Abstract BACKGROUND Brome grass ( Bromus diandrus ) is a prevalent weed in southern Australian cropping regions which causes significant economic damage in crops and pastures. Before harvest 2023, two populations (named 6‐24 and 7‐24) suspected of resistance to imidazolinone herbicides were collected from separate fields. Preliminary screening indicated that these populations were resistant to several different acetolactate synthase (ALS)‐inhibiting herbicides. Therefore, further characterisation of the level and mechanism of resistance was carried out. RESULTS There was high survival of the resistant populations at the maximum tested rates of sulfometuron (600 g ha −1 ) and imazamox + imazapyr (74 + 34 g ha −1 , 2× the label rate for use in imidazolinone‐tolerant crops). All examined individuals (15 from each population) carried the same point mutation causing a tryptophan‐to‐leucine substitution at position 574 in the ALS protein. Application of the cytochrome P450 inhibitor malathion prior to herbicide treatment did not affect survival rates. CONCLUSIONS This is the first study reporting a Trp574–Leu mutation as the basis for cross‐resistance to ALS‐inhibiting herbicides (sulfonylureas, imidazolinones and triazolopyrimidines) in Australian populations of B. diandrus . Control of these populations will require strategic use of pre‐emergence herbicides and herbicide‐tolerant crops in combination with harvest weed seed control. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.
Busi et al. (2026) studied this question.