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Abstract Palmer amaranth is an increasing concern for producers in the northeastern United States. A new Palmer amaranth population (NYPA) was identified from a soybean field in Ontario County, NY in 2024. The main objectives of this research were to (1) confirm whether this NYPA population was resistant to glyphosate and atrazine (multiple herbicide-resistant, MHR), and (2) determine the effectiveness of various postemergence (POST) herbicides alone or in mixtures for its control. Along with the NYPA population, two previously known glyphosate-resistant Palmer amaranth populations from Connecticut (CTPA) and Kansas (KSPA), and a known glyphosate-susceptible population from Alabama (ALSUS) were also evaluated. Results from the quantitative polymerase chain reaction (qPCR) assay revealed that the NYPA population had 165 to 199 copies of the 5-Enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene as compared to a single EPSPS gene copy in the ALSUS population. A greenhouse dose-response study revealed that the NYPA and CTPA Palmer amaranth populations had 7- to 11-fold resistance to atrazine. Nearly all POST herbicides tested, including 2, 4-D, dicamba, saflufenacil, glufosinate, lactofen alone or in mixtures with 2, 4-D, dicamba, and glufosinate, provided effective control (90 to 100%) of Palmer amaranth from NY, CT, and KS. All these POST herbicides, alone or in mixtures, reduced shoot dry biomass of all three populations by 82 to 97% as compared to the nontreated control. These results confirm the first report of MHR (resistant to glyphosate and atrazine) Palmer amaranth populations from NY and CT. Effective POST herbicides tested in this research can be used to manage these MHR Palmer amaranth populations.
Rieley et al. (Mon,) studied this question.