Characterization of Fungicide Resistance in Cercospora beticola Populations from Sugarbeet ( Beta vulgaris ) in Michigan

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Sugarbeets produce over half of domestic sugar in the United States, and Michigan is the fourth-highest U.S. producer. Cercospora beticola causes one of the most impactful foliar diseases in sugarbeet, Cercospora leaf spot (CLS). CLS management relies heavily on timely and repeated fungicide applications, and resistance to several fungicide groups has been observed. In 2021 and 2022, the in vitro fungicide sensitivity of Michigan C. beticola isolates was tested for five demethylation inhibitors (DMIs; difenoconazole, fenbuconazole, mefentrifluconazole, prothioconazole, and tetraconazole), a quinone outside inhibitor (QoI; pyraclostrobin), a methyl benzimidazole carbamate (MBC; thiophanate-methyl), and triphenyltin hydroxide. Cross-resistance was confirmed among most DMIs, with the strongest correlations between difenoconazole and mefentrifluconazole ( r = 0.58, P < 0.0001) and fenbuconazole and prothioconazole or tetraconazole ( r = 0.51 or 0.54, P < 0.0001). Sensitivity to thiophanate-methyl and prothioconazole was affected by the number of MBC and DMI applications, respectively ( P < 0.05). Mutations associated with QoI, MBC, and DMI resistance were evaluated using polymerase chain reaction–restriction fragment length polymorphism. Whereas the E198A mutation was significantly associated with C. beticola responses to thiophanate-methyl ( P < 0.0001), G143A was not as strongly associated with responses to pyraclostrobin, nor was Glu169 strongly associated with responses to prothioconazole or tetraconazole. Overall, in vitro insensitivity (based on k-means cluster analyses) was observed for all tested active ingredients; however, elevated frequencies were observed for thiophanate-methyl (72 to 86%), tetraconazole (28 to 56%), prothioconazole (78 to 92%), and pyraclostrobin (9 to 33%). Identifying more explanatory resistance mutations for DMI and QoI fungicides in C. beticola populations would improve timely monitoring efforts. [Formula: see text]