Botrytis is a fungus that affects peonies, especially in a rainy year. Symptoms include blackening of flower buds that then fail to open as well as blackening of leaves, starting at the edges and moving backwards to the stem. Eventually, if left untreated, botrytis will cause all of the stems to blacken and shrivel.
The first line of defense is proper garden sanitation. If you spot botrytis, cut off all affected plant parts and DISCARD them in the garbage- do not compost them! (In earlier times, gardeners would burn diseased plant parts.) After pruning off the diseased foliage, dip your shears in bleach or isopropyl alcohol to sterilize them. Otherwise, moving from peony to peony will spread the disease. Monitor your plants for the entire season; do not let the botrytis go unchecked.
In the fall, be sure to remove all of the leaves and stems from your peonies and DISCARD them, do not compost them! Clean cultivate around the base of the plants, removing fallen plant debris, and possibly even removing the existing mulch and adding fresh mulch.
In the spring, spray the emerging shoots with modern Bordeaux mix, a copper-sulphur organic fungicide so named because it was used to treat grapes in the Bordeaux region of France, for fungus. Spray again when the plants have leafed out, and weekly or bi-weekly in the spring as the buds are forming. As a preventative measure, you can use Actinovate on plants that have not had active Botrytis the previous year. This is especially useful in a rainy, wet year/exceptionally cold, wet spring. After the peonies bloom, monitor them. If they remain clean and disease free, you can stop spraying them. If you notice the blackening of the leaves beginning to return, clean them and spray them weekly or bi-weekly for the rest of the season.
Your best defense is to wander through your garden every few days and observe what is happening. If you catch a fungus problem in the early stages, simply removing and destroying the affected parts may be all you need.
In an effort to provide horticultural information, these educational documents are written by Nancy DuBrule-Clemente and are the property of Natureworks Horticultural Services, LLC. You are granted permission to print/photocopy this educational information free of charge as long as you clearly show that these are Natureworks documents.