Resistance to atrazine (a photosystem II [PSII] inhibitor) is prevalent in waterhemp [Amaranthus tuberculatus(Moq.) J. D. Sauer] across the U.S. Midwest. Previous research suggests that target-site mutation or rapid metabolism of atrazine mediated by glutathioneS-transferase (GST) conjugation confers resistance inA. tuberculatusfrom Illinois. The distribution and mechanism of resistance to atrazine inA. tuberculatuspopulations from Nebraska (NE) are unknown. In this research we (1) evaluated the response and frequency of resistance in NEA. tuberculatusto soil-applied PSII (metribuzin and atrazine) and protoporphyrinogen oxidase (sulfentrazone) inhibitors, as well as POST-applied atrazine; and (2) determined the mechanism of atrazine resistance in NEA. tuberculatus. The chloroplasticpsbAgene, coding for a D1 protein (the target site of atrazine) was sequenced in 85 plants representing 27 populations ofA. tuberculatus. Furthermore, 24 plants selected randomly from four atrazine-resistant (AR) populations were used to determine the metabolism of atrazine via GST conjugation. Results from the soil-applied herbicide evaluation suggest that metribuzin (0.56 kg ai ha−1) and sulfentrazone (0.28 kg ai ha−1) were effective onA. tuberculatusmanagement. PRE and POST screenings against atrazine in the greenhouse indicate that atrazine (1.345 kg ai ha−1) was not effective on 39% and 73% of theA. tuberculatuspopulations evaluated (total of 109 and 85 populations, respectively), suggesting the prevalence of atrazine resistance inA. tuberculatusin NE. Sequence analysis of thepsbAgene found no known point mutations conferring atrazine resistance. However, the AR plants conjugated atrazine via GST activity faster than the known atrazine-susceptibleA. tuberculatus. Overall, the outcome of this study demonstrates the predominance of metabolism-based resistance to atrazine inA. tuberculatusfrom NE, which may predispose this species to evolve resistance to other herbicides. The use of integrated management strategies forA. tuberculatusis crucial for the control of this troublesome species.

The continued dispersal of Palmer amaranth can impose detrimental impacts to cropping systems in Wisconsin. Our objective was to characterize the response of a recently introduced Palmer amaranth accession in southern WI to postemergence (POST) and preemergence (PRE) herbicides commonly used in corn and soybean. Greenhouse experiments were conducted with the WI putative herbicide-resistant accession (BRO), and two additional control accessions from Nebraska, a glyphosate-resistant (KEI2) and a glyphosate-susceptible (KEI3). Postemergence treatments were 2,4-D, atrazine, dicamba, glufosinate, glyphosate, imazethapyr, lactofen, and mesotrione at 1× and 3× label rates. Preemergence treatments were atrazine, mesotrione, metribuzin, S-metolachlor, and sulfentrazone at 0.5×, 1×, and 3× label rates. Plant survival of each accession was ≥ 63% after exposure to imazethapyr POST 3× rate. Survival of BRO and KEI2 was 44% (±13) and 50% (±13), respectively, after exposure to atrazine POST 3× rate. Survival of BRO was 69% (±12) after exposure to glyphosate POST 1× rate, whereas survival of KEI2 was 44% (±13) after exposure to glyphosate POST 3× rate. After exposure to 2,4-D POST 1× rate, KEI2 and KEI3 survival was 38% (±13) and 50% (±13), respectively. Survival of all accessions was ≤ 31% after exposure to 2,4-D POST 3× rate, or dicamba, glufosinate, lactofen, and mesotrione POST at either rate. Plant density reduction of KEI2 was 77% (±13) after exposure to atrazine PRE 1× rate, whereas density reduction of BRO was 56% (±13) after exposure to atrazine PRE 3× rate. Plant density reduction of all accessions was ≥ 94% after exposure to mesotrione PRE 1× and 3× rates, or metribuzin, S-metolachlor, and sulfentrazone PRE at either rate. Our results suggest that each accession is resistant (≥ 50% survival) to imazethapyr POST, that BRO and KEI2 are resistant to atrazine and glyphosate POST, and KEI2 and KEI3 are resistant to 2,4-D POST. The recently introduced BRO accession exhibited multiple resistance to imazethapyr, atrazine, and glyphosate POST. In addition, atrazine PRE was ineffective for BRO control, suggesting that diversified resistance management will be critical for its effective management.

A comprehensive WI state-wide assessment of waterhemp response to a diverse group of herbicide sites of action (SOAs) has not been conducted. Our objective was to characterize the response of a WI state-wide collection of waterhemp accessions to postemergence (POST) and preemergence (PRE) herbicides commonly used in corn and soybean. Greenhouse experiments were conducted with more than 80 accessions from 27 counties. POST treatments were 2,4-D, atrazine, dicamba, fomesafen, glufosinate, glyphosate, imazethapyr, and mesotrione at 1× and 3× label rates. PRE treatments were atrazine, fomesafen, mesotrione, metribuzin, and S-metolachlor at 0.5×, 1×, and 3× label rates. Ninety-eight and 88% of the accessions exhibited ≥ 50% plant survival after exposure to imazethapyr and glyphosate POST 3× rate, respectively. Seventeen, 16, and 3% of the accessions exhibited ≥ 50% plant survival after exposure to 2,4-D, atrazine, and dicamba POST 1× rate, respectively. Survival of all accessions was ≤ 25% after exposure to 2,4-D or dicamba POST 3× rate, or glufosinate, fomesafen, and mesotrione POST at either rate evaluated. No plant of any accession survived exposure to glufosinate at either rate. Forty-five and 3% of the accessions exhibited < 90% plant density reduction after exposure to atrazine PRE 3× rate and fomesafen PRE 1× rate, respectively. Plant density reduction of all accessions was ≥ 96% after exposure to fomesafen PRE 3× rate, or metribuzin, S-metolachlor, and mesotrione PRE 1× rate. Our results suggest that waterhemp resistance to imazethapyr and glyphosate POST is widespread in WI, whereas resistance to 2,4-D, atrazine, and dicamba POST is present to a lower extent. One accession (A75, Fond du Lac County) exhibited multiple resistance to imazethapyr, atrazine, glyphosate, and 2,4-D POST. Overall, atrazine PRE was ineffective for waterhemp control in WI. Proactive resistance management and the use of effective PRE and POST herbicides are fundamental for waterhemp management in WI.