Multiple herbicide resistance among kochia (<i>Bassia scoparia</i>) populations in the southcentral Great Plains

Abstract Multiple herbicide–resistant (MHR) kochia [ Bassia scoparia (L.) A.J. Scott] is a concern for farmers in the Great Plains. A total of 82 B. scoparia populations were collected from western Kansas (KS), western Oklahoma (OK), and the High Plains of Texas (TX) during fall of 2018 and 2019 (from the various locations), and their herbicide resistance status was evaluated. The main objectives were to (1) determine the distribution and frequency of resistance to atrazine, chlorsulfuron, dicamba, fluroxypyr, and glyphosate; and (2) characterize the resistance levels to glyphosate, dicamba, and/or fluroxypyr in selected B. scoparia populations. Results indicated that 33%, 100%, 48%, 30%, and 70% of the tested B. scoparia populations were potentially resistant (≥20% survival frequency) to atrazine, chlorsulfuron, dicamba, fluroxypyr, and glyphosate, respectively. A three-way premixture of dichlorprop/dicamba/2,4-D provided 100% control of all the tested populations. Dose–response studies further revealed that KS-9 and KS-14 B. scoparia populations were 5- to 10-fold resistant to dicamba, 3- to 6-fold resistant to fluroxypyr, and 4- to 5-fold resistant to glyphosate as compared with the susceptible (KS-SUS) population. Similarly, OK-10 and OK-11 populations were 10- to 13-fold resistant to dicamba and 3- to 4-fold resistant to fluroxypyr and glyphosate compared with the OK-SUS population. TX-1 and TX-13 B. scoparia populations were 2- to 4-fold resistant to dicamba, and TX-1 was 5-fold resistant to glyphosate compared with the TX-SUS population. These results confirm the first report of dicamba- and fluroxypyr-resistant B. scoparia from Oklahoma and glyphosate- and dicamba-resistant B. scoparia from Texas. These results imply that adopting effective integrated weed management strategies (chemical and nonchemical) is required to mitigate the further spread of MHR B. scoparia in the region.