Answer: What you describe sounds like Phytophthora. Cool, wet conditions during the spring favor the development of Phytophthora blight on peony. The fungus Phytophthora, common in most soils, initially attacks either the roots or the developing shoots at the soil level, causing a blackening and decay of stem tissue. These black often sunken areas, usually several inches long, may appear on upper stem tissue as well. Stems tend to fall over at the stem lesions. Flowers, buds, and leaves may also turn a dark brown or black color. The tissue will appear somewhat leathery.
The disease is most serious in soils that are poorly drained. The disease can be spread by splashing rain or contaminated tools, soil, or plant material.
To prevent Phytophthora blight, plant peonies in well-drained soils and thin crowded plantings. If disease symptoms appear, destroy infected plant parts. Peony plants with rotted roots need to be removed together with the adjacent soil. Fungicides may help control the disease in situations where the roots are not rotted. Spray the foliage, bases of shoots, and nearby soil at intervals of 7 to 10 days during rainy periods with a fungicide containing mancozeb or maneb.
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