BACKGROUND: Bromus tectorum L. is one of the most troublesome grass weed species in cropland and non-cropland areas ofthe northwestern USA. In summer 2016, a B. tectroum accession (R) that survived imazamox at the field-use rate (44 g ha–1)inan imidazolinone-tolerant ( IMI-tolerant or Clearfield™) winter wheat field was collected from a wheat field in Carter County, MT,USA. The aim of this study was to determine the resistance profile of the B. tectroum R accession to imazamox and other ALSinhibitors, and investigate the mechanism of resistance to imazamox.

RESULTS: The R B. tectorum accession had a high-level resistance (110.1-fold) to imazamox (IMI) and low to moderate-levelscross-resistance to pyroxsulam (TP) (4.6-fold) and propoxycarbazone (SCT) (13.9-fold). The R accession was susceptible tosulfosulfuron (SU) and quizalofop and clethodim (ACCase inhibitors), paraquat (PS I inhibitor), glyphosate (EPSPS inhibitor)and glufosinate (GS inhibitor). Sequence analysis of the ALS gene revealed a single, target-site Ser653Asn mutation in R plants.Pretreatment of malathion followed by imazamox at 44 or 88 g ha–1did not reverse the resistance phenotype.

CONCLUSION: This is the first report of evolution of cross-resistance to ALS-inhibiting herbicides in B. tectorum. A single-pointmutation, Ser653Asn, was identified, conferring the high-level resistance to imazamox.