Comparative Studies of the in Vitro Activity of the Fungicides Oxadixyl and Metalaxyl

Abstract

The two phenylamide fungicides oxadixyl and metalaxyl were compared in vitro with respect to their activity against a diversity of zoosporic fungi and representative higher fungi. The spectrum of activity of these fungicides, formerly thought to inhibit only members of the Peronosporales, was expanded to include Pythiopsis cymosa in the Saprolegniales and Lagenidium giganteum in the Lagenidiales, both fungi that cannot synthesize their own sterols as is the case with members of the Peronosporales. Zoophagus insidians, a member of the Peronosporales that makes ergosterol, was insensitive to oxadixyl and metalaxyl. The Hyphochytriomycetes showed a sensitivity intermediate between that of sensitive fungi and that of the higher fungi. Different genera and different species among sensitive fungi had varying degrees of sensitivity to oxadixyl and metalaxyl. Sensitivity was also influenced by incubation time, growth medium, and incubation temperature. Stages in the life cycle of Phytophthora palmivora that were sensitive to oxadixyl and metalaxyl included vegetative growth, sporan? gium formation, and sporangium germination. Zoospore behavior, encystment, and cyst germination were not influenced by the two fungicides. Inhibition of germling growth by metalaxyl began less than 90 min after cyst germination. Mitotic divisions occurred at con? centrations ofthe fungicides that gave a 50% inhibition of growth. The results are compared with those of other workers and the significance of the results with respect to control and possible mechanisms of action is discussed.