α-tubulin Mutation Thr239-Ile in Poa annua Induces Variable Responses to Prodiamine and Dithiopyr

Mitotic-inhibiting herbicides, like prodiamine and dithiopyr, are used to control annual bluegrass (Poa annua L.) preemergence in managed turfgrass; however, resistance to mitotic-inhibiting herbicides has evolved due to repeated applications of herbicide from a single mechanism of action. Three suspected resistant populations (R1, R2, and R3) were collected in Alabama and Florida and screened for resistance to prodiamine. Part of the α-tubulin gene was sequenced for known target-site mutations. Target-site mutations were reported in all three R populations, with each of them containing an amino acid substitution at position 239 from threonine to isoleucine (Thr239-Ile). Previous research has indicated that the Thr239-Ile mutation confers resistance to dinitroaniline herbicides in other species. Dose response screens using prodiamine and dithiopyr were conducted and I50 values were calculated for R1, R2, and R3 using regression models based on seedling emergence. For prodiamine, I50 values for R1, R2, and R3 were 35.3, 502.7, and 91.5 g ai ha-1, respectively, resulting in 2.9-, 41.9-, and 7.6-fold resistance, respectively, when compared to a susceptible (S) population. For dithiopyr, I50 values for R1, R2, and R3 were 154.0, 114.2, and 190.1 g ai ha-1, respectively, resulting in 3.6-, 2.7-, and 4.5-fold resistance, respectively, when compared to a S population. When comparing I90 values to the highest labeled use rates, R2 had a 2.9-fold level of resistance to prodiamine and R1, R2, and R3 had a 2.4-, 2.0-, and 3.2-fold level of resistance to dithiopyr, respectively. This is the first report of a variable response in P. annua to prodiamine despite each R population possessing the same mutation.