We, the *Paeonia* genus, are herbaceous or woody perennial plants prized for our magnificent, often fragrant blooms. Our resilience is legendary; we can thrive for decades in a single location with minimal care. However, our primary weakness, our botanical Achilles' heel, is our susceptibility to a specific fungal pathogen. This condition, known as Peony Wilt or Botrytis Blight, is caused primarily by the fungus *Botrytis paeoniae* and sometimes *Botrytis cinerea*. Our succulent new shoots, fat flower buds, and sturdy stems are particularly vulnerable to invasion, especially during the cool, wet weather of spring in many US gardens.
When infected, our systems send clear distress signals that attentive gardeners can observe. The attack often begins at the soil line or on young shoots. You may see dark brown, leathery lesions that girdle and cause our young stems to wilt and collapse suddenly, as if we have been deprived of water. Our prized flower buds may turn black and fail to open, often becoming covered in a fuzzy, grayish-brown mold—this is the sporulation of the fungus. On more mature leaves, large, dark brown, irregular blotches appear. In severe cases, the entire plant can succumb, as the fungus moves from the foliage down into our crown and roots, compromising our ability to regenerate the next season.
The fungus *Botrytis paeoniae* overwinters comfortably in our plant debris and in the soil on sclerotia (dense, hardened masses of fungal mycelium). In spring, these structures produce spores (conidia) that are splashed by rain or carried by wind onto our new, tender growth. Infection requires a film of moisture on our tissue surfaces to germinate and penetrate. Therefore, the environmental conditions most conducive to an outbreak are prolonged periods of cool temperatures (55-65°F is ideal for the fungus), high humidity, and poor air circulation around our foliage. Crowded planting beds create a perfect microclimate for this pathogen to thrive and spread rapidly from one plant to another.
The most effective treatment is a rigorous cultural practice focused on sanitation and environmental control. At the end of each growing season, our above-ground foliage must be cut back to the ground and all plant material removed from the garden and destroyed—do not compost it. This eliminates the primary overwintering site for the fungus. In spring, ensure we are not smothered by heavy mulch; keep it away from our emerging crowns to reduce humidity. Improve air circulation by spacing us appropriately and thinning surrounding vegetation. When watering, avoid splashing our leaves; instead, use soaker hoses or drip irrigation at the soil level.
If an infection is severe or recurrent, a targeted fungicide application can be a useful tool. The treatment must be preventative and applied in early spring as our red shoots first emerge from the soil, before the fungus becomes active. Fungicides containing chlorothalonil or mancozeb can create a protective barrier on our tissue. It is crucial to cover all surfaces thoroughly and to follow the label instructions exactly. For gardeners preferring organic options, a product containing *Bacillus subtilis* or neem oil may offer some suppression, though they are generally less effective than chemical controls and require very diligent application.
