First Report of Reduced Sensitivity to a QoI Fungicide in Isolates of Alternaria solani Causing Early Blight of Potato in Canada.

Early blight of potato (Solanum tuberosum L.) caused by Alternaria solani Sorauer is a frequent concern for potato growers in Canada. Management of early blight has relied on foliar fungicides that often include quinone outside inhibitor (QoI) fungicides such as azoxystrobin. In recent years, isolates of A. solani with reduced sensitivity to QoI fungicides, conferred by the presence of the F129L mutation (in the cytochrome b gene causing amino acid substitution of phenylalanine with leucine at position 129), have become widespread in potato-production areas of the United States, leading to a reduced efficacy of these products (3). Observations of reduced fungicide efficacy, following application of QoI fungicides to commercial fields in Manitoba, Canada in 2007, prompted an examination of the fungicide sensitivity of isolates of A. solani collected from fields in this province. Nine isolates of A. solani were obtained from potato foliage with typical early blight symptoms from four fields in Manitoba using standard protocols (2). Isolates were maintained on clarified V8 agar (1) and identified to species level based on conidial morphology (4). The sensitivity of each isolate to azoxystrobin was determined by assessing conidial germination on water agar plates amended with 0, 0.001, 0.01, 0.1, 1.0, or 10.0 mg/liter of azoxystrobin with protocols described previously (1). Two reference isolates of A. solani from North Dakota with known sensitivities to azoxystrobin and one isolate from Prince Edward Island (PEI), Canada, (a province yielding only isolates sensitive to azoxystrobin in previous surveys; R. D. Peters, unpublished data) were included in the assays. Calculated effective concentration (EC₅₀) values (azoxystrobin concentration inhibiting conidial germination by 50%) were determined for each isolate response from two replications of the assays. The reference isolates of A. solani from North Dakota were sensitive or had reduced sensitivity to azoxystrobin with mean EC₅₀ values of 0.02 and 0.2 mg/liter, respectively. The isolate from PEI was sensitive to azoxystrobin with a mean EC₅₀ value of 0.04 mg/liter. By contrast, isolates of A. solani from Manitoba had reduced sensitivity to azoxystrobin with mean EC₅₀ values from 0.2 to 0.8 mg/liter. Real-time PCR analysis of each isolate was performed (2) and confirmed the presence of the F129L mutation in the Manitoba isolates and the isolate with reduced sensitivity to azoxystrobin from North Dakota. The F129L mutation was absent in the azoxystrobin-sensitive wild-type isolates from PEI and North Dakota. To our knowledge, this is the first report of isolates of A. solani with reduced sensitivity to azoxystrobin in Canada. Since cross resistance among QoI fungicides has been demonstrated in A. solani isolates with the F129L mutation (3), adoption of resistance management strategies, including alternating QoI fungicides with fungicides having different modes of action and further monitoring pathogen populations for QoI sensitivity in Canadian production areas, is recommended. References: (1) J. S. Pasche et al. Plant Dis. 88:181, 2004. (2) J. S. Pasche et al. Plant Dis. 89:269, 2005. (3) J. S. Pasche and N. C. Gudmestad. Crop Prot. 27:427, 2008. (4) J. Rotem. The Genus Alternaria: Biology, Epidemiology, and Pathogenicity. The American Phytopathological Society, St. Paul, MN, 1994.