Evidence of a potential adaptation of Magnaporthe oryzae for increased phosphorothiolate-fungicide resistance on rice

In this study, we evaluated and compared the virulence of Magnaporthe oryzae isolates sensitive and resistant to the phosphorothiolate fungicides, edifenphos and iprobenfos on rice plants treated with or without fungicides. Monoconidial isolates of M. oryzae sensitive and resistant to the fungicides were selected from our previous relative mycelial growth assay of 1080 isolates on fungicide-amended media. When six-leaf stage rice plants treated with different concentrations of the fungicides were inoculated with four resistant and four sensitive isolates of M. oryzae , the lowest concentrations showing virulence differences were twice the minimum inhibitory concentrations (MICs) of edifenphos (40 μg a.i. ml −1 ) and iprobenfos (110 μg a.i. ml −1 ). Therefore, two MICs were used as discriminatory concentrations for further virulence tests of 20 sensitive and 20 resistant isolates of M. oryzae . Isolates resistant to edifenphos produced greater disease severity and susceptible lesion number per leaf area (SLNLA) than sensitive isolates on rice plants regardless of treatments. Reduction of disease severity and SLNLA was greater for sensitive than resistant isolates on edifenphos-treated plants compared to untreated controls. Similar results were obtained with iprobenfos, but disease severity or SLNLA of resistant isolates were not affected by iprobenfos compared to untreated controls. In this study, it was evident that resistant isolates of M. oryzae were more virulent than sensitive isolates on rice plants and were less affected by the phosphorothiolate fungicides. Therefore, rice blast management programs should consider a potential adaptation in M. oryzae populations for increased phosphorothiolate-fungicide resistance.