Soil Microbial Root Colonization of Glyphosate-Treated Giant Ragweed (Ambrosia trifida), Horseweed (Conyza canadensis), and Common Lambsquarters (Chenopodium album) Biotypes

Abstract

Root colonization by soil microorganisms has been shown to increase the activity of glyphosate in resistant and susceptible biotypes of giant ragweed and a susceptible common lambsquarters biotype, but not in horseweed biotypes. The objective of this study was to investigate the colonization of roots in glyphosate-resistant and -susceptible giant ragweed and horseweed biotypes, and glyphosate-tolerant and -susceptible biotypes of common lambsquarters after a sublethal glyphosate application. The three weed species were grown separately in sterile and unsterile field soil and treated with glyphosate at two sublethal rates. Soil microbes were isolated from the roots onto sterile media 3 d after the glyphosate treatment. The susceptible biotypes of giant ragweed and horseweed grown in unsterile soil were colonized by more soil microbes at the higher rate of glyphosate, compared to the resistant biotype grown in unsterile soil. Oomycetes were isolated separately on a selective media and they were also more prevalent in the roots of the susceptible biotypes of each weed species grown in the unsterile soil when glyphosate was applied at the highest rate. Therefore, the ability of these three weed species to tolerate a glyphosate application may involve differences in the susceptibility to soil microbial colonization, especially oomycetes.

Rhizosphere Microbial Community Dynamics in Glyphosate-Treated Susceptible and Resistant Biotypes of Giant Ragweed (Ambrosia trifida)

“…By using traditional culturing techniques, Pythium spp. were frequently isolated directly from the root tissue of glyphosate-treated GS giant ragweed; therefore this root-invading pathogen was proposed to play a role in glyphosate efficacy, and possibly glyphosate resistance (Schafer et al 2013). We anticipated that the molecular techniques used here would allow us to evaluate interactions among GS and GR giant ragweed biotypes to gain a deeper understanding of the role of the rhizosphere microbial community in the mode of action of glyphosate and glyphosate resistance.…”

“…We cannot, however, highlight any perturbations that stand out as likely contributors to the herbicide-disease response observed in the plant. The microbial sequencing techniques used did not show that the soil-borne plant pathogen Pythium spp., typically implicated in this pathosystem with the use of classical techniques (Schafer et al 2013), were affected in the rhizosphere microbial community. We note, however, that in our previous studies Pythium spp.…”

“…Schafer et al (2013) reported that the roots of a GS giant ragweed biotype grown in unsterile field soil were heavily colonized by oomycete pathogens (e.g., Pythium spp. and Phytophthora spp.)…”

Noncompetitive Growth and Fecundity of Wisconsin Giant Ragweed Resistant to Glyphosate

“…Root growth was not evaluated in our research, as it was by Alcorta et al (2011), who found that root mass was more than twofold greater for glyphosate-resistant than -susceptible horseweed. Furthermore, microbial communities in the rhizosphere have been shown to play a role in the differential response of glyphosate-resistant andsusceptible giant ragweed to glyphosate (Schafer et al 2012(Schafer et al , 2013. Schafer et al (2013) found that a glyphosate-resistant giant ragweed biotype from Indiana may be capable of withstanding soil microbial colonization, specifically oomycete colonization (predominantly Pythium spp.…”

“…Furthermore, microbial communities in the rhizosphere have been shown to play a role in the differential response of glyphosate-resistant andsusceptible giant ragweed to glyphosate (Schafer et al 2012(Schafer et al , 2013. Schafer et al (2013) found that a glyphosate-resistant giant ragweed biotype from Indiana may be capable of withstanding soil microbial colonization, specifically oomycete colonization (predominantly Pythium spp. ), or that the defense mechanism of the roots may not be suppressed greatly by glyphosate compared to a glyphosatesusceptible biotype.…”