The molecular basis for acetolactate synthase (ALS)–inhibitor resistance was determined for 6 susceptible (HS) and 24 resistant (HR) kochia populations from western Canada. The latter included 3 HR populations from Alberta (AB), 3 from Manitoba (MB), and 18 from Saskatchewan (SK). HR plants survived application of the ALS-inhibitor herbicide thifensulfuron–tribenuron mixture in the greenhouse. Most of the HR populations were heterogeneous and contained both HR and HS individuals. The molecular basis for resistance was determined in 273 HR individuals by sequencing the ALS gene (2,270 base pair [bp]) or by conducting a TaqMan genotyping assay developed in this study using real-time polymerase chain reaction (PCR) for single nucleotide polymorphism (SNP) 1709, where a G to T substitution resulted in a Leu for Trp substitution at amino acid position 574 (Trp₅₇₄Leu mutation). A total of 16 SNPs were identified in the ALS gene sequences (0.7% polymorphism), 5 of which resulted in amino acid changes that confer resistance to ALS-inhibiting herbicides. The SNPs correspond to three target-site mutations: Pro₁₉₇ (SNPs 565 and 566), Asp₃₇₆ (SNP 1116), and Trp₅₇₄ (SNPs 1708 and 1709). The Trp₅₇₄Leu mutation was predominant (189 HR plants). The next most common mutation was the highly variable residue Pro₁₉₇ (44 HR plants) with substitution by one of nine amino acids. The least-frequent were Asp₃₇₆Glu (9 plants) and Trp₅₇₄Arg (3 plants) substitutions. The presence of two ALS target-site mutations was found in 30 individual kochia plants, the first report from field-selected weed populations. These include combinations Pro₁₉₇ Trp₅₇₄ (23 plants) and Pro₁₉₇ Asp₃₇₆ (7 plants). The detection of Pro₁₉₇, Asp₃₇₆, and Trp₅₇₄ mutations, as well as both combinations, from geographically separate regions suggests multiple origins of these mutations.

Nomenclature: Thifensulfuron; tribenuron; kochia, Kochia scoparia (L.) Schrad. KCHSC, synonym: Bassia scoparia (L.) A.J. Scott