Multiple herbicide resistance in waterhemp (Amaranthus tuberculatus) accessions from Wisconsin

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