ALS-Resistant Spotted Spurge Confirmed in Georgia.

Metsulfuron is used for postemergence control of spotted spurge in many warm-season turfgrasses.  A suspected resistant (R)-biotype of spotted spurge was collected from turfgrass in Georgia with a history of exclusive metsulfuron use.  Research was conducted to evaluate the resistance level of this biotype to metsulfuron, efficacy of other mechanisms of action for control, and the molecular basis for resistance.  Compared to a susceptible (S)-biotype, the R-biotype required >90 and >135-times greater metsulfuron rates to reach 50% injury and reduce biomass 50% from the nontreated, respectively. The R-biotype was also resistant to trifloxysulfuron, but was injured equivalent to the S-biotype from dicamba, glyphosate, and triclopyr.  Gene sequencing of the R-biotype revealed a Trp₅₇₄ to Leu substitution that has conferred resistance to ALS-inhibitors in previous research. This is the first report of ALS-resistance in spotted spurge.