Confirming resistance to PPO-inhibiting herbicides applied preemergence and postemergence in a Georgia Palmer amaranth population

Protoporphyrinogen oxidase- (PPO) inhibiting herbicides are used in over 40 agronomic and specialty crops across Georgia to manage weeds through residual and postemergence (POST) control. In 2017, a population of Palmer amaranth exhibiting reduced sensitivity to POST applications of PPO-inhibiting herbicides was identified by the University of Georgia. Seed were collected from the site along with a known sensitive population; distance between the samples was 200 m, increasing the likelihood of similar environmental and genetic characteristics. To quantify sensitivity for both preemergence (PRE) and POST uses, 21 greenhouse dose-response assessments were conducted from 2017 to 2022. After conducting initial rate response studies, 13 doses per herbicide were chosen for the POST experiment; field use rates of fomesafen (420 g ai ha-1), lactofen (219 g ai ha-1), acifluorfen (420 g ai ha-1), and trifludimoxazin (25 g ai ha-1) at rates ranging from 0 to 4X the field use rate for the susceptible population, and 0 to 40X for the suspect population were applied. Herbicide treatments included adjuvants and were applied to plants 8 to 10 cm in height. Relative resistance factors (RRF) were calculated for control ratings, mortality, and biomass, and ranged from 105 to 318, 36 to 1477, 215 to 316, and 9 to 49 for fomesafen, lactofen, acifluorfen, and trifludimoxazin, respectively. In the PRE experiment, herbicide applications included five to nine doses of fomesafen (1X=210 g ai ha-1), flumioxazin (1X=57 g ai ha-1), oxyfluorfen (1X=561 g ai ha-1), and trifludimoxazin (1X=38 g ai ha-1); doses ranged from 0 to 6X for the suspect population and 0 to 2X for the susceptible population. Visual control, mortality, and biomass RRF ranged from 3 to 5 for fomesafen, 21 to 31 for flumioxazin, 6 to 22 for oxyfluorfen, and 8 to 38 for trifludimoxazin. Results confirm that a Georgia Palmer amaranth population is resistant to PPO-inhibiting herbicides applied both PRE and POST.