Evaluation of QoI Fungicide Application Strategies for Managing Fungicide Resistance and Potato Early Blight Epidemics in Wisconsin.

Potato early blight (Alternaria solani) is a yield-limiting disease and control depends primarily on multiple fungicide applications. Azoxystrobin, registered in the United States in 1999, initially provided outstanding early blight control. Within 3 years, approximately 80% of the total potato acreage was being treated with azoxystrobin and other quinone outside inhibitor (QoI), fungicides registered subsequently. Alternaria solani isolates with decreased in vitro sensitivity to azoxystrobin were detected in Wisconsin during 2001. Field experiments were conducted in 2001 to 2003 to evaluate season-long fungicide programs and test fungicide resistance management strategies. The fungicide program recommended to growers at that time, which consisted of three applications of azoxystrobin for weeks 1, 3, and 5 alternated with applications of chlorothalonil at label recommended rates, was effective in controlling early blight when conditions were conducive to disease development. Mean sensitivity in vitro of A. solani isolates from fungicide efficacy field experiments in 2001 to 2003 was numerically highest for isolates from the untreated control plots, chlorothalonil-alone plots, or plots treated with three applications of azoxystrobin alternated with chlorothalonil compared with other treatments tested. Three single-nucleotide polymorphisms (SNPs) can cause the F129L substitution (TTC to TTA, CTC, or TTG) that results in decreased sensitivity to azoxystrobin of A. solani. The TTA mutant was the most frequently recovered mutant type in the field experiments. The frequency of recovery of wild-type isolates in experiments was 22% in 2001, 4% in 2002, and 22% in 2003.