Comparison of acetolactate synthase enzyme inhibition among resistant and susceptible Xanthium strumarium biotypes

Failure to control Xanthium strumarium with acetolactate synthase (ALS) inh ibitor herbicides has been reported in Iowa and surrounding states. Single- seed descent techniques were used to isolate three X. strumarium biotypes: CAM-10 from near Cambridge, Iowa; Cole-25 from near Cole, Iowa; and Ohio-1 from Fulton County, Ohio. Ohio-1 and Cole-25 were selected because of appar ent resistance to imazethapyr, whereas CAM-10 was selected for observed sen sitivity to imazethapyr. The biotypes were assayed in vitro with three diff erent ALS inhibitor herbicides, and ALS activity was measured. The 50% inhi bition values (I-50) of ALS for imazethapyr were determined to be ninefold or higher for the Ohio-1 and Cole-25 biotypes compared to the CAM-10 biotyp e. The I-50 for imazaquin was determined to be about ninefold higher for th e Cole-25 biotype and sixfold higher for the Ohio-1 biotype when compared t o the CAM-10 biotype. All biotypes were equally sensitive to chlorimuron et hyl. The resistance was due to a single dominant nuclear gene.