The fungus overwinters as sclerotia (Figure 7) or mycelium in plant debris. When conditions are favorable, the mycelium produces conidiophores and conidia. On potatoes, B. cinerea can be seed-borne and develop from mycelium on the mother tuber. In Michigan with such a wide crop range, mycelium on plant debris in the upper layers of the soil may serve as inoculum sources of the pathogen. Conidia are readily airborne and may also be carried on the surface of splashing rain or irrigation water. Relative humidity around 90 percent is necessary for spore production. Spores landing on potato plants germinate and infect in the presence of free water from dew, rain, mists or irrigation. The optimum temperature for infection is between 65 and 75 °F and can occur within five hours. However, temperatures above 82 °F suppress growth and spore production, although at temperatures this high,
secondary bacterial infections can enhance the development of other disease complexes. Infected flowers can spread infection to leaves and stems. In addition, direct contact with moist, infested soil or plant debris can promote infection.