Antibacterial Activity of the Herbicide Glufosinate on Pseudomonas syringae Pathovar Glycinea

The antibacterial activity of the herbicide glufosinate on Pseudomonas syringae pathovar (pv.) glycinea (L-529), causal agent of soybean blight, was examined in culture and in planta. Bacteria were grown in medium containing either formulated or technical-grade glufosinate from 0.01 to 100 mM, and growth was monitored by turbidity measurements over time. Glufosinate inhibited bacterial growth at concentrations of 1 mM or greater and delayed growth at concentrations as low as 0.01 mM. Metabolism of [14C]glufosinate by P. syringae pv. glycinea grown in Davis minimal medium containing either a sublethal (1 mM) or a lethal (100 mM) glufosinate concentration was measured incrementally up to 45 h after inoculation. Thin-layer chromatography analysis revealed the presence of three glufosinate metabolites in addition to the parent compound. The rate of [14C]glufosinate metabolism by bacteria was greater in medium treated with 1 mM than in medium treated with 100 mM glufosinate. More 14C was recovered as 3-methylphosphinico-propionic acid and as an unidentified metabolite with an Rf value of 0.45 in medium treated with 1 mM than in medium treated with 100 mM glufosinate. These data suggest that P. syringae pv. glycinea can slowly metabolize glufosinate into less toxic metabolites, resulting in greater bacterial growth in medium treated with 1 mM glufosinate. Treatment of transgenic glufosinate-resistant (GR) soybeans with field rates of glufosinate reduced the number of live P. syringae pv. glycinea at 90 h after inoculation. Inoculated GR soybeans showed a 45;pc reduction of colony forming units after treatment with 0.5 kg/ha glufosinate and a 60;pc reduction after treatment with 1.0 kg/ha glufosinate. These results show that glufosinate exhibits some antibacterial activity against P. syringae pv. glycinea, but at concentrations greater than those of registered bactericides.