What do I do about the whitish powdery mold on my peonies?

Question

Very early this year (starting in mid-May) my peonies' foliage was covered in whitish-gray powdery mold. What is it and what if anything should I do about it?

Answer 1

Sclerotinia sclerotiorum can cause stem rot on peony as well as on many different herbaceous plants found in the garden. The entire plant or portion of the plant may wilt. The infected part of the stem turns a light tan color and may become dry and stringy. Fluffy white mycelium often appears under humid conditions, thus the name, white mold. Slice the stem lengthwise to help you diagnose this disease. Various sizes of irregularly shaped, hard black sclerotia may be found inside the tan area of the stem. Sclerotia are the overwintering structures for this fungus and they can survive for many years in the soil. Remove and dispose of any infected plant parts being careful to not drop the sclerotia. Sclerotinia is a soil-inhabiting fungus that is nearly impossible to remove. Replant infested areas with nonsusceptible plants. Wider spacing to improve air circulation will minimize infection by this fungus.

From the article: University of Minnesota - "Diseases of Peony"

Compare how your plants look to pictures of the disease, and you can judge if you believe that is what your plants are infected with.

Answer 2

Without a visual, I think it sounds like powdery mildew, which is a fungus that is encouraged by dry, perhaps shaded, conditions and lack of air movement. Depending on the host plant, leaving the infection untreated can lead to leaf loss and weakening of the plant. To control it organically, prune out infected tissue, being sure not to shake it around as this can spread the fungal spores, and dispose of the prunings by burning them or putting them out with the rubbish rather than adding them to your compost. If you want to treat it with fungicide, check with your local garden centre to see what's available, or investigate online. To prevent it in the future, ensure that your peonies are well-watered and mulched, and improve air circulation around them, if necessary. Perhaps they need dividing?