Insights into ALS-inhibiting herbicide resistance in Poa annua in an arable cropping system

Dependence on a single herbicide type to control a broad-spectrum of weeds has resulted in cases of resistance to acetolactate synthase (ALS) inhibitors in weeds normally considered lower priority such as Poa annua, being recorded in winter wheat fields in Ireland. This study characterizes resistance to ALS-inhibiting sulfonylureas (SU), sulfonylamino-carbonyl-triazolinones (SCT) and triazolopyrimidine (TP) herbicides in five resistance-suspect populations (POAAN-R1 to POAAN-R5).

Variability in cross-resistance levels were noted in the POAAN-R populations, correlating with target-site mutations and specific amino acid substitutions. Mutations Pro-197-Thr in POAAN-R1, Pro-197-Thr + Trp-574-Leu in POAAN-R4, and Trp-574-Leu in POAAN-R5 confer high levels of resistance to SU, SU + SCT and TP herbicides (GR₅₀ resistance index, RI >10). In POAAN-R2, Pro-197-Gln confers high resistance to SU and SU + SCT (RI >10) and low resistance to TP (RI = 3.2). POAAN-R3 with Pro-197-Thr + Pro-197-Gln, confers moderate resistance to SU and SU + SCT (RI >5) and high resistance to TP (RI >10). All sequenced plants had homozygous mutations that evolved independently in ALS1 (Pro-197-Gln) or ALS2 (Pro-197-Thr or Trp-574-Leu) isoforms. Among the alternative non-residual herbicides evaluated, clethodim and glyphosate effectively controlled P. annua populations.

Pro-197-Thr and/or Pro-197-Gln, as well as the combination of Pro-197-Thr + Trp-574-Leu were identified for the first time in P. annua in this study. As cultural/non-chemical management of P. annua is challenging, the primary control in cereal crops will likely be the use of non-ALS herbicide modes of action, including the application of pre-emergence or autumn residual-type herbicides. © 2025 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.