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Gray Mold on Peonies

The most common disease of herbaceous peonies is Gray Mold

By: Sandy Feather 2008
Penn State Extension


Q. An established row of peonies is growing along our property line in full sun. We had beautiful blooms the past couple years, although the plants have been neglected. This year, the plants look fine and are full of buds, but the buds are not opening. What is the problem, and what can we do about it?

A. If the buds look as though they are drying up and dying without opening, it could be peony botrytis. We had cool, wet weather in May as the peonies were starting to grow, creating ideal conditions for this fungal disease also known as gray mold, the most common disease of herbaceous peonies. The warm spell that followed brought many peonies into bloom, followed by rains that shattered those delicate flowers into moldering masses of goo that stuck to the wet plants, enhancing the development of mold.


Symptoms of Gray Mold Disease

Irregular dark brown leaf spots are characteristic of this disease. Buds form but often turn dark brown or black and dry up rather than opening. On buds and petals, you can see a brownish-gray fuzz. This is the fruiting body of the fungus and comprises thousands of microscopic spores that are spread by wind, splashing rain or irrigation water, and on tools or even your hands when you work on infected plants. The fungus survives on plant debris and can persist in the soil for many years.

pink peonies
Pink peonies in full bloom

Remove and dispose of all the decaying and infected plant parts. Do not compost it. Thoroughly deadhead them when they finish blooming, and do not hesitate to remove badly spotted stems to the ground while doing so. Clean up plant debris around your peonies during the growing season and again before putting the garden to bed in the fall. Cut herbaceous peonies (but NEVER tree peonies!) to the ground and dispose of the cuttings.

 

While you cannot keep them dry when it rains, avoid wetting the foliage when you water. If your peonies are very old and overcrowded, it may be helpful to divide them. This will improve air circulation inside the plant, which helps the foliage dry faster after rain. Late summer (mid-September) is the best time to divide herbaceous peonies. Try to have four or five "eyes" per division, and avoid planting them deeper than 1-1/2 to 2 inches. If peonies are planted too deeply, they will not bloom in future years.

red peony

You can also make fungicide applications to the new growth next spring to protect it from botrytis, especially if the weather is cool and wet when young shoots start poking up through the ground. Mancozeb is labeled to control botrytis on peonies. Organic gardeners can use Serenade biofungicide (Bacillus subtilis, QST 713 strain). Follow label directions regarding application rate and intervals between applications. Both products perform best if used preventatively when environmental factors are favorable for disease development. Remember that fungicide applications are not a substitute for good garden sanitation and maintenance, but they are a partner to them.
  
Peonies prefer a slightly acid soil pH, between 6.0 and 7.0, so it would be wise to obtain a soil test kit from your local cooperative extension office so that you can amend your soil back into the preferred range. Plants grown in accordance with the cultural preferences are less likely to suffer serious insect or disease problems than those growing where they do not belong.
  
It is helpful to realize that not all peonies bloom at the same time. I was at Fellows Riverside Garden in Youngstown, Ohio, the first Saturday in June, and not all of the peonies in its sizable collection were open yet.

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