RESISTANCE TO ACETOLACTATE SYNTHASE INHIBITORS AND QUINCLORAC IN A BIOTYPE OF FALSE CLEAVERS (GALIUM-SPURIUM)

A false cleavers population that survived treatment with triasulfuron/ brornoxynil in 1996 was identified in central Alberta, Canada, in a fi eld that had been treated with acetolactate synthase (ALS) inhibitors in 3 of the previous 6 yr. In greenhouse studies, this biotype was hig hly resistant ro the ALS inhibitors triasulfuron, thifensulfuron/tribe nuron, and sulfometuron and moderately resistant to imazerhapyr; GR(50 ) values were > 16, > 5, > 1.0, and 9.9, respectively In addition, cro ss-resistance was identified to the auxin-type herbicide quinclorac (G R(50) value > 6.7) but not to fluroxypyr (GR50 value 1) or MCPA/mecopr op/dicamba. Quindorac had not been used previously in this field. Anal ysis of ALS extracted from the resistant biotype and a susceptible bio type from a nearby location indicated that resistance to ALS inhibitor s was due to an altered target site with reduced sensitivity to a broa d range of ALS inhibitors. The ALS I-50 values for triasulfuron, metsu lfuron, chlorsulfuron, thifensulfuron, and imazethapyr were 36, 34, 92 , 36, and 14 times higher, respectively, for the resistant compared to the susceptible biotype. The mechanism of resistance to quinclorac is unknown. This is die first report of high-level herbicide resistance in this weed species.