Biological and Molecular Characterizations of Pydiflumetofen-Resistant Isolates in Fusarium fujikuroi

Management of rice bakanae disease, caused by Fusarium fujikuroi, faces significant challenges arising from the development of fungicide resistance in the pathogen. Pydiflumetofen (Pyd) is a novel succinate dehydrogenase inhibitor (SDHI) and might offer potential in alleviating this crisis. In this study, the establishment of a sensitivity baseline revealed a unimodal distribution (EC₅₀ = 0.0623 ± 0.0108 μg/ml), suggesting that the sensitive population is relatively concentrated. To assess the risk of resistance to Pyd in F. fujikuroi, two high-resistance mutants (Pyd^HR), six moderate-resistance mutants (Pyd^MR), and two low-resistance mutants (Pyd^LR) were generated by fungicide mutagenesis. The Pyd^HR mutants exhibited similarities in mycelial growth rate, sporulation capacity, germination rate of conidium, various osmotic stresses, and virulence, indicating that the Pyd resistant (PydR) mutants might form subpopulations with competitive advantages in the field. Analysis of nucleotide sequences of succinate dehydrogenase subunit-coding genes revealed three types of mutations in PydR isolates: FfSdhB^H248Y + FfSdhC2^A83V, FfSdhC2^A83V, and FfSdhD^D133G. The expression levels of FfSdhA/B/C1/C2/D in resistant double mutations RSS24-13 (FfSdhB^H248Y and FfSdhC2^A83V) all exhibited significant upregulation change; FfSdhC2 was dramatically upregulated by nearly 200-fold. These results indicate that point mutations and FfSdh gene upregulation expression may codetermine the Pyd resistance mechanisms in F. fujikuroi. Cross-resistance analysis revealed that the PydR isolates also exhibited resistance to SDHIs isofetamid and fluopyram, but there was no cross-resistance between benzovindiflupyr and fungicides from other classes. These results indicate that the resistance risk of Pyd is elevated to medium-high, and reasonable monitoring and management of resistance to Pyd should be carried out in F. fujikuroi.