White mycelium and black sclerotia of S. sclerotiorum on an infected soybean stem. Photo X.B. Yang White mold, also called sclerotinia stem rot, is caused by the fungus Sclerotinia sclerotiorum. You can see some sclerotia embedded in the white mycelium on the soybean stem in Figure 1. Sclerotia resume growth as mycelium, or as mushroom-like structures called apothecia. Apothecia of the white mold fungus on the soil surface.
Spores form in apothecia and are ejected into the air and disseminated by wind to a host.
Apothecia are found on the soil surface and form from sclerotia when the soil is moist and dim light is filtered through the crop canopy. The number of apothecia is related to the number of sclerotia near the soil surface, soil moisture and temperature, and the timing and density of crop canopy closure. Apothecia will readily form under the canopy of a nonhost. Bird's nest fungus on the soil surface.