The critical period for control of early blight (Alternaria solani) of potato

The purpose of this study was to identify when the critical period for control of early blight occurs and thus the need to initiate a spray schedule of protectant fungicides. The initiation of weekly fungicide spray schedules was staggered to identify the critical period. The accumulation of 400 P-Days, environmental conditions favorable for sporulation, spore dissemination and plant infection and airborne spores ofA. solani were detected prior to the critical period each year. Since early blight is a disease of old plants, host susceptibility appeared to be involved in the timing of the critical period in addition to favorable weather and the presence of the pathogen. A measure of potato plant development, and therefore susceptibility toA. solani, is needed to adequately predict the critical period. The accumulation of 400 P-Days may aid in the prediction of the critical period by estimating the time of plant susceptibility and therefore the timing of the first necessary fungicide spray.