Detection and confirmation of atrazine-resistant Palmer amaranth (Amaranthus palmeri) in the northeastern United States

Authors: BESANÇON, THIERRY - Rutgers University; BRUNHARO, CAIO - Pennsylvania State University; Polashock, James; SOSNOSKIE, LYNN - Cornell University

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/27/2026
Publication Date: 3/6/2026
Citation: Besançon, T.E., Brunharo, C.A., Polashock, J.J., Sosnoskie, L.M. 2026. Detection and confirmation of atrazine-resistant Palmer amaranth (Amaranthus palmeri) in the northeastern United States. Weed Science. 74(1). Article e25. https://doi.org/10.1017/wsc.2026.10102.
DOI: https://doi.org/10.1017/wsc.2026.10102

Interpretive Summary: One of the most important limiting factors in agricultural production is weed pressure. Weeds compete with the crop for resources such as nutrients and water as well as by attracting and harboring harmful insects. Primary control of weeds is by the careful use of herbicides. Palmer amaranth is a weed native to the southeastern United States, but it has spread well beyond it's native range and has become one of the most troublesome weeds in major crops such as corn, cotton and soybean. It is now found in New Jersey and threatens vegetable and small fruit production. Unfortunately, many weed species, including Palmer amaranth, are exhibiting resistance to commonly used herbicides. Here we report the occurrence of atrazine-resistant Palmer amaranth in the northeastern United States. Atrazine controls weeds by targeting a protein important in photosynthesis. We demonstrate that resistance in the weed samples we collected is not associated with the target site. Thus, atrazine resistance in the Palmer amaranth samples we tested, is associated with a different mechanism. This has important implications for continued control of this weed species. The information we present will be useful to extension agents, scientists, and the grower community.

Technical Abstract: Weeds in agricultural systems compete with crops for resources, harbor insects and diseases, and can impede harvesting. While herbicides are the primary method of weed control, the chemistries available are limited. This has led to widespread resistance development in many troublesome weed species. Palmer amaranth is a weed species native to the southeastern United States, but it has spread well beyond it's native range and has become one of the most important weeds in major crops such as corn, cotton and soybean. It is now found in New Jersey and other parts of the northeastern United States and threatens production of vegetables and small fruits. Control of Palmer amaranth using herbicides is failing due to resistance development. Populations of this weed are reported to be resistant to at least nine herbicide sites of action (SOAs), including acetolactate synthase (ALS), microtubule, auxin-mimicking, photosystem II (PSII), 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), glutamine synthetase, protoporphyrinogen oxidase (PPO), very long-chain fatty acid synthesis (VLCFA), and hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors. Here we report the occurrence of atrazine-resistant Palmer amaranth in the northeastern United States. Atrazine controls weeds by targeting the psbA gene-encoded protein D1, which is an important component of Photosystem II. Mutations in the psbA gene that are known to be associated with resistance were not found. Thus, atrazine resistance in the Palmer amaranth samples we tested, is not target-site associated and likely occurs via a different mechanism. Determining the mechanism(s) of resistance has important implications in the effective control of this invasive weed.