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

Management of rice bakanae disease (RBD), 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 (EC 50 = 0.0623 ± 0.0108 μg/mL), suggesting that sensitive population is relatively concentrated. To assess the risk of resistance to Pyd in F. fukikoroi, two high resistance mutants (Pyd HR ), six moderate resistant mutants (Pyd MR ), and two low resistance mutants (Pyd LR ) were generated by fungicide mutagenesis. The Pyd HR mutants exhibited similarities in the 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 all exhibited significant up-regulation change, FfSdhC2 was dramatically upregulated by nearly 200-fold. These results indicate that point mutations and FfSdh genes up-regulation expression may be codetermine the Pyd resistance mechanisms in F. fujikuroi. 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.