Peonies: disease control

Credit: Wikipedia/Peony-Fanghong

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Q: I have two peonies that have developed black stems. It seems to start at ground level and goes up about 3 inches. The stems cannot hold themselves upright any longer. What causes this and can I get rid of it for next year?

There are two possible diseases, depending on when the problem with the peonies occurred.
The first one is common during cloudy, cool, moist weather and attacks stems, leaves and buds. This disease, caused by fungus, can infect peonies at any time of year, especially when new shoots are just 15 cm (6 in.) tall in the early spring. They are a complex group fungi causing “Blight” (Botrytis paeoniae and B. cinerea), but are distinct species that are easy to identify. A distinctive diagnostic clue is a mass of grey fuzzy spores appearing above ground on the plant canopy. There are a number of good methods to control this problem. You’ll need proper site selection to plant peonies in a sunny location. Cultural management, including improved air circulation and watering midday at ground level, will also help.

Finally, you’ll want to engage in good sanitation practices, such as working during the dry time of day, disposing of diseased tissue not compost, cutting stems to ground level in fall, and avoiding moisture retentive mulch too close to emerging shoots.

The second disease, Crown Rot (Phytophthora cactorum), usually occurs in poorly drained soil. Its affects are seen mainly closer to roots and lower stems, but no spore masses are evident. To control, remove infected plant material and contaminated soil. Plant new, healthy material in well drained soil.