Peonies (Paeonia spp.) are prized for large, colorful blooms that appear in late spring or early summer. Multiple disease pathogens and pests may occasionally plague peonies, including powdery mildew fungi. Good sanitation and care practices discourage powdery mildew or limit its impact. Where powdery mildew does appear, efficiently identifying this disease and treating it properly minimizes its negative impact on overall peony appearance and health.
Identifying Powdery Mildew
Powdery mildew first appears on peonies as patches of white, powdery growth on leaves, stems buds and flowers. Patches may grow and completely cover both sides of the leaves. Later in the growing season, the patches may turn gray to tan and develop tiny dark dots. Affected peonies may suffer leaf curling or dwarfing, yellowing, browning and premature flower drop. Powdery mildew will not kill the peony, and damage is primarily aesthetic, but may shorten the plant's growing season.
About Powdery Mildew Pathogens
More than 1,000 species of fungi in six genera cause powdery mildew. Powdery mildew fungi over winter in leaves, within dormant buds, or on the bark of branches or stems. When the plant resumes growth as temperatures rise in spring, the overwintering bodies release spores that spread to new plants or portions of plants by wind or contact. The fungus then grows and appears as the recognizable thin layer of white mycelium on the surface of affected plant parts. Under a hand lens, chains of spores are visible on the upper and lower surfaces of infected leaves. Powdery mildew grows best in shady conditions when temperatures are between 60 and 80 degrees Fahrenheit and may die if leaf temperatures rise above 95 F. The fungi are also sensitive to sunlight and the prolonged presence of water on the leaf surface.
Preventative Care and Cultural Control
Prevention of powdery mildew begins with buying or transplanting only healthy, disease-free stock. The growing site should give the peony with full sun, well-drained soil and ample air circulation. Remove and destroy infected leaves as they appear. In fall, cut back diseased peonies to the ground and remove all debris from the site to curb powdery mildew the following year. Do not cut tree to the ground, as they will not recover. Do not fertilize peonies too much, as it encourages vulnerable new growth. Divide overgrown or crowded peonies in fall to improve spacing and air circulation.
Both protectant and eradicant fungicides can control powdery mildew. Where peonies or nearby plants have been affected by powdery mildew in previous seasons, you may want to apply a preventative fungicide. Otherwise, apply the fungicide as soon as possible after the first signs of powdery mildew appear to achieve control. Properly applied products that contain sulfur, biological fungicides or synthetic fungicides like myclobutanil prevent severe powdery mildew problems. Horticultural or neem oils treat peonies already infected by powdery mildew.
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- University of California Statewide Integrated Pest Management Program: Peony—Paeonia spp.
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- University of Illinois Extension: Powdery Mildew of Ornamentals
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- University of Illinois Extension: Peony Powdery Mildew
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