Sensitivity toBoscalid and Trifloxystrobin and Fitnessof Corynespora cassiicola Associatedwith <i>CcSdh</i> & <i>CcCytb</i> in Cucumber

The extensive use of succinate dehydrogenase inhibitors (SDHIs) and quinone outside inhibitors (QoIs) has led to the development of resistance in Corynespora cassiicola, which causes corynespora leaf spot (CLS) in cucumber. In this study, the sensitivity and the resistance phenotypes to SDHIs and QoIs were monitored, and the fitness of C. cassiicola carrying <i>CcSdh</i> & <i>CcCytb</i> mutations was measured. The average 50% effective suppressive concentration (EC₅₀) values of C. cassiicola to boscalid from 2019 to 2021 ranged from 6.65 to 24.16 μg/mL, while the average EC₅₀ values of C. cassiicola to trifloxystrobin from 2007 to 2021 ranged from 105.11 to 462.02 μg/mL. Ten fungicide resistance phenotypes and 19 resistance genotypes were found. The D-D95E&Cytb-G143A and B–H278L&Cytb-G143A mutants presented increased fitness, while the B–H278Y&Cytb-G143A mutant showed significant fitness costs, and the other mutants presented no or slight fitness costs. These findings provide a basis for the formulation of resistance management strategies of CLS.