Although many fungal diseases in the flower garden do not get going until the warm, humid days of summer, Botrytis blight attacks peonies even as they emerge from the ground.
Gardeners with Botrytis blight will notice that young peony shoots will grow 6-8 inches and then suddenly wilt and fall over. Close examination of these fallen shoots will reveal a dark brown to black soft sunken area on the stem right around the soil line. Under humid conditions, a dense velvety mat of gray spores will be present on the infected part of these stems. These spores are characteristic of Botrytis blight, caused by Botrytis paeoniae, and should be taken as a warning of future problems.
Botrytis paeoniae only attacks peony plants. It thrives in cool wet weather (60F is perfect) and often attacks young shoots as they emerge from the ground in the spring. The spores that are produced on these early infections can spread to other parts of the plant by wind, rain, or insects. Botrytis blight often blasts young flower buds, causing them to turn black and dry up before they ever open. Large irregular brown spots can also occur on petals of open flowers and on leaves. Infections from buds and flowers often continue down the peony stem turning it brown or black. In severe cases the infection, it can move into the roots and crown of the plant resulting in rot, although this is not common. Botrytis blight can spread throughout the summer whenever cool wet weather is present. It may not be seen at all in hot dry years.
For gardeners dealing with Botrytis blight of peony, sanitation is the best defense against future flower and leaf infections. The Botrytis blight fungus produces an abundance of spores on infected plant material whenever moisture is present. These spores spread the disease to other plant parts. It is therefore critical to remove all infected plant parts from the garden as soon as possible.
Choose a dry day when spores are less likely to be present. Take a paper bag to the garden and cut out any infected plant parts, placing them immediately in the bag. For infected young shoots, cut out as much of the infected stem as possible without hurting the crown of the plant. This may mean excavating an inch of soil from the base of the infected shoot to make a clean cut.
If any dried stalks from last year remain around the base of the plant, remove these as well. At the end of the summer, the Botrytis blight fungi makes hard resting structures called sclerotia that allow it to survive MN's winter on last years infected stems.
If flower buds or leaves become infected later in the season, they should also be promptly removed from the garden. In gardens where Botrytis blight has been a problem in the past, it is a good idea to collect and remove faded flowers as well, since the fungi can easily colonize the old flower petals and produce spores. Do not compost these diseased plant clippings, as the Botrytis blight fungi can survive and even grow within the compost pile. Instead, throw these infected plant parts into the trash.
Reducing moisture on and around the peonies will also help minimize problems with Botrytis blight. Avoid using dense wet mulches like composting leaves, around the base of the plant. Choose instead mulches that allow air to pass through to the soil like large woodchips or bark pieces. Pull mulch away from the immediate base of the plant so that air moves by stems. In plants suffering from many infected shoots, it may be worthwhile to remove old mulch and discard it like the infected plant material, since the fungi can survive in organic material.
Replace the old mulch with fresh airy mulch. As with any fungal leaf blight disease, using drip irrigation or soaker hose instead of sprinkler irrigation will help to keep foliage dry and reduce disease.
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