Prevalence, competitive fitness and impact of the F129L mutation in Alternaria solani from the United States

Reduced sensitivity to quinone outside inhibitor (QoI) fungicides conferred by the presence of the F129L mutation was identified in Alternaria solani isolates collected in Nebraska in 2000 and in isolates collected from the Midwestern states of Minnesota and North Dakota in 2001. Over 4200 isolates of A. solani collected over a 5-year period from 2002 to 2006 from 11 potato-producing states were evaluated for the presence of the F129L mutation utilizing azoxystrobin spore germination assays and real-time PCR. From this population, 96.5% of isolates were determined to have reduced sensitivity to QoI fungicides and/or to contain the F129L mutation. In addition to Midwestern states of Minnesota, Nebraska and North Dakota, A. solani isolates containing the F129L mutation were collected from Colorado, Michigan and Texas in 2003, Wisconsin in 2005, as well as from the Western United States including Idaho, Washington and Wyoming in 2005 and Oregon in 2006. The detection of these isolates in areas outside the Midwest suggests that the F129L mutation is stable and present under conditions that are less conducive for the pathogen and where it is under less selection pressure by QoI fungicides. Competitive fitness tests confirm that, while isolates of A. solani with the F129L mutation had a decrease in in vitro percentage germination compared with wild-type isolates, F129L mutant isolates produced higher disease severity in greenhouse trials than wild-type isolates. Field trials were performed in central Minnesota and central North Dakota in 2000 and 2001 when the A. solani population was dominated by wild-type isolates as well as in 2002, 2003, 2005 and 2006 when F129L mutant isolates dominated. These trials support in vitro and greenhouse results indicating that the F129L mutation has affected the field performance of all QoI fungicides. Field trial results suggest that these fungicides do not provide improved disease control over standard protectant fungicides such as chlorothalonil and mancozeb. (C) 2007 Elsevier Ltd. All rights reserved.