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ARS Home » Pacific West Area » Pullman, Washington » WHGQ » Research » Publications at this Location » Publication #428402
Research Project: Ecology and Genomics of Soilborne Pathogens, Beneficial Microbes, and the Microbiome of Wheat, Barley, and Biofuel Brassicas

Location: Wheat Health, Genetics, and Quality Research

Title: Palmer amaranth (amaranthus palmeri) in the Pacific Northwest: Glyphosate-resistance confirmation and efficacy of selected herbicides

Author
item ADJESIWOR, ALBERT - University Of Idaho
item FELIX, JOEL - Oregon State University
item ALDER, CLARKE - Amalgamated Sugar Company
item Landau, Olivia
item ARAUJO, ANDRE - Colorado State University
item GAINES, TODD - Colorado State University
item LAWRENCE, NEVIN - University Of Nebraska

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/26/2026
Publication Date: N/A
Citation: N/A

Interpretive Summary: Palmer amaranth (Amaranthus palmeri) was recently introduced to Pacific Northwest (PNW) in 2022. This is a troublesome finding because this species is a prolific seed-producer and has developed resistance to many commonly utilized herbicides in the PNW, which makes eradication through chemical control more difficult. Extensive surveys in Idaho and Oregon were conducted in order to acquire seed for subsequent greenhouse assays where eight populations displayed glyphosate-resistance and reduced sensitivity to 2,4-D, dicamba, and mesotrione. However, glufosinate and saflufenacil effectively controlled Palmer amaranth populations. Tissue was collected from these glyphosate-resistant plants for 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene copy number analysis, which confirmed increased EPSPS gene copies in these suspected glyphosate-resistant populations relative to sensitive with a single EPSPS gene. These results suggest the PNW Palmer amaranth has been introduced from regions in North America where glyphosate-resistant plants are already prevalent. This information is beneficial to growers because it allows them to avoid wasting time and money on herbicides that will not provide control for Palmer amaranth. Furthermore, this information will encourage them to be incorporate preventative and non-chemical weed management options, such as cleaning field equipment between fields, field scouting, cultivation, tillage, and harvest weed seed control.

Technical Abstract: Palmer amaranth (Amaranthus palmeri) has been recently introduced to the Pacific Northwest (PNW), which prompted extensive surveys in 2023 and 2024 to track Palmer amaranth in the PNW, collect tissue, and collect seed samples for herbicide resistance screening. Greenhouse dose-response bioassays were conducted in Kimberly, ID, during the summer of 2024 to assess the response of Palmer amaranth populations to selected postemergence herbicides. Resistance to glyphosate predominated among the populations, and reduced sensitivity to 2,4-D, dicamba and mesotrione was also observed. In contrast, glufosinate and saflufenacil provided effective control of PNW Palmer amaranth populations. Based on the dose response bioassays, the effective dose required to provide 90% control of the suspected glyphosate-resistant populations was 21 to 63-fold more compared to the susceptible population. Subsequent 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene duplication analysis was conducted to confirm glyphosate resistance in the Palmer amaranth populations. About 74% of the Palmer amaranth tissue samples showed gene duplication of up to 184 EPSPS gene copies. The EPSPS gene amplification analysis of plants that survived 2X rate of glyphosate (2,520 g ae ha-1) showed up to 150 EPSPS genes in glyphosate-resistant populations. The widespread glyphosate resistance in the samples collected suggests that Palmer amaranth populations being introduced into the PNW are coming from states where this weed has developed resistance to some herbicide sites of action.