QoI Resistance and Phenotypic Variability in Cercospora beticola Isolates from Sugar Beet in the Russian Federation

Sugar beet cercospora leaf spot (CLS), caused by the fungus Cercospora beticola, is among the most economically important diseases of sugar beet, and the effectiveness of chemical disease management depends on the susceptibility of pathogen populations to fungicides. In this study, isolates of C. beticola from the Russian Federation were characterized based on QoI resistance, molecular detection of the G143A mutation, and phenotypic variability in radial growth, colony morphology, and aggressiveness. A total of 46 leaf samples were surveyed, and 196 isolates were obtained, from which a representative subset of 48 isolates was selected for detailed analysis. The selected isolates were characterized for in vitro radial growth and colony morphology, assessed for aggressiveness on sugar beet leaves, and tested for sensitivity to azoxystrobin based on EC50 values. The G143A mutation was detected by allele-discriminating real-time PCR and confirmed by sequencing of the cytB region. The G143A mutation was identified in 41 of 48 isolates (85.4%). In the mycelial bioassay, all isolates had EC50 values above 0.2 µg/mL, and 38 of 48 isolates (79.2%) had EC50 values exceeding 100 µg/mL. Additional validation with SHAM in a subset of 35 isolates did not alter the qualitative interpretation of resistance. Considerable variability was also observed in radial growth rate, aggressiveness, and colony appearance among isolates. These findings indicate widespread QoI resistance in the analyzed Russian isolate collection and provide a structured baseline for regional resistance monitoring and for more cautious use of FRAC 11 fungicides in integrated sugar beet disease-management programs.