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Effect of Soilborne Plant-Pathogenic Fungi on the Herbicidal Action of Glyphosate on Bean Seedlings. Gurmukh S. Johal, Graduate student, Pestology Centre, Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6; James E. Rahe, associate professor, Pestology Centre, Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6. Phytopathology 74:950-955. Accepted for publication 9 March 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-950.

The apparent herbicidal action of glyphosate can be separated from its growth-inhibitory action by the use of vermiculite or sterilized soil as a growth medium. The results suggest that glyphosate-linked death of bean plants in unsterilized soil was actually due to their parasitization by fungal root-rot pathogens in the growth medium. This effect occurred at treatment levels several-fold lower than those required to cause death in the absence of fungal root-rot pathogens. Although bean seedlings grown in the absence of soil-associated root pathogens did not die, they showed markedly inhibited growth when treated with doses of glyphosate that were lethal in unsterilized soil. However, when Pythium sp. or Fusarium sp. was added to sterilized soil or vermiculite, glyphosate-treated plants died within 6 or 11- 12 days, respectively. The herbicidal, but not the growth-inhibitory, action of glyphosate on bean seedlings grown in sterilized medium reinfested with Pythium was blocked by metalaxyl, a phycomycete-specific, systemic fungicide. However, metalaxyl did not block the herbicidal action of glyphosate on plants growing in untreated field soil, probably because such soils contain root-rot pathogens other than Pythium that are not suppressed by metalaxyl.