Resistance risk assessment of mefentrifluconazole in Corynespora cassiicola and the control of cucumber target spot by a two-way mixture of mefentrifluconazole and prochloraz.

The cucumber target spot, caused by Corynespora cassiicola, is a major cucumber disease in China. Mefentrifluconazole, a new triazole fungicide, exhibits remarkable efficacy in controlling cucumber target spot. However, the resistance risk and mechanism remain unclear. In this study, the inhibitory activity of mefentrifluconazole against 101 C. cassiicola isolates was determined, and the results indicated that the EC50 values ranged between 0.15 and 12.85 µg/mL, with a mean of 4.76 µg/mL. Fourteen mefentrifluconazole-resistant mutants of C. cassiicola were generated from six parental isolates in the laboratory through fungicide adaptation or UV irradiation. The resistance was relatively stable after ten consecutive transfers on a fungicide-free medium. No cross-resistance was observed between mefentrifluconazole and pyraclostrobin, fluopyram, prochloraz, mancozeb, or difenoconazole. Investigations into the biological characteristics of the resistant mutants revealed that six resistant mutants exhibited an enhanced compound fitness index (CFI) compared to the parental isolates, while others displayed a reduced or comparable CFI. The overexpression of CcCYP51A and CcCYP51B was detected in the resistant mutants, regardless of the presence or absence of mefentrifluconazole. Additionally, a two-way mixture of mefentrifluconazole and prochloraz at a concentration of 7:3 demonstrated superior control efficacy against the cucumber target spot, achieving a protection rate of 80%. In conclusion, this study suggests that the risk of C. cassiicola developing resistance to mefentrifluconazole is medium, and the overexpression of CcCYP51A and CcCYP51B might be associated with mefentrifluconazole resistance in C. cassiicola. The mefentrifluconazole and prochloraz two-way mixture presented promising control efficacy against the cucumber target spot.

Resistance to pydiflumetofen in Botrytis cinerea: risk assessment and detection of point mutations in sdh genes that confer resistance.

BACKGROUND: Gray mold caused by Botrytis cinerea Pers. is one of the most significant airborne diseases. It can infest a wide range of crops, causing significant losses in yield and quality worldwide. Pydiflumetofen, a new generation succinate dehydrogenase inhibitor (SDHI), is currently being registered in China to control gray mold in a variety of crops. The baseline sensitivity, resistance risk, and resistance mechanism of Botrytis cinerea to pydiflumetofen were assessed in this study. RESULTS: A total of 138 strains of B. cinerea from 10 different regions were tested for their sensitivity to pydiflumetofen, and the mean EC50 value was 0.0056 µg mL-1 . Eight mutants were obtained by fungicide adaption from five sensitive parental isolates, and the resistance factor (RF) ranged from 51 to 135. The mutants exhibited strong adaptive traits in conidial production, conidial germination, and pathogenicity. Positive cross-resistance was only observed between other SDHIs (i.e. boscalid, fluopyram, and isopyrazam). Two different types of pydiflumetofen-resistant mutants were identified: point mutation P225L in sdhB and double mutation G85A and I93V in sdhC. The in vivo control efficacy of pydiflumetofen on the resistant mutants carrying P225L in sdhB as well as G85A and I93V in sdhC was significantly decreased to 52.62% and 32.27%, respectively. CONCLUSION: The fitness was significantly higher for all pydiflumetofen-resistant mutants than the corresponding parental. Two types of point mutations, sdhB-P225L and sdhC-G85A and I93V, might confer resistance to pydiflumetofen in B. cinerea. A precautionary resistance management strategy should be implemented. © 2021 Society of Chemical Industry.