Temporal sensitivity of Alternaria solani to foliar fungicides

Alternaria solani Sorauer (Ellis) is a causal agent of early blight of potato (Solanum tuberosum L.). Sensitivity to chlorothalonil, triphenyl tin hydroxide (TPTH), and mancozeb was determined for field isolates of A. solani in 1998 and 1999. Specifically, the relative sensitivity of A. solani to foliar fungicides in field populations collected from several commercial fields was evaluated over the course of two growing seasons. Sensitivity to these foliar fungicides was determined with an in vitro spore-germination assay method. The fungicide concentration that inhibited conidial germination by 50% (EC50) was estimated for each isolate. Sensitivity of A. solani isolates to TPTH and mancozeb varied little during the growing season, possibly because there were few TPTH and mancozeb, applications made to the fields studied. At several locations, however, repeated exposure of A. solani populations to chlorothalonil resulted in considerable variability in sensitivity, frequently causing isolates to have decreased sensitivity to this fungicide at the end of the growing season. In five of seven fields, isolates of A. solani collected at the end of the season were significantly less sensitive to chlorothalonil than isolates collected at the beginning of the season.