Cyanide-insensitive respiration of phytopathogenic fungi demonstrated by antifungal joint action of respiration inhibitors

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 inhibition of 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 necessary 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.