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Accepted manuscript

Palmer Amaranth (Amaranthus palmeri) in the Pacific Northwest: Glyphosate-Resistance Confirmation and Efficacy of Selected Herbicides

Published online by Cambridge University Press:  06 February 2026

Albert T. Adjesiwor Open the ORCID record for Albert T. Adjesiwor [Opens in a new window] ,
Joel Felix Open the ORCID record for Joel Felix [Opens in a new window] ,
Clarke G. Alder ,
Olivia Landau Open the ORCID record for Olivia Landau [Opens in a new window] ,
Andre Lucas Simoes Araujo ,
Todd A. Gaines Open the ORCID record for Todd A. Gaines [Opens in a new window]  and
Nevin C. Lawrence Open the ORCID record for Nevin C. Lawrence [Opens in a new window]
Albert T. Adjesiwor*
Affiliation:
Assistant Professor, University of Idaho, Kimberly Research & Extension Center, Kimberly, ID, USA
Joel Felix
Affiliation:
Professor, Oregon State University, Department of Crop and Soil Science, Malheur Experiment Station, Ontario, OR, USA
Clarke G. Alder
Affiliation:
Amalgamated Sugar Company, Boise Idaho, USA
Olivia Landau
Affiliation:
USDA-ARS Wheat Health, Genetics and Quality Research Unit, Pullman, WA, USA
Andre Lucas Simoes Araujo
Affiliation:
Graduate Student, Colorado State University, Agricultural Biology Department, Fort Collins, CO, USA
Todd A. Gaines
Affiliation:
Professor, Colorado State University, Agricultural Biology Department, Fort Collins, CO, USA
Nevin C. Lawrence
Affiliation:
Associate Professor, Department of Agronomy and Horticulture, University of Nebraska-Lincoln, Panhandle Research and Extension Center, Scottsbluff, NE, USA
*
Author for correspondence:Albert T. Adjesiwor; Email: aadjesiwor@uidaho.edu
Rights & Permissions [Opens in a new window]

Abstract

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Herbicide-resistant Palmer amaranth has been problematic within the United States for the past 30 years. 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, ID, during the summer of 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 PNW Palmer amaranth populations. Based on the dose-response bioassays, the effective dose required to provide 90% control (ED90) of the suspected glyphosate-resistant populations was 20 to 63-fold 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% (17 of 23) of the Palmer amaranth tissue samples showed gene duplication, with up to 150 copies of the EPSPS gene. 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 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.

Keywords

2,4-Ddicambaglufosinateglyphosate-resistancemesotrionepigweed

Information

Type
Research Article
Information
Weed Technology , Accepted manuscript , pp. 1 - 23
Creative Commons
Creative Common License - CCCreative Common License - BYCreative Common License - NCCreative Common License - ND
This is an Open Access article, distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives licence (http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is unaltered and is properly cited. The written permission of Cambridge University Press must be obtained for commercial re-use or in order to create a derivative work.
Copyright
© The Author(s), 2026. Published by Cambridge University Press on behalf of Weed Science Society of America