The disease is characterized by obvious lesions which appear on the surface of ripe fruit. Lesions are light to dark brown and vary from small flecks affecting only the epidermal tissue to large, more or less circular, sunken lesions with decay extending into the carpel wall and often into the seed locule.

Black mold appears in the field after rain or dew. Free water must be present three to five hours before fungus spores germinate ; infection by direct penetration of the epidermis follows soon thereafter. Thus dews of even short duration provide conditions favorable for disease establishment. However, rainfall usually is far more conducive to blackmold than are dews: total destruction of a tomato crop may occur within four to five days following a period of rain and high humidity. In the absence of rain, fruits protected by a leaf canopy seldom develop blackmold because dew is deposited only on fruit surfaces fully exposed to the sky.