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

Palmer amaranth (Amaranthus palmeri S. Watson) poses a significant threat to northeastern US crop production due to its rapid growth, prolific seed production, and evolving herbicide resistance. This study characterized the response of four A. palmeri populations from New York (NY) and New Jersey (NJ) to postemergence (POST) applications of atrazine, a photosystem II (PSII) inhibitor, and mesotrione, a hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor. Dose-response bioassays revealed that two NY populations (NY-GEN and NY-STE) exhibited high-level atrazine resistance, 31- to 42-fold based on ED₉₀ estimates, whereas NY-ORA and NJ-CMB populations remained susceptible. Target-site sequencing of the psbA gene revealed no mutations, indicating that resistance is conferred by a non-target-site mechanism. Metabolic assays demonstrated that resistant populations retained 20-21% less intact atrazine 48 h post-treatment compared to the susceptible reference, suggesting enhanced metabolism likely mediated by glutathione S-transferase enzymes. All populations were susceptible to mesotrione, with the field rate of 105 g ai ha^-1 providing ≥94% control. Tank mixtures of atrazine plus mesotrione applied postemergence provided near-complete control (≥97% biomass reduction relative to nontreated checks) across the tested populations, including those resistant to atrazine alone, which is consistent with synergistic interactions between PSII and HPPD inhibitors. This study documents two new cases of atrazine-resistant A. palmeri in NY and shows that resistance is mediated by enhanced metabolism, consistent with findings from other states. These results have important implications for northeastern corn (Zea mays L.) production, where atrazine remains foundational to weed management. The sustained efficacy of atrazine-mesotrione combinations offers an immediate management option, but integrated strategies incorporating multiple herbicide sites of action and cultural practices are critical to prevent further resistance evolution.