Back-mutation of an organophosphorus thiolate-resistant mutant of Pyricularia oryzae

Abstract

From a large number of conidia of a mutant of Pyricularia oryzae resistant to organophosphorus thiolate (PTL) fungicides (mutant I), a new mutant was selected on agar plates containing a phosphoramidate (PA). The latter compound was found to be more fungicidal to mutant I than to the wild-type strain. The PA-resistant mutant obtained by this selection (mutant II) was as sensitive as the wild-type strain to PTL fungicides. A synergistic joint action of PTL and PA, as shown against the wild-type strain but no against mutant I, was also shown against mutant II. The wild-type strain and mutant I and II were compared regarding their metabolism of PTL and PA compounds. Marked differences were hardly found among the three strains when they were incubated with low dosages of either PTL or PA. At higher dosages of either PTL or PA which affected the three strains differentially, the wild-type strain as well as mutant II metabolized PTL and PA far more rapidly than mutant I did. Therefore, in the present expriments mutant II was similar to the wild-type strain with regard to sensitivity to PTL and PA and to their joint action, and with respect to ability to metabolize these compounds.