MBI-D resistance management of Pyricularia oryzae using an application program incorporating benomyl

Various amino acid substitutions in succinate dehydrogenase complex regulating differential resistance to pydiflumetofen in Magnaporthe oryzae

Rice blast, caused by Magnaporthe oryzae, is a devastating fungal disease worldwide. Pydiflumetofen (Pyd) is a new succinate dehydrogenase inhibitor (SDHI) that exhibited anti-fungal activity against M. oryzae. However, control of rice blast by Pyd and risk of resistance to Pyd are not well studied in this pathogen. The baseline sensitivity of 109 M. oryzae strains to Pyd was determined using mycelial growth rate assay, with EC50 values ranging from 0.291 to 2.1313 μg/mL, and an average EC50 value of 1.1005 ± 0.3727 μg/mL. Totally 28 Pyd-resistant (PydR) mutants with 15 genotypes of point mutations in succinate dehydrogenase (SDH) complex were obtained, and the resistance level could be divided into three categories of very high resistance (VHR), high resistance (HR) and moderate resistance (MR) with the resistance factors (RFs) of >1000, 105.74-986.13 and 81.92-99.48, respectively. Molecular docking revealed that all 15 mutations decreased the binding-force score for the affinity between Pyd and target subunits, which further confirmed that these 15 genotypes of point mutations were responsible for the resistance to Pyd in M. oryzae. There was positive cross resistance between Pyd and other SDHIs, such as fluxapyroxad, penflufen or carboxin, while there was no cross-resistance between Pyd and carbendazim, prochloraz or azoxystrobin in M. oryzae, however, PydR mutants with SdhBP198Q, SdhCL66F or SdhCL66R genotype were still sensitive to the other 3 SDHIs, indicating lack of cross resistance. The results of fitness study revealed that the point mutations in MoSdhB/C/D genes might reduce the hyphae growth and sporulation, but could improve the pathogenicity in M. oryzae. Taken together, the risk of resistance to Pyd might be moderate to high, and it should be used as tank-mixtures with other classes of fungicides to delay resistance development when it is used for the control of rice blast in the field.

Management of melanin biosynthesis dehydratase inhibitor (MBI-D)-resistance in Pyricularia oryzae using a non-MBI-D fungicidal application program for nursery boxes and a diclocymet and ferimzone mixture for field foliar applications

We compared the risk of reselecting melanin biosynthesis dehydratase inhibitor (MBI-D)-resistant Pyricularia oryzae isolates between two treatment programs, a nursery box application of diclocymet and a rotational program in which nursery boxes received a non-MBI-D fungicide application and then later, in the field, a foliar application of a diclocymet and ferimzone mixture. Both were effective against panicle blast. However, the latter prevented the reselection of MBI-D-resistant isolates more effectively than the former.