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Cercospora leaf spot (CLS), caused by the fungal pathogen Cercospora beticola, is the most important foliar disease of sugarbeet. The disease is managed in part by timely applications of quinone outside inhibitor (QoI) fungicides. However, pathogen resistance to QoI fungicides is associated with the exchange of glycine to alanine at amino acid position 143 (G143A) in the C. beticola cytochrome b gene. To assess whether QoI resistance has developed in C. beticola in the Red River Valley (RRV) of Minnesota and North Dakota, a real-time PCR procedure was used to determine whether the G143A mutation could be identified in samples harvested from 922 fields across the RRV. In total, 12 fields located in diverse locations within the RRV contained the G143A mutation, suggesting that QoI resistance arose independently at each location and in several genetic backgrounds. This is the first report of QoI resistance in the RRV. Careful monitoring of the G143A mutation will be necessary to ensure that QoI fungicides remain efficacious for CLS management in the RRV region. Accepted for publication 31 May 2013. Published 12 August 2013.

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