Antifungal activity of metalaxyl and furalaxyl

The systemic acylalanines metalaxyl and furalaxyl proved in vitro highly selective to fungi of the order of the Peronosporales, the former being somewhat more active than the latter. Various other fungal, and bacterial, species did not show sensitivity to the toxicants. Metalaxyl was strikingly more active on Pythium splendens and Phytophthora cinnamomi at low than at high temperature. With Phytophthora species metalaxyl displayed improved activity in combination with sodium dodecylsulphate and Triton X-100. Neither the composition of the medium nor its pH influenced activity. None of many compounds tested could antagonize their effect. The fungitoxicity caused by metalaxyl in the gas phase was strikingly better than that of furalaxyl against P. splendens. The toxicants had little effect on germination of the hyphal swellings of P. splendens, but had significant influence on the subsequent hyphal development. This included reduced growth, abnormal variation in the fungal thickness, and inhibition of the formation of hyphal swellings. Metalaxyl and furalaxyl were fungistatic at minimal inhibitory concentration (MIC) values during the first 24 hr. Thereafter, the toxicants became fungicidal. Metalaxyl and furalaxyl did not inhibit respiration of P. splendens at MIC values or somewhat higher concentrations. Neither did metalaxyl inhibit the ¹⁴C]carbon dioxide evolution from externally supplied radiolabeled glucose.