Within-Season Changes in Genetic Diversity and Population Structure of Cercospora beticola Associated with Repeated Fungicide Exposure

Cercospora leaf spot, caused by Cercospora beticola , is a devastating foliar disease affecting sugar beet worldwide. Effective disease management strategies often rely on several applications of demethylation inhibitor (DMI) fungicides combined with broad-spectrum tank-mix partners. However, the relationship between short-term fungicide exposure and the genetic composition of C. beticola populations remains poorly understood. In this study, simple sequence repeat (SSR) markers were used to evaluate changes in genetic diversity and structure of C. beticola populations associated with repeated exposure to DMI fungicides, with and without broad-spectrum tank-mix partners, within a single growing season. Among 237 isolates collected before fungicide exposure and 497 isolates collected after, 59 and 120 multilocus genotypes (MLGs) were identified, respectively. Changes in the relative abundance of standing genetic variation and DMI-associated genetic differentiation were observed. Isolates exposed to prothioconazole and tetraconazole grouped together, and a similar pattern was observed for mefentrifluconazole and difenoconazole. Several predominant MLGs had sensitivity profiles corresponding with the treatments in which they were more abundant. Within the representative subset evaluated for CbCyp51 variation, haplotype composition differed among nine discriminant analysis of principal component clusters. Exposure to broad-spectrum tank-mix partners did not result in significant genetic differentiation. These findings provide novel insight into short-term changes in MLG frequencies and population structure associated with repeated fungicide exposure in C. beticola . The observed patterns among DMI active ingredients, with and without broad-spectrum tank-mix partners, provide a foundation for further evaluation of fungicide rotation and mixture strategies for sustainable CLS management in sugar beet.