Stem lesions on seedling tomatoes can occur at, or just below, the soil level. Entry to the stem may occur through senescent cotyledons or damaged tissue. Stem lesions can also occur later during the growth of the crop. Stems can become infected through leaf scars, dead leaves, or any form of stem damage. Stem lesions often partially girdle the stem, but sometimes the whole stem is affected and the plant is killed.

The fungus overwinters as sclerotia or as mycelium in plant debris and may be seedborne as spores or mycelium in a few crops. Other crops may also serve as sources of the pathogen and are likely to cross-infect. Conidia are airborne and may also be carried on the surface of splashing rain drops. High relative humidities are necessary for prolific spore production. In the field, spores landing on tomato plants germinate and produce an infection when free water from rain, dew, fog, or irrigation occurs on the plant surface. Optimum temperatures for infection are between 65° and 75° F (18° and 24° C), and infection can occur within 5 hours. High temperatures, above 82° F (28° C), suppress growth and spore production. Dying flowers are a favorable site for infection, but infections can also result from direct contact with moist infested soil or plant debris. In the greenhouse, stem lesions develop either by direct colonization of wounds or through infected leaves. The presence of external nutrients, such as pollen grains in the infection droplet, can markedly increase infection. The type of wound is said to influence stem lesion development; breaking off leaves is reported to give a lower incidence of stem lesions than cutting off leaves with a knife, leaving a stub.