Work

Palmer amaranth ( Amaranthus palmeri ) in the Pacific Northwest: glyphosate-resistance confirmation and efficacy of selected herbicides

Year: 2026

Type: article

Abstract: Herbicide-resistant Palmer amaranth has been problematic within the United States for the past 30 yr. The recent introduction of Palmer amaranth into the Pacific Northwest (PNW) prompted extensive surveys in 2023 and 2024 to collect seed samples for herbicide-resistance screening and leaf tissue for resistance-mechanism genotyping. Greenhouse dose-response bioassays were conducted in Kimberly, Idaho, during summer 2024 to assess the response of Palmer amaranth populations to selected postemergence herbicides. Resistance to glyphosate predominated across populations, and reduced sensitivity to 2,4-D, dicamba, and mesotrione was also observed. In contrast, glufosinate and saflufenacil provided effective control of Palmer amaranth populations in the PNW. Based on the dose-response bioassays, the effective dose required to provide 90% control (ED 90 ) of the suspected glyphosate-resistant populations was 20-fold to 63-fold compared with the susceptible population. Subsequent duplication analysis of the 5-enolpyruvylshikimate-3-phosphate synthase ( EPSPS ) gene was conducted to confirm glyphosate resistance in the Palmer amaranth populations. About 74% (17 of 23) of the Palmer amaranth tissue samples exhibited gene duplication, with up to 150 copies of the EPSPS gene. The EPSPS gene amplification analysis of plants that survived a 2× 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 collected samples suggests that Palmer amaranth populations are being introduced into the PNW from locations where resistance to herbicide sites of action has previously evolved.

Source: Weed Technology

DOI: 10.1017/wet.2026.10081