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.
gardening.stackexchange.com/users/1380/Bamboo
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?
gardening.stackexchange.com/users/133/Shanna
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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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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I've found some useful information from Colorado State University: Prevention:
Avoid late-summer applications of nitrogen fertilizer to limit the production of succulent tissue, which is more susceptible to infection.
Avoid overhead watering to help reduce the relative humidity.
Selectively prune overcrowded plant material to help increase air circulation.
This helps reduce relative humidity and infection.
Control: Remove and destroy all infected plant parts (leaves, etc.).
For infected vegetables and other annuals, remove as much of the plant and its debris in the fall as possible.
This decreases the ability of the fungus to survive the winter.
Do not compost infected plant debris.
Temperatures often are not hot enough to kill the fungus.
Also, you can kill it with:
sulfur fungicide From here :
Sulfur is broad-spectrum with little detrimental environmental effects.
Can be irritating to the eyes, ears, and nose during application.
Used for the control of powdery mildew on grapes and other crops, it is effective against most species of pest mites, brown rot, rust, and scab.
Often used in the dormant season.
neem oil
From here : Neem oil is best as a preventative, or in low-disease-pressure situations, for fungus problems.
Use it to control diseases such as powdery mildew, anthracnose, botrytis, rust, leaf spot, or flower blight.
copper fungicide From here : The strongest and most broad-spectrum (organic) fungus control...
.
Use only when other techniques have failed.
Copper can be detrimental to plants and soil when applied in rates above the recommended amounts.
Often used in the dormant season.
Read the label!
gardening.stackexchange.com/users/419/J. Musser
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?
What do I do about the whitish powdery mold on my peonies?
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<fungus><diseases><peonies><mold>
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gardening.stackexchange.com/users/63/Scott Stafford
Try using a mix of milk and water against powdery mildew. Prune off infected leaves and burn them. You can try neem oil as a preventive measure. To control it organically, prune out infected parts and dispose of by burning them or putting out with the rubbish. Do not add to your compost.  You can also try neem oil
There is a 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. To control it organically, prune out infected parts and dispose of by burning them or putting out with the rubbish. Do not add to your compost.   Try neem oil.