Cause The fungus Sclerotinia sclerotiorum causes stem rot in many plants and occasionally attacks peony. Overwintering sclerotia can survive many years in soil. The disease develops in cool, wet conditions and is more prevalent when plants form a complete, compact canopy that holds high relative humidity. The fungus can colonize dead organic plant matter and survive many years. Plants can be infected when host tissue colonized by the fungus comes in contact with healthy tissues.
