Metsulfuron is used for POST 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 with 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 acetolactate synthase (ALS) inhibitors in
previous research. This is the first report of ALS resistance in spotted
spurge. More importantly, this is the first report of a herbicide-resistant
broadleaf weed from a turfgrass system in the United States.