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 field that had been treated with acetolactate synthase (ALS) inhibitors in 3 of the previous 6 yr. In greenhouse studies, this biotype was highly resistant ro the ALS inhibitors triasulfuron, thifensulfuron/tribenuron, and sulfometuron and moderately resistant to imazerhapyr; GR(50) values were > 16, > 5, > 1.0, and 9.9, respectively In addition, cross-resistance was identified to the auxin-type herbicide quinclorac (GR(50) value > 6.7) but not to fluroxypyr (GR50 value 1) or MCPA/mecoprop/dicamba. Quindorac had not been used previously in this field. Analysis of ALS extracted from the resistant biotype and a susceptible biotype from a nearby location indicated that resistance to ALS inhibitors was due to an altered target site with reduced sensitivity to a broad range of ALS inhibitors. The ALS I-50 values for triasulfuron, metsulfuron, 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.