A Bioassay to Determine Poa annua Responses to Indaziflam

Herbicide resistance within Poa annua is widespread in managed turfgrass systems. In 2020, a P. annua collection from a golf course in the southeastern United States was reported to be resistant to indaziflam as well as six other mode-of-action groups. Considering P. annua is the most troublesome weed in turfgrass, a bioassay to screen other collections with putative indaziflam resistance is needed. A dose-response experiment was conducted with ten concentrations of indaziflam (0, 250, 500, 750, 1000, 1250, 1500, 2000, 4500, and 9000 pM) in Gelrite® culture during 2021 and 2022. An herbicide-susceptible (S1) collection of P. annua, a resistant standard (Site 3A), and a collection with putative-resistance to indaziflam (Site 18) were included in this experiment. Petri dishes were filled with 80 mL of Gelrite®(3.75 g/L) containing technical grade (≥ 98%) indaziflam (Sigma-Aldrich, St. Louis, MO) and rifampicin (1000 ppm). Each plate was sealed with parafilm after placing 15 seeds of a single collection on the Gelrite®surface. During the experiment, all plates were placed at a 75-degree angle to facilitate gravitropic root growth and stored in a growth chamber set to a constant air temperature of 16 °C. At 14 days after seeding (DAS), the length of root tissue (mm) protruding from each seed was recorded with digital calipers. Root length data from each P. annua collection were expressed as a percentage of the non-treated and subjected to non-linear regression analysis to calculate indaziflam concentrations required to reduce root growth by 70% (EC₇₀). Statistically significant differences were detected among P. annua collections with the EC₇₀ for the herbicide-susceptible collection measuring 742 pM [95% confidence interval (CI) = 686 to 803 pM] compared to 2226 pM (CI = 1851 to 2759 pM) for Site 3A and 4263 pM (CI = 3471 to 5382) for Site 18. Overall, these findings indicate that a discriminatory dose of 750 pM can be used to differentiate among susceptible and resistant individuals when screening additional P. annua collections from field sites where poor control is observed following broadcast applications of indaziflam.