A survey was conducted for involvement of cyanide-insensitive respiration of phytopathogenic fungi by testing a joint fungitoxic action between salicylhydroxamic acid (SHAM) as an inhibitor of the cyanide-insensitive pathway and the inhibitors of cytochrome pathway of respiratory chain such as methoxyacrylate derivatives or antimycin A. Remarkable synergism in fungitoxic action was shown in Botrytis cinerea, Monilinia fructicola and Pyricularia oryzae. The synergism may result from the induction of the alternative pathway by the latter inhibitors and the inhibitionof the induced alternative pathway by SHAM. To confirm the induction of the alternative pathway, effects of SHAM on oxygen consumption by hyphal cells of three fungi were tested by pretreating with methoxyacrylate or potassium cyanide. These results confirmed the induction of cyanide-insensitive respiration by the pretreatment with methoxyacrylate or potassium cyanide as expected from the joint fungitoxic action. The synergism in fungitoxic action was also observed in Cochliobolus miyabeanus, though it was less remarkable. In Rhizoctonia solani, the synergism was observed with methoxyacrylates but hardly with antimycin A, so that cyanide-insensitive respiration was induced with the methoxyacrylates but hardly with antimycin A in Rhizoctonia solani. In Gibberella fujikuroi, Pythium sp. and Valsa ceratosperma the synergism was not necessarily clear. A phytopathogenic bacterium, Xanthomonas campestris pv. citri and a yeast, Saccharomyces cerevisiae did not show any synergism in joint antimicrobial action of the above inhibitors.

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.

Transmethylation from methionine into choline in the intact mycelia of Pyricularia oryzae and the effects of fungicides on it were observed by 13C NMR. P. oryzae was incubated with [methyl-13C]methionine. After 3 hr incubation, signal at 54.9 ppm, corresponding to the chemical shift of -N-CH3, was observed in the 13C NMR spectrum of the mycelia and its intensity increased with incubation time up to 24 hr. From the resultant mycelial cells, choline and phosphatidylcholine were isolated, and labeled N-methyl group was found in them. The signal intensity of the N-methyl was most remarkable in the water soluble fraction, suggesting that the signal at 54.9 ppm in the spectra of the mycelia was attributable to N-methyl group of choline. The methyltransfer from [methyl-13C]methionine into choline could be observed by 13C NMR. When 100 ppm of IBP was added to the incubation mixture, this methyltransfer was inhibited during 3 to 6 hr. Addition of 40 ppm of isoprothiolane had a similar effect.

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