Annual bluegrass (Poa annua L.) is a globally distributed weed species with the ability to evolve resistance to herbicides. Oregon hazelnut (Corylus avellana L.) growers have recently reported poor control of P. annua with clethodim, pendimethalin, paraquat, and glyphosate, raising concerns about new herbicide-resistance cases. To investigate these reports, we conducted a herbicide resistance survey of field-collected accessions using seed-based and whole-plant dose–response bioassays. Based on dose–response estimates, resistance to all four herbicides was confirmed. Clethodim-resistant accessions had resistance indices (RIs) of 2 to 10 compared with susceptible accessions with seed-based LD50 values of 0.4 to 0.5 µM and whole-plant LD50 values of 14 to 19 g ha⁻¹. Pendimethalin-resistant accessions had RIs of 3 to 47 compared with susceptible accessions with seed-based LD50 values of 0.5 to 1 µM and whole-plant LD50 values of 360 to 590 g ha⁻¹, and cross-resistance to pronamide was also confirmed (RI = 7 to 16; susceptible accessions LD50 = 550 to 600 g ha⁻¹). The glyphosate-resistant accession had RIs of 2 to 6 compared with susceptible accessions with seed-based LD50 values of 340 to 490 µM and whole-plant LD50 values of 60 to 95 g ha⁻¹. Paraquat-resistant accessions had RIs of 2 to 85 compared with susceptible accessions with seed-based LD50 values of 0.6 to 1 µM and whole-plant LD50 values of 30 to 50 g diquat ha⁻¹. This study documents the first global case of clethodim resistance in P. annua, pendimethalin and glyphosate resistance in Oregon, and paraquat resistance in P. annua in the United States. We also demonstrate, for the first time, that seed-based bioassays can detect clethodim and paraquat resistance in P. annua. Seed assays provided a rapid (2 wk), cost-effective diagnostic tool suitable for on-farm implementation and complementary to molecular diagnostics. These findings underscore the urgent need for integrated weed management in perennial systems and adoption of resistance diagnostics and stewardship programs to mitigate further resistance evolution.
