Baseline Pydiflumetofen Sensitivity of Fusarium pseudograminearum Isolates Collected from Henan, China, and Potential Resistance Mechanisms.

Fusarium crown rot (FCR), caused by Fusarium pseudograminearum, is one of the most important diseases impacting wheat production in the Huanghuai region, the most important wheat-growing region of China. The current study found that the SDHI fungicide pydiflumetofen, which was recently developed by Syngenta Crop Protection, provided effective control of 67 wild-type F. pseudograminearum isolates in potato dextrose agar, with an average EC50 value of 0.060 ± 0.0098 µg/ml (SE). Further investigation revealed that the risk of fungicide resistance in pydiflumetofen was medium to high. Four F. pseudograminearum mutants generated by repeated exposure to pydiflumetofen under laboratory conditions indicated that pydiflumetofen resistance was associated with fitness penalties. Mutants exhibited significantly (P < 0.05) reduced sporulation in mung bean broth and significantly (P < 0.05) reduced pathogenicity in wheat seedlings. Sequence analysis indicated that the observed pydiflumetofen resistance of the mutants was likely associated with amino acid changes in the different subunits of the succinate dehydrogenase target protein, including R18L and V160M substitutions in the FpSdhA sequence; D69V, D147G, and C257R in FpSdhB; and W78R in FpSdhC. This study found no evidence of cross-resistance between pydiflumetofen and the alternative fungicides tebuconazole, fludioxonil, carbendazim, or fluazinam, which all have distinct modes of action and could therefore be used in combination or rotation with pydiflumetofen to reduce the risk of resistance emerging in the field. Taken together, these results indicate that pydiflumetofen has potential as a novel fungicide for the control of FCR caused by F. pseudograminearum and could therefore be of great significance in ensuring high and stable wheat yields in China.

Baseline Sensitivity and Potential Resistance Mechanisms for Fusarium pseudograminearum to Fludioxonil.

Fusarium crown rot (FCR), which is caused by Fusarium pseudograminearum, is one of the most important diseases affecting wheat production in the Huanghuai wheat-growing region of China. Although the phenylpyrrole fungicide fludioxonil is known to have a broad-spectrum activity against a wide range of plant pathogens, including F. pseudograminearum, it has not yet been registered for the control of FCR in China, and further research is needed to assess the biological characteristics and molecular mechanisms associated with fludioxonil resistance, and especially the potential for highly resistant isolates to emerge. The current study demonstrated that the baseline fludioxonil sensitivity of 61 F. pseudograminearum isolates collected from the Henan province of China during the summers of 2019 to 2021 conformed to a unimodal distribution with a mean effective concentration for 50% inhibition (EC50) value of 0.021 ± 0.003 µg/ml, which indicated that none of the isolates exhibited natural resistance to fludioxonil. Nevertheless, four fludioxonil-resistant mutants were attained after repeated exposure to fludioxonil under laboratory conditions. All resistant mutants exhibited significantly lower growth rates on potato dextrose agar (PDA) and lower levels of sporulation and pathogenicity in wheat seedlings. In addition, the resistant mutants also exhibited less growth on PDA amended with either 0.5 M mannitol, 0.5 M glucose, 0.5 M MgCl2, or 0.5 M NaCl, which indicated that they had greater sensitivity to osmotic stress. Molecular analysis of the proposed fludioxonil target protein FpOs1 indicated that the predicted sequences of the resistant mutants contained none of the characteristic amino acid changes previously associated with fludioxonil resistance in other species. Further investigation via quantitative real-time PCR analysis revealed that expression of the FpOs1 gene was significantly altered in the resistant mutants in both the absence and presence of fludioxonil. Meanwhile, plate assays found evidence of cross-resistance between fludioxonil and cyprodinil, as well as with the triazole fungicides tebuconazole and difenoconazole, but not with other commonly used fungicides including prochloraz, fluazinam, and carbendazim. Taken together, these results provide new insights into the mechanism and biological characteristics associated with fludioxonil resistance in F. pseudograminearum and indicate that fludioxonil could provide effective and sustained control of FCR during wheat production.