MUTUAL ANTAGONISM BETWEEN STEROL DEMETHYLATION INHIBITORS AND PHOSPHOROTHIOLATE FUNGICIDES ON PYRICULARIA-ORYZAE AND THE IMPLICATIONS FOR THEIR MODE OF ACTION

Joint action between a phosphorothiolate (PTL) fungicide, iprobenfos, and a sterol demethylation inhibitor (DMI), pefurazoate, was tested by crossed paper technique on three types of field isolates of Pyricularia oryzae Cavara that differed in PTL sensitivity and metabolism. Mutual antagonism in anti-fungal action between iprobenfos and pefurazoate was observed in a wild-type field isolate of the fungus sensitive to PTL and in an isolate moderately resistant to PTL, but not in a PTL-resistant isolate lacking the ability to metabolize PTL. Antagonism of the antifungal action of iprobenfos by pefurazoate seemed to be a result of inhibition of activation by cleavage of the P-S bond of iprobenfos mediated by mixed-function oxygenase (mfo) activity, while antagonism of the anti-fungal action of pefurazoate by iprobenfos may be caused by the binding of pefurazoate by large amounts of an iprobenfos-induced mfo which results in reduced inhibition of ergosterol biosynthesis. In the PTL-resistant isolate, the mutually antagonistic action was not observed, presumably because the induction of the mfo-metabolizing iprobenfos was lacking. Similar antagonism was also observed when another PTL, edifenphos, was used instead of iprobenfos, and when other DMIs, propiconazole, prochloraz and hexaconazole were used instead of pefurazoate. The results of the present experiment indicate that DMIs may also bind to and inhibit an inducible type of fungal mfo which metabolizes xenobiotics, and that PTLs may be activated by an mfo prior to their anti-fungal action.