Greenhouse experiments were conducted to evaluate the response to halosulfuron of several smooth pigweed populations that had been shown to be resistant to acetolactate synthase (ALS, EC 2.2.1.6)-inihibiting herbicides. Five ALS-resistant smooth pigweed populations (R1, R2, R3, R4, and R5) and one susceptible (S) population were treated with halosulfuron POST at 0.27, 2.7, 27, 270, and 2,700 g ai/ha. Percentage injury and dry weight were used to determine resistance of smooth pigweed populations to halosulfuron. Populations of smooth pigweed with previous reports of resistance to ALS-inhibiting herbicides showed varying degrees of resistance to halosulfuron compared with the susceptible population. Concentrations of halosulfuron required to reduce ALS-resistant smooth pigweed dry weights 50% were 2 to 12-fold higher than that of the susceptible population. One population, designated R2, had increased resistance to halosulfuron applications, requiring 97 g/ha halosulfuron to reduce shoot dry weight 50% compared with only 8 g/ha for S. Our results show that populations of smooth pigweed with a history of ALS-inhibiting resistance can have differing degrees of resistance to halosulfuron.

Acetolactate synthase–- (ALS-) inhibiting herbicides are frequently used to control annual bluegrass in managed turfgrass systems. Recently, the number of annual bluegrass populations resistant to these herbicides has increased on golf courses. Previous research involving annual bluegrass resistance to ALS inhibitors has included elucidating the mechanism of resistance and in vivo response of ALS to ALS-inhibiting herbicides. The purpose of this research was to generate baseline information on enzymatic parameters of resistant (R) and susceptible (S) annual bluegrass biotypes and further distinguish these biotypes by comparing vegetative growth and reproductive characteristics. The R biotype contained a mutation in the ALS gene resulting in a Trp574 to Leu amino acid substitution. Compared to the S biotype, the R biotype exhibited a 27- and 10-fold resistance to trifloxysulfuron at the whole-plant level and under in vitro conditions, respectively. No significant differences were observed in substrate concentration at one-half maximum rate of enzyme activity (pyruvate) or extractable ALS activity between biotypes, but the maximum rate of enzyme activity was higher for the R biotype. The feedback inhibition of ALS activity by the branched-chain amino acids was higher for the R biotype than the S biotype, with leucine, valine, and isoleucine inhibiting ALS activity 20, 6, and 4% more in the R biotype, respectively. The R biotype produced more inflorescences and seeds per plant in comparison with the S biotype, but relative growth rates between biotypes were similar at all harvest intervals. Our research provides baseline information regarding ALS enzyme response, vegetative growth, and reproductive characteristics of annual bluegrass biotypes resistant and susceptible to ALS-inhibiting herbicides.

Annual bluegrass is commonly controlled by acetolactate synthase (ALS)-inhibiting herbicides in managed turfgrass. An annual bluegrass population with suspected resistance to ALS-inhibiting herbicides was collected from Grand National Golf Course in Opelika, AL (GN population). Subsequent testing confirmed resistance of the GN population to foramsulfuron, trifloxysulfuron, bispyribac-sodium (bispyribac), and imazaquin when compared to a susceptible population collected locally at Auburn University (AU population). Sequencing of the ALS gene revealed a point mutation resulting in an amino acid substitution at Trp574. Cloning of the ALS gene surrounding the Trp574 region yielded two distinct ALS gene sequences: one producing Trp574 and one producing Leu574. Trp574 to Leu has been previously correlated with resistance to ALS-inhibiting herbicides. Both AU and GN gene sequences contained other similar silent and missense mutations. This research confirms resistance of annual bluegrass to ALS-inhibiting herbicides with Trp574 to Leu amino acid substitution being the most likely mode of resistance based on past literature.