Survival Cost and Diverse Molecular Mechanisms of Magnaporthe oryzae Isolate Resistance to Epoxiconazole

Rice blast caused by Magnaporthe oryzae is one of the most destructive diseases on rice worldwide. Epoxiconazole is a 14α-demethylation inhibitor (DMI) with excellent control on rice blast; to date, no resistant isolates have been observed in the field. Four mutants resistant to epoxiconazole were generated from three parental isolates via fungicide adaptation. Resistance was stable after 10 weekly consecutive transfers on fungicide-free medium. Three parameters, including growth rate, sporulation in vitro, and aggressiveness, were significantly lower for mutants compared with their parental isolates, with the exception of the low-resistance isolate. Sporulation and aggressiveness were negatively correlated with effective concentration values for 50% inhibition of mycelial growth for parental isolates and mutants (P < 0.05). Cross-resistance was found between epoxiconazole and prochloraz (ρ = 0.863, P = 0.000) or difenoconazole (ρ = 0.861, P = 0.000). The resistance factor for mutants was positively correlated with the relative expression of MoCYP51A in epoxiconazole treatment (r = 0.977, P = 0.02). In addition, two putative amino acid substitutions in MoCYP51A were found in two resistant mutants: Y126F in the high-resistance mutant and I125L in the low-resistance mutant. Mutation Y126F reduced the affinity of MoCYP51A with epoxiconazole, whereas I125L was not in the binding pocket of epoxiconazole. No amino acid change or overexpression in MoCYP51B was found in any of the mutants studied. To our knowledge, this is the first study to report DMI resistance observed in M. oryzae. The survival cost of M. oryzae resistance to epoxiconazole might be the reason why DMI resistance has not yet emerged in field populations worldwide.