Divergent Target‐Site Substitutions at Pro197 Confer Variable Degrees of Resistance to Tribenuron‐Methyl and Florasulam in Tripleurospermum inodorum Populations Across Europe

ABSTRACT Tripleurospermum inodorum (L.) Sch. Bip. is a widespread weed in cereal production systems across Europe and has evolved resistance to acetolactate synthase (ALS)‐inhibiting herbicides in several Northern and Central European countries. This study identified and characterised resistance to the ALS‐inhibiting herbicides tribenuron‐methyl and florasulam in eight populations of T. inodorum from the Czech Republic, Germany, Norway and Sweden. The two Czech populations, with Pro‐197‐Gln + Pro‐197‐Ala substitutions in one population (CZ1) and a Pro‐197‐Thr substitution in the second population (CZ2), differed in their response to tribenuron‐methyl: CZ1 showed low resistance (resistance factor, RF: 5.2), while CZ2 exhibited high resistance (RF: > 53). However, both showed similar and low resistance to florasulam (RF: 2.5 and 3.9, respectively). The two German populations also showed contrasting responses: one population, with a Pro‐197‐Leu substitution, exhibited low resistance to both ALS inhibitors (RF: 2.8 for tribenuron‐methyl and 3.3 for florasulam), whereas the other population, with a Pro‐197‐Thr substitution, displayed high resistance to both herbicides (RF: > 53 and 12.9, respectively). Norwegian populations with a Pro‐197‐Tyr substitution and Swedish populations with Pro‐197‐Thr or Pro‐197‐Gln substitutions exhibited high resistance to tribenuron‐methyl (RF: 15.2–> 53), but only low resistance to florasulam (RF: 2.5–4.8). Geographic patterns in substitution types were evident, with Nordic populations predominantly exhibiting polar substitutions and Central European populations showing a mix of polar and non‐polar substitutions, suggesting divergent resistance evolution pathways. Notably, except for the Pro197Gln mutation, all other identified mutations have not been previously reported in T. inodorum . Overall, these results highlight the need for region‐specific resistance management strategies.