Suppression of metalaxyl activity by glyphosate: evidence that host defence mechanisms contribute to metalaxyl inhibition of Phytophthora megasperma f. sp. glycinea in soybeans

Etiolated soybean hypocotyls (cultivar Altona) treated with the systemic fungicide metalaxyl [N-(2,6-dimethylphenyl)-N-(methyl-acetyl) alanine methyl ester] developed restricted brown lesions with high levels of the phytoalexin, glyceollin, when inoculated with zoospores of Phytophthora megasperma f. sp. glycinea (race 6, compatible). These responses were very similar to those in interactions of untreated hypocotyls with incompatible races. When hypocotyls were supplied with a mixture of metalaxyl, at marginally inhibitory concentrations, and glyphosate (N-phosphonomethylglycine), an inhibitor of the shikimic acid pathway, the effectiveness of the metalaxyl treatment was reduced, the lesions spread and glyceollin concentrations were lower. The effectiveness of metalaxyl added directly to the inoculum drop was also less in glyphosphate-treated hypocotyls than in control hypocotyls. Glyphosate did not affect the fungitoxicity of metalaxyl in in vitro growth assays. It is concluded that at marginally inhibitory concentrations host defence mechanisms contribute to metalaxyl activity. This is most probably due to glyceollin accumulation, but as levels frequently remained above ED₉₀ values in spreading lesions and other aspects of host metabolism are known to be influenced by glyphosate, alternative explanations are possible. The possibility that the stimulation of host defence responses in metalaxyl-treated tissue may be due to inhibition of pathogen activities that suppress such responses in untreated tissues is discussed.