First Report of QI-insensitive Powdery Mildew (Erysiphe polygoni) on Sugar Beet in the United States.

The $2.1 billion United States sugar beet (Beta vulgaris L.) industry is the primary provider of domestic sucrose. Sugar beet powdery mildew is caused by Erysiphe polygoni DC and occurs principally in sugar beet growing regions in the western United States. In these regions, the quinone outside inhibitor (QI) fungicides pyraclostrobin (Headline, BASF, NC) and trifloxystrobin (Gem, Bayer Crop Science, NC) have been important tools to manage powdery mildew since registration in 2002 and 2005, respectively. However, researchers in Idaho reported poor disease management despite QI application starting in 2011. In 2013, a research plot near Parma, ID, containing natural powdery mildew infection received treatments of pyraclostrobin, trifloxystrobin, or was untreated (control). Since there was no significant reduction in disease levels between QI-treated blocks and untreated control blocks, experiments were conducted to clone a partial fragment of the E. polygoni cytochrome b (cytb) gene to gain insight into the molecular basis of QI resistance in this pathosystem. The primers MDB-920 (5'-CACATCGGAAGAGGTTTATA-3') and MDB-922 (5'-GGTATAGATCTTAATATAGCATAG-3') were designed based on consensus sequences of several fungal cytb genes obtained from GenBank (data not presented) and used to amplify a 575-bp fragment of the E. polygoni cytb gene using DNA isolated from 12 infected leaf samples collected in September 2013 from the Parma research plot. Each sample consisted of three leaves harvested from three plants (one leaf per plant) in an experimental block. All DNA extraction, PCR, and sequencing procedures were as described previously (1). PCR products derived from six QI-treated samples and six untreated samples were sequenced directly. Without exception, all QI-treated samples harbored a point mutation at nucleotide position 143 that encoded a G143A mutation compared with cytb sequence from untreated samples. The two identified cytb haplotypes have been deposited in GenBank under accession numbers KF925325 and KF925326. This is the first report of QI resistance and the associated cytb G143A mutation in E. polygoni. The G143A mutation has been reported in most QI-resistant pathogens to date (2). When the G143A mutation dominates in a pathogen population, there is a consistent association with a loss of disease management despite QI application (3). Careful monitoring and judicious use of QI fungicides will be necessary to ensure QI fungicides remain efficacious for sugar beet powdery mildew management in the United States. References: (1) M. D. Bolton et al. Pest Manag. Sci. 69:35, 2013. (2) N. Fisher and B. Meunier. FEMS Yeast Res. 8:183, 2008. (3) U. Gisi et al. Pest Manag. Sci. 58:859, 2002.