Botrytis blight on peonies

Botrytis blight is the most common and damaging disease of peonies in the eastern US, and it strikes at exactly the wrong moment -- on the emerging stems and developing buds when the plant is most visible. I grow peonies at my Long Island property, and Botrytis is the one disease I watch for every.

Botrytis blight is the most common and damaging disease of peonies in the eastern US, and it strikes at exactly the wrong moment — on the emerging stems and developing buds when the plant is most visible. I grow peonies at my Long Island property, and Botrytis is the one disease I watch for every spring. In years when April and May bring persistent cold rain, it can destroy a substantial portion of the season's flower display. In dry springs, I barely see it.

The pattern at my property follows what the research predicts exactly: cool temperatures (55–70°F, 13–21°C) combined with rain and high humidity create conditions where the gray mold spreads rapidly through emerging tissue.

The pathogen

Botrytis blight is caused by Botrytis cinerea (perfect stage: Botryotinia fuckeliana). Per Cornell Cooperative Extension, B. cinerea is a generalist pathogen that infects hundreds of plant species but causes distinctive damage on peonies due to their susceptibility during the cool, moist conditions of spring emergence.

The fungus overwinters as sclerotia (hardened survival structures) in infected plant debris and as mycelium in the crown and surrounding soil. Per Penn State Extension, infected stems and leaves left in the garden at season's end are the primary inoculum source for the following spring.