Development of lesions results primarily from infection through hydathodes at the margins of the leaf, and to a lesser extent through wounds. Secondary stem lesions can develop from leaf infections by growing through the petiole into the stem, and from bud infections late in the season. Stem lesions first appear on current season's growth as dark red circular to elliptical lesions around leaf scars. As the lesions enlarge, the affected stems turn brown and eventually become gray and die. On highly susceptible cultivars such as Jersey, the disease results in severe dieback, measuring up to 20 inches.
Damage is also serious in rooting beds, causing severe defoliation and poor growth.
The fungus overwinters in infected leaves on the ground and in stem lesions. Spores are produced in abundance on new lesions throughout the season with heaviest infection occurring between June and September. Stem lesions are flat or slightly sunken with a tan to gray center and a reddish brown margin. Stem lesions and leaf spots are most severe on small plants or on the lower parts of large plants. In late summer, the fungus develops a secondary necrotic area around the original lesion spreading in an irregular to fan-shaped pattern.