Metabolism of a phosphoramidate by Pyricularia oryzae in relation to tolerance and synergism by a phosphorothiolate and isoprothiolane

Metabolism of dibutyl N-methyl-N-phenylphosphoramidate (BPA) by mycelial cells of Pyricularia oryzae was studied to elucidate the mechanism of synergism and negatively correlated cross-resistance in fungicidal action between phosphoramidates and phosphorothiolate derivatives. Rapid metabolism of BPA by a wild-type strain through hydroxylation and N-demethylation was observed. The metabolism was inhibited by diisopropyl S-benzyl phosphorothiolate (IBP; Kitazin P) and by isoprothiolane (diisopropyl 1,3-dithiolan-2-ylidenemalonate; Fuji-One). This inhibition of BPA metabolism is probably the mechanism of synergistic fungicidal action between the phosphoramidate and the thiol derivatives. The metabolism was, however, not inhibited by S-1358 (S-butyl S′-p-tert-butylbenzyl N-3-pyridyldithiocarbonimidate; Denmert) or triarimol α-(2,4-dichlorophenyl)-α-phenyl-5-pyrimidinemethanol; EL-273], both of which are considered to be inhibitors of hydroxylation of a methyl radical in ergosterol biosynthesis. The metabolism of BPA by P. oryzae was much slower when mutants selected for IBP resistance and for isoprothiolane resistance were used. This phenomenon probably explains the differential sensitivity to phosphoramidate of wild-type strains and mutants resistant to the thiol derivatives.