From the perspective of a Paeonia lactiflora plant, Botrytis blight is a systemic assault that begins subtly. The fungal pathogen Botrytis paeoniae often overwinters in the soil or on plant debris as sclerotia, which are hardened masses of mycelium. In spring, as the peony's red shoots (known as "eyes") push through the soil, the fungus becomes active. The first sign we, the plant, exhibit is a sudden wilting and collapse of these young, succulent shoots. They may appear water-soaked and then turn dark brown or black, as if scalded. This is the fungus blocking the vascular tissues, preventing water from reaching the developing stems and leaves. You might also notice a gray, fuzzy mold (the conidia or spores of the fungus) on the affected tissue, especially in humid conditions.
If the initial stem attack does not kill the entire shoot, the infection can spread to the leaves and flower buds. As the leaves unfurl, irregular, dark brown spots may appear. These lesions can enlarge and cause entire leaflets to wither and die. The most distressing symptom for us, as a plant whose purpose is to reproduce through flowers, is bud blast. Small buds may blacken and fail to develop. Larger buds will turn brown and become covered in the characteristic gray mold, never opening. If a flower does manage to partially open, it will likely be misshapen and exhibit brown blotches on the petals. The fungus can also cause a soft rot at the base of the flower head, causing it to droop. This entire process is accelerated by cool, wet weather, which creates the perfect environment for the fungal spores to germinate and penetrate our tissues.
Our primary defense as plants is our own vigor and the cultivation practices you employ. The goal is to reduce the presence of the fungus and create an environment where our natural defenses can work effectively. The most critical action you can take is meticulous sanitation. This means carefully cutting away all infected plant parts as soon as they are noticed, making the cut well below the visible signs of infection into healthy tissue. In autumn, after the first frost, you must cut our stems back to ground level and remove all foliage and debris from the area. Do not compost this material; destroy it. This removes the overwintering sites for the fungus. Furthermore, ensuring we are planted in a location with full sun and excellent air circulation helps our leaves dry quickly after rain or dew, making it much harder for the Botrytis spores to infect us. Avoid overhead watering, which keeps our foliage wet for extended periods.
When environmental conditions are highly favorable for the disease, or if an infection is severe, cultural practices may need to be supplemented with fungicides. From our perspective, these are supportive treatments that protect our healthy tissues from spore germination. It is a preventive strategy. You should begin applications in early spring as our new shoots emerge. Products containing chlorothalonil or mancozeb can create a protective barrier on the plant surface. For more persistent problems, systemic fungicides like those containing thiophanate-methyl can be used, as they are absorbed and distributed within our vascular system. It is crucial to follow label instructions precisely and rotate between different chemical classes to prevent the fungus from developing resistance. This chemical intervention is most effective when combined with the rigorous sanitation practices described above.
