Confirmation and characterisation of ALS inhibitor resistant Poa trivialis from Ireland

Relying on one broad-spectrum product to control grass weeds has resulted in cases of resistance to acetolactate synthase (ALS) inhibitors in species that were not the primary target such as Poa trivialis (POATR), in wheat fields in Ireland. In this study, we have characterised ALS inhibitor resistance in two populations of POATR-R, suspected of being resistant, to (i) sulfonylurea (SUs)-mesosulfuron + iodosulfuron (the selecting agent), (ii) SU + sulfonylamino‑carbonyl-triazolinone (SCT)-mesosulfuron + propoxycarbazone, and (iii) triazolopyrimidine (TP)-pyroxsulam ALS chemistries. Resistant POATR-R populations showed ALS inhibitor cross-resistance associated with target-site resistance (TSR) mutations. Combined mutations (Pro-197 and Trp-574) were found in POATR-R plants; Trp-574-Leu in POATR-R1 conferring greater SUs, SU + SCT and TP (resistance index, RI >214) resistance, while Pro-197-Thr in POATR-R2 (RI >37) was associated with less resistance. The high levels of ALS inhibitor cross-resistance in POATR-R populations caused by TSR mutations are consistent with the ploidy status, as cytogenetic tests revealed that both the R and S populations were diploid (2n = 2× = 14). Cytochrome P450 inhibitor assays did not detect metabolism-based ALS resistance in POATR-R. Alternative herbicide modes of action, including acetyl-CoA carboxylase (pinoxaden, fenoxaprop, propaquizafop, cycloxydim or clethodim) and 5-enolpyruvylshikimate-3-phosphate synthase (glyphosate) inhibitors applied at the recommended label rate were highly effective on these resistant populations. Residual herbicides and cultural methods, as well as the judicious use of alternative in-crop herbicide options, should be prioritized as sustainable options for managing these ALS-resistant populations. This is first worldwide characterisation of resistance mechanisms in P. trivialis to ALS inhibitor chemistries.