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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?

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4 Answers 4

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.

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Sclerotinia doesn't leave white powdery substance on leaves. It is a very bad fungal infection and the entire plant should be removed and bagged as soon as one finds this in the stems of a plant. To id positively, cut into the rotted stem. You can see shiny, blackish capsules that look like rabbit pellets. Just cutting off the infected part won't do any good. The whole plant is infected. –  stormy Jul 23 at 19:37

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?

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There is another non chemical treatment which should work, particularly for powdery mildew - 1 part milk to 9 parts water mixed in a sprayer, spray all leaves and stems, including under the leaves. Doesn't matter whether the milk is full fat or skimmed. The 'scientific' explanation as to why it works is there's something about the lactic acid which seems to destroy the fungal spores. Increasing the ratio of milk to water doesn't make it more efficacious, though 2 parts to 8 parts water will be okay.

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Try neem oil. My peonies get infected with powdery mildew every year and this is the only organic solution that seems to really work. It works immediately but you have to start applying it before the mildew sets in. Spray the whole plant liberally from all directions making sure to get the undersides of leaves. The powdery mildew washes right off. Follow the instructions on the bottle regarding dilution and not applying when it's very hot and sunny. A follow up treatment may be necessary a week later.

When I say liberally i mean drench the plant with spray and dig around to make sure you have every surface treated.

After that if you hose them off from time to time you'll wash away the spores that haven't attached firmly to the leaves and as mentioned previously wet conditions discourage powdery mildew as counter intuitive as that may sound.

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