The disease usually appears first on lower leaves. New spores are produced on the surface of mature lesions. These spores cause secondary infections and serve to spread the disease up the plant, further within a field, and to surrounding fields. Many secondary cycles of infection may occur during the growing season if weather conditions are favorable. Corn inbreds and hybrids appear to become more susceptible to gray leaf spot after tasseling, thus the disease sometimes seems to explode later in the season.
Heavy rains tend to aid in dissemination of the pathogen.
When spores of the fungus land on leaf or husk tissue, they germinate if humidity is high and temperatures are in the optimum range. After growing over the leaf or husk tissue for a time, the fungus penetrates living tissue and initiates a new infection that results in another lesion. The longer the optimum conditions persist, the more rapidly new infections develop, and the more severe the disease. Once spores of most leaf-infecting fungi germinate, they must penetrate the leaf within a few hours or they die. Initial symptoms of gray leaf spot, often surrounded by a yellow halo.