Pro-197 mutations confer resistance to thiencarbazone but not to foramsulfuron in ALS-resistant Poa annua populations

[EN] 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.