Cause The fungus, Botrytis cinerea, is an opportunistic pathogen that easily invades weak, damaged, or senescing tissue. Disease usually starts on senescing flowers and flower infections are important for subsequent spread to leaves and pods. Spores (conidia) are produced throughout the growing season under a wide range of temperatures and humidity, and stem infections are a durable inoculum source, producing conidia throughout bloom. Cool temperatures, high humidity, and free water on plant surfaces favor the disease, as do close plant spacing and irrigation practices that keep plants wet for a longer time. Durable sclerotia overwinter in soil.
Symptoms The initial symptom is a dark, water-soaked, slimy lesion, with concentric zonation and sometimes a yellowish margin. Long brown streaks form on stems and petioles. As tissues dry out, gray, powdery spore masses form. Sclerotia may also develop.
- Rotate grasses, cereals, or corn for at least 2 years.
- If the placement of irrigation equipment allows, orientating the plant rows in the direction of prevailing winds helps control gray mold as well as white mold.
- Limit irrigation during and after bloom. Irrigate in the morning so plants do not stay wet more than 12 hours. In some areas, growers may be able to irrigate heavily before bloom and as little as possible thereafter.
- Avoid over-fertilization with nitrogen.
- Deep plowing will bury sclerotia in infected debris or weeds but plowing the field in later years may return viable sclerotia to the surface where they can produce apothecia or conidia.