible to eradicate once it infects your plant, since no chemical or biological control effectively treats it. Learn to identify white mold's symptoms to keep it from affecting other peony plants and to keep your plant as healthy as possible once infection strikes.
White mold on peony plants is caused by Sclerotinia sclerotiorum, a soil-borne fungus. This fungus can survive in the soil for many years and is nearly impossible to remove from the soil once it's established.
Peony plants infected with white mold develop white mycelium (or fungal tendrils) in humid weather; this symptom gives the disease its common name. While this symptom is not the only one, the bright white color makes it easy for busy gardeners to notice the mycelium, compared with other signs of infection.
Part or all of the peony plant can wilt. Peony plant stems dry out, fade from green to tan and become stringy. Gardeners who cut open infected stems lengthwise may find black sclerotia, a hard mass that serves as the dormant state of the fungus. Sclerotia help the white mold fungus survive in the soil for years.
Prune off infected portions of the plant and dispose of them. Do not drop sclerotia onto the ground. Dig up your peony plant and replant it in a container to avoid spreading white mold to other parts of your garden. This may keep the plant healthy. Prune plants for improved air circulation, and do not crowd that part of your garden to minimize fungal growth.
White mold was once believed to infect only plants in cool, moist regions, notes the University of Hawaii. While this disease is more common in temperate regions, white mold has been confirmed in arid regions and tropical areas, too.