Qin Peng,Xinchang Hao,Chunyue Liu,Xiuhuan Li,Xingxing Lu,Xili Liu
Florylpicoxamid, a QiI fungicide, demonstrates broad-spectrum activity against a wide range of phytopathogenic organisms belonging to the phyla Basidiomycota and Ascomycota. Nevertheless, the potential for resistance and the underlying resistance mechanisms of Corynespora cassiicola against florylpicoxamid are still not fully understood. We determined the baseline sensitivity levels of 101 C. cassiicola isolates to florylpicoxamid. The EC 50 values varied from 0.01 to 1.18 μg/mL with an average of 0.50 μg/mL. Laboratory-induced fungicide adaptation of nine wild-type isolates generated seven C. cassiicola mutants exhibiting high level of resistance to florylpicoxamid, all originating from a single parental isolate. The mutants maintained their resistance even after undergoing ten successive cultivations on a medium devoid of fungicides. No cross-resistance was detected between florylpicoxamid and pyraclostrobin, fluopyram, prochloraz, or propineb. Five of the resistant mutants showed an improved compound fitness index (CFI) relative to their parental isolate, whereas the remainder displayed either a reduced or comparable CFI. All seven of the resistant mutants displayed an A37V substitution within the CcCytb protein, which was responsible for the resistance to florylpicoxamid, as validated through molecular docking analysis. Furthermore, an allele-specific PCR (AS-PCR) method for detecting the CcCytb A37V mutation was successfully established. In summarize, the findings of this study indicate a moderate risk of C. cassiicola developing resistance to florylpicoxamid, with the A37V substitution in CcCytb playing a key role in this resistance, detectable through the use of specific AS-PCR primers.
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.
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