Palmer amaranth is the most problematic weed in agronomic crop production fields in the United States. A Palmer amaranth biotype was not controlled with sequential applications of glyphosate in glyphosate-resistant (GR) soybean production field in south-central Nebraska. The seeds of the putative GR Palmer amaranth biotype were collected in the fall of 2015. The objectives of this study were to (1) confirm GR Palmer amaranth and determine the level of resistance in a whole-plant dose-response bioassay, (2) determine the copy number of 5-enolpyruvylshikimate-3-phosphate (EPSPS) gene, the molecular target of glyphosate, and (3) evaluate the response of GR Palmer amaranth biotype to POST corn and soybean herbicides with different modes-of-action. Based on the effective dose required to control 90% of plants (ED90), the putative GR Palmer amaranth biotype was 37- to 40-fold resistant to glyphosate depending on the glyphosate-susceptible (GS) used as a baseline population. EPSPS gene amplification was present in the GR Palmer amaranth biotype with up to 32 to 105 EPSPS copies compared to the known GS biotypes. Response of GR Palmer amaranth to POST corn and soybean herbicides suggest reduced sensitivity to atrazine, hydroxyphenylpyruvate dioxygenase (HPPD)- (mesotrione, tembotrione, and topramezone), acetolactate synthase (ALS)- (halosulfuron-methyl), and protoporphyrinogen oxidase (PPO)- (carfentrazone and lactofen) inhibitors. GR Palmer amaranth was effectively controlled (>90%) with glufosinate applied at 593 g ai ha−1 with ≥95% reduction in biomass. More research is needed to determine whether this biotype exhibits multiple resistant to other group of herbicides and evaluate herbicide programs for effective management in corn and soybean.

In a previous study, glyphosate-susceptible and -resistant giant ragweedbiotypes grown in sterile field soil survived a higher rate of glyphosatethan those grown in unsterile field soil, and the roots of the susceptiblebiotype were colonized by a larger number of soil microorganisms than thoseof the resistant biotype when treated with 1.6 kg ae ha−1glyphosate. Thus, we concluded that soil-borne microbes play a role inglyphosate activity and now hypothesize that the ability of the resistantbiotype to tolerate glyphosate may involve microbial interactions in therhizosphere. The objective of this study was to evaluate differences in therhizosphere microbial communities of glyphosate-susceptible and -resistantgiant ragweed biotypes 3 d after a glyphosate treatment. Giant ragweedbiotypes were grown in the greenhouse in unsterile field soil and glyphosatewas applied at either 0 or 1.6 kg ha−1. Rhizosphere soil wassampled 3 d after the glyphosate treatment, and DNA was extracted, purified,and sequenced with the use of Illumina Genome Analyzer next-generationsequencing. The taxonomic distribution of the microbial community,diversity, genera abundance, and community structure within the rhizosphereof the two giant ragweed biotypes in response to a glyphosate applicationwas evaluated by metagenomics analysis. Bacteria comprised approximately 96%of the total microbial community in both biotypes, and differences in thedistribution of some microbes at the phyla level were observed. Selectsoil-borne plant pathogens (Verticillium and Xanthomonas) and plant-growth–promoting rhizobacteria (Burkholderia) present in the rhizosphere wereinfluenced by either biotype or glyphosate application. We did not, however,observe large differences in the diversity or structure of soil microbialcommunities among our treatments. The results of this study indicate thatchallenging giant ragweed biotypes with glyphosate causes perturbations inrhizosphere microbial communities and that the perturbations differ betweenthe susceptible and resistant biotypes. However, biological relevance of therhizosphere microbial community data that we obtained by next-generationsequencing remains unclear.