In years when summers are cooler and wetter than
normal, the disease can develop earlier. Under favorable
conditions, lesions can form seven to 12 days after
infection. Each lesion can produce many spores, which are splashed or wind-blown to upper leaves or to other
plants. Due to the length of the infection process, symptoms
may not be noticeable for one or two weeks after
infection occurs, depending on weather conditions and
hybrid susceptibility. Hot, dry weather restricts disease
development and spread.