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Sweet Woodruff (Galium odoratum)
QCan you tell me why the bottom leaves of our peonies turn brown, as if they have a blight?
APeonies are susceptible to several fungal diseases that can cause spots or blotches on their leaves and stems. Botrytis blight is most prevalent during wet, cool weather, although it can appear at other times as well. This fungus affects new, growing shoots in the spring and can continue to affect mature foliage. You may see large brown patches that are covered with a fuzzy grayish mass -- these are the fungal spores. Phytophthora is another fungal blight that can infect peony leaves and stems. It can cause large dark patches on the leaves and can also kill whole shoots. There are several other fungal diseases that may cause various spots on leaves and stems.
Good sanitation is the first step in preventing these diseases. Make sure the peonies are growing in a site with good soil drainage and good air circulation. Consider moving the plants to a better site if necessary. Cut back and dispose of the foliage in the fall. If you see diseased leaves during the growing season, cut them out and put them in the trash.
Fungicides may prevent some of these fungal organisms. It's important to remember that fungicides work to prevent an infection before it starts, not to cure it once it's there. This means that you need to apply fungicides before a fungal infection starts. Start spraying early in the season and be sure that you cover all of the plant. Read the fungicide label carefully so that you know it's approved for peonies, and follow the instructions carefully for quantities and frequency of sprays.
-- Nancy Rose is a research horticulturist at the Minnesota Landscape Arboretum. To ask her a gardening question, call 612-673-9073 and leave a message. She will answer questions in this column only.