Herbicide-resistant kochia is an increasing concern for growers in the northwestern United States. Four suspected glyphosate-resistant (Gly-R) kochia accessions (referred to as GIL01, JOP01, CHES01, and CHES02) collected in fall 2012 from four different chemical-fallow fields in northern Montana were evaluated. The objectives were to confirm and characterize the level of glyphosate resistance in kochia accessions relative to a glyphosate-susceptible (Gly-S) accession and evaluate the effectiveness of various POST herbicides for Gly-R kochia control. Whole-plant dose–response experiments indicated that the four Gly-R kochia accessions had 7.1- to 11-fold levels of resistance relative to the Gly-S accession on the basis of percent control ratings (I50 values). On the basis of shoot dry weight response (GR50 values), the four Gly-R kochia accessions exhibited resistance index (R/S) ratios ranging from 4.6 to 8.1. In a separate study, the two tested Gly-R accessions (GIL01 and JOP01) showed differential response (control and shoot dry weight reduction) to various POST herbicides 21 d after application (DAA). Paraquat, paraquat linuron, carfentrazone 2,4-D, saflufenacil alone or with 2,4-D, and bromoxynil fluroxypyr effectively controlled (99 to 100%) and reduced shoot dry weight (88 to 92%) of the GIL01 accession, consistent with the Gly-S kochia accession; however, bromoxynil MCPA and bromoxynil pyrasulfotole provided 76% control and 83% shoot dry weight reduction of the GIL01 accession and were lower compared with the Gly-S accession. The JOP01 accession exhibited lower control or shoot dry weight reduction to all herbicides tested, except dicamba, diflufenzopyr dicamba 2,4-D, paraquat linuron, and bromoxynil pyrasulfotole, compared with the Gly-S or GIL01 population. Furthermore, paraquat linuron was the only treatment with ≥ 90% control and shoot dry weight reduction of the JOP01 kochia plants. Among all POST herbicides tested, glufosinate was the least effective on kochia. This research confirms the first evolution of Gly-R kochia in Montana. Future research will investigate the mechanism of glyphosate resistance, inheritance, ecological fitness, and alternative strategies for management of Gly-R kochia.
Nomenclature: 2,4-D; bromoxynil; carfentrazone; dicamba; diflufenzopyr; fluroxypyr; glufosinate; glyphosate; linuron; MCPA; paraquat; pyrasulfotole; saflufenacil; kochia, Kochia scoparia (L.) Schrad.
Kochia scoparia resistente a herbicidas es una preocupación creciente para los productores del noroeste de Estados Unidos. Se evaluaron cuatro accesiones de K. scoparia (referidas como GIL01, JOP01, CHES01, y CHES02) que se sospechaban resistentes a glyphosate (Gly-R) y que fueron colectadas en el otoño 2012, en cuatro campos en barbecho químico en el norte de Montana. Los objetivos fueron confirmar y caracterizar el nivel de resistencia a glyphosate en accesiones de K. scoparia en relación con una accesión susceptible a glyphosate (Gly-S) y evaluar la efectividad de varios herbicidas POST para el control de K. scoparia Gly-R. Experimentos de respuesta a dosis en plantas enteras indicaron que las cuatro accesiones de K. scoparia Gly-R tuvieron 7.1 a 11 veces el nivel de resistencia en relación a la accesión Gly-S con base en evaluaciones de porcentaje de control (valores I50). Con base en la respuesta del peso seco del tejido aéreo (valores GR50), las cuatro accesiones de K. scoparia Gly-R mostraron ratios de índice de resistencia (R/S) entre 4.6 y 8.1. En un estudio aparte, las dos accesiones Gly-R e
