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