Henbit is a facultative broadleaf winter annual in the Lamiaceae family.
Acetolactate synthase (ALS) inhibitors are primarily used to control a broad
spectrum of weeds, including henbit. During 2012 to 2013, field applications
of ALS-inhibiting herbicides were ineffective in controlling a henbit
population from Marion County, KS (MCK). To confirm field-evolved resistance
to ALS inhibitors, response of MCK henbit and a known susceptible henbit
population from Kansas (DPS) to varying doses of three different ALS
inhibitors were examined: chlorsulfuron, imazamox, and propoxycarbazone.
Results of the dose–response experiments suggest that the MCK population is
highly resistant to chlorsulfuron (resistance index [R/S] > 1,000) and
propoxycarbazone (R/S = 331) but is susceptible to imazamox. A full-length
ALS gene sequence obtained using the 5′- and 3′- rapid
amplification of complementary DNA ends approach revealed a
Pro197 to Arg point mutation (a common mutation that confers
resistance to sulfonylurea herbicides, e.g., chlorsulfuron) in the MCK
henbit. No other known resistance-conferring mutations were found in the
study. Evolved resistance to major classes of ALS inhibitors in the MCK
henbit will reduce herbicide options for its control. To our knowledge, this
is the first case of evolution of herbicide resistance in henbit.