Investigating the potential mechanism of pydiflumetofen resistance in Sclerotinia sclerotiorum.

Abstract

The necrotrophic pathogen Sclerotinia sclerotiorum is one of the most damaging and economically important plant pathogens. Pydiflumetofen, which was recently developed by Syngenta Crop Protection, has already been registered in China for the management of Sclerotinia stem rot (SSR) which was caused by S. sclerotiorum in oilseed rape. In an attempt to preempt and forestall the development of resistance to this useful fungicide, the current study was initiated to investigate potential mechanism of resistance in laboratory mutants. Five pydiflumetofen-resistant S. sclerotiorum mutants were successfully generated by repeated exposure to the fungicide under laboratory conditions. Although the mutants had greatly reduced sensitivity to pydiflumetofen, they were also found to have significantly (p < 0.05) reduced fitness exhibiting reduced mycelial growth and sclerotia formation on PDA medium. However, three of the four mutants had significantly (p < 0.05) increased pathogenicity on detached soybean leaves compared to their respective parental isolates, indicating a moderate to high level of fungicides resistance risk according to the criteria of Fungicide Resistance Action Committee (FRAC). Sequence analysis of four succinate dehydrogenase (Sdh) target genes identified several nucleotide changes in the sequences of the pydiflumetofen-resistant mutants, most of which were synonymous and caused no changes to the predicted amino acid sequences. However, all of the pydiflumetofen-resistant mutants had two amino acid point mutations (A11V and V162A) in their predicted SsSdhB sequence. No similar changes were found in the SsSdhA, SsSdhC, and SsSdhD genes of any of the mutants tested. In addition, there was a positive cross-resistance between pydiflumetofen and boscalid, and no cross-resistance between pydiflumetofen and other commonly used fungicides, including tebuconazole, fludioxonil, cyprodinil, dimethachlone, prochloraz, pyraclostrobin, fluazinam, procymidone, and carbendazim. These results indicate that pydiflumetofen has great potential as an alternative fungicide for the control of S. sclerotiorum, especially where resistance to other fungicides has already emerged. Mixing or alternate application with fludioxonil, prochloraz, and fluazinam could be used to limit the risk of resistance to pydiflumetofen.