Poa annua becomes the first weed to evolve resistance to indaziflam applied preemergence and early-postemergence.

Herbicide resistance evolution is a major challenge in agriculture. Poa annua L., a globally distributed and genetically diverse weed, has repeatedly evolved resistance to multiple herbicide sites of action due to its genetic plasticity and rapid life cycle. Indaziflam is widely used for P. annua control in several agroecosystems, with reports of postemergence resistance and a few confirmed cases of resistance to preemergence application. This study investigated suspected resistance to indaziflam in P. annua accessions from Oregon hazelnut orchards. Whole-plant dose-response assays confirmed indaziflam resistance in eight accessions ranging from 2.5- to 51-fold relative to susceptible accessions, which had a lethal dose to 50% of the accession (LD50) of 1.0-3.4 g ha-1. Resistance increased with plant developmental stage and was most pronounced when indaziflam was applied early-postemergence, with some accessions surviving two- to four-fold the labeled rate in tree nut orchards (50-95 g ha-1). Field experiments confirmed reduced efficacy of preemergence and early-postemergence indaziflam treatments up to 195 g ha-1. Indaziflam efficacy declined significantly across all accessions at air temperatures 9 °C:1 °C (day/night), with LD50 values increasing up to eight-fold across accessions compared to 25 °C:12 °C (day/night). Enzyme inhibitor seed-based assays with cytochrome P450 and glutatione transferase inhibitors did not reverse resistance, suggesting resistance pathways other than enhanced metabolism may be involved. This study provides the first confirmed cases of field-evolved resistance to indaziflam applied both preemergence and early-postemergence in P. annua. Resistance was most severe under cooler temperatures and postemergence use, highlighting environmental and developmental effects on indaziflam activity.