Pro‐197 mutations confer resistance to thiencarbazone but not to foramsulfuron in
ALS
‐resistant
Poa annua
Vijaya Bhaskar Alwarnaidu Vijayarajan, Iniya Vijayakumar, Rafa Ruiz‐Partida, Fabian Runge, Joel Torra, Jorge Lozano‐Juste, Patrick Dermot Forristal Abstract
BACKGROUND
Poa annua is currently a lower‐priority arable weed in Ireland; however, increasing target‐site resistance to acetolactate synthase (ALS) inhibitors and declining availability of non‐ALS herbicides highlight emerging control challenges. ALS‐inhibiting co‐formulations of foramsulfuron + thiencarbazone have been specifically developed for use in herbicide‐tolerant beet varieties. While foramsulfuron can control grass weeds carrying specific target‐site mutations, its efficacy within the co‐formulation on ALS‐resistant populations remains unknown. We addressed this knowledge gap using genetic characterisation, dose–response and interaction analyses, and ligand docking modelling in 12 independently evolved, ALS‐resistant, Poa annua populations carrying Pro‐197 or Trp‐574 mutations.
RESULTS
All Pro‐197 variants (Thr, Gln or Ala) were thiencarbazone‐resistant (resistance index (RI) = 20.9–33.3) but remained foramsulfuron‐sensitive (2.5–4), while the co‐formulation showed reduced efficacy (5.5–6.2). Interaction analyses showed no overall deviation from additivity for Pro‐197 populations ( P > 0.05), although antagonism occurred at reduced rates ( P < 0.05). Structural modelling and ligand docking analyses indicated that most Pro‐197 substitutions did not alter the ALS catalytic tunnel. Foramsulfuron retained extensive hydrogen‐bonding and electrostatic interactions, with a more favourable binding free energy than thiencarbazone, which showed fewer interactions and a less favourable free energy. Trp‐574‐Leu variants exhibited cross‐resistance to all treatments, consistent with disruption of key binding interactions.
CONCLUSION
This study demonstrates that foramsulfuron + thiencarbazone efficacy on Pro‐197‐resistant Poa annua populations is poor, despite these populations being sensitive to foramsulfuron used alone. Consequently, this co‐formulation, developed for ALS‐tolerant beet, should be restricted to fields without ALS‐resistant weeds to ensure effective in‐crop control and avoid selection for resistance. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.