Antifungal spectrum of cyclobutrifluram and multi-point mutations in CcSdh proteins confer resistance in Corynespora cassiicola

Cucumber target spot, a major disease that threatens cucumber production, is caused by Corynespora cassiicola. Cyclobutrifluram, a novel succinate dehydrogenase inhibitor (SDHI) developed by Syngenta, has demonstrated strong inhibitory activity against various plant pathogenic fungi and nematodes. However, its antifungal spectrum, resistance risk as well as underlying mechanisms of resistance in C. cassiicola remain poorly understood. In this study, cyclobutrifluram exhibited potent inhibitory activity against anamorphic fungi and selected ascomycetes, with the mean sensitivity of C. cassiicola isolates to the fungicide being 0.98 ± 1.26 μg/mL. Additionally, five laboratory-derived cyclobutrifluram-resistant mutants showed comparable or lower biological fitness than their respective parental isolates. The resistant mutants and field isolates were also found to possess nine distinct point mutations in the CcSdhB, CcSdhC or CcSdhD genes. Finally, cyclobutrifluram exhibited positive cross-resistance with other SDHIs, with the resistance levels varying depending on the specific mutations present. In conclusion, cyclobutrifluram was found to be effective against anamorphic fungi and selected ascomycetes. C. cassiicola's risk of resistance development to cyclobutrifluram was assessed as moderate to high and was primarily associated with mutations in CcSdh genes.