Submitted to: Communications in Soil Science and Plant Analysis
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/30/2007
Publication Date: 7/15/2007
Citation: Means, N.E., Kremer, R.J. 2007. Influence of soil moisture on root colonization of glyphosate-treated soybean by fusarium species. Communications in Soil Science and Plant Analysis. 38:1713-1720. Interpretive Summary: Soybeans genetically modified (GM) for resistance to the herbicide glyphosate (Roundup®) are planted on about 90% of the soybean production area in the United States. Although GM soybeans and glyphosate pose little or no human health concerns, their impacts on soil biology and productivity, and on the growth and health of the soybean plant itself have received little attention. Important biological processes in the root zone (rhizosphere), including beneficial and detrimental effects of root-inhabiting microorganisms, have been largely neglected in environmental assessments of GM soybeans. Previous studies suggested a link between infection of glyphosate-resistant (GR) soybean roots with Fusarium, a potential fungal disease agent, and the use of glyphosate. However, under rainfed (non-irrigated) field conditions, we noted inconsistent infection of roots by Fusarium due to year-to-year variability in rainfall and soil moisture. Therefore, our objective was to determine the impact of varied soil moisture levels on root infection of glyphosate-treated soybean under controlled conditions in the greenhouse. Ten days after we sprayed glyphosate on soybean plants, infection of soybean roots by Fusarium was highest under optimum soil moisture compared with root infection at low soil moisture. Fusarium infection of roots from soybean with no glyphosate treatment was consistently lower at all soil moisture levels. This study confirms our previous field observations that reported increased incidence of Fusarium on GR soybean under high soil moisture and documents for the first time that root infection is depressed under soil moisture stress. These quantitative findings are useful to partly explain apparent root diseases that had been widely but superficially observed in GR soybean growing in wet soils. This information has important implications for scientists, extension personnel, producers, and the agricultural industry because it helps in understanding the occasional production problems observed with GR soybean, and in developing improved crop management systems for avoiding potential crop growth reductions that may be related to variable soil moisture patterns in production fields.
Technical Abstract: The widespread use of glyphosate-resistant (GR) cropping systems may impact rhizosphere microbial associations and crop productivity. We previously reported that glyphosate accumulation in the rhizosphere may stimulate colonization of soybean [Glycine max (L.) Merr.] roots by soilborne Fusarium. Field studies often reveal inconsistent root colonization by Fusarium especially during growing seasons characterized by contrasting rainfall patterns. Therefore, this study was conducted to determine the impact of different soil moisture contents on root colonization of glyphosate-treated soybean by Fusarium species. Glyphosate (0.84 kg ae ha-1) was applied to greenhouse-grown GR soybean at the V2-V3 growth stage growing in a Mexico silt loam at 27%, 13%, and 10% soil moisture contents. Soil and plant samples were sampled periodically after herbicide application and selectively cultured for Fusarium. Highest Fusarium colonization was associated with the glyphosate treatment with maximum populations occurring at the highest soil moisture. Thus, glyphosate interactions with root colonization by Fusarium in glyphosate-resistant soybean are greatly influenced by soil moisture content.