Ascospores are produced within the perithecia on these dead leaves during the winter. The ascospores are forcibly ejected in the spring and are carried by wind or water to new leaf tissue. Infection by both spore types occurs only through the lower leaf surface. The incubation period between infection and the appearance of lesions on the upper leaf surface is from 10 to 14 days. Large numbers of new conidia, which appear as a downy white growth on the underleaf surface, are produced and cause secondary infections during fairly prolonged periods of damp to wet, moderately warm weather.
Infections continue to occur throughout the growing season - except during hot, dry weather.
Young, expanding leaves are much more susceptible to infection than mature leaves. The leaf scorch fungus also produces ascospores in the early spring, within disk-shaped apothecia that appear as black dots in old lesions on the lower surface of dead leaves. Conidia are also produced on both leaf surfaces in glistening, speck-sized, black acervuli. These acervuli form on diseased leaves that survive the winter. Infection by the leaf scorch fungus, like the leaf spot pathogen, occurs almost entirely through the lower leaf surface.