DIFFERENTIAL HERBICIDE RESPONSE AMONG SULFONYLUREA-RESISTANT KOCHIA-SCOPARIA L ACCESSIONS

Three chlorsulfuron-resistant kochia accessions were tested for levels of resistance to sulfonylurea and imidazolinone herbicides, based on whole plant response and sensitivity of the target enzyme. The resistant Minot and Chester accessions were not affected by treatment with 175 g ha-1 chlorsulfuron, and I50 values for the Chester accession ranged from 22-fold (metsulfuron-methyl) to 196-fold (chlorsulfuron) higher than the susceptible Bozeman accession. However, the Chester accession was 1.5- to 2-fold more resistant than Minot to five sulfonylurea herbicides, as determined by acetolactate synthase (ALS) I50 values. The third resistant accession (Power) displayed an intermediate response and was only 2- to 5-fold more resistant than the susceptible Bozeman accession to all sulfonylurea herbicides tested. The Minot and Chester accessions were slightly cross-resistant to four imidazolinone herbicides, ranging from 2-fold (imazamethabenz, imazethapyr, and imazaquin) to 6-fold (imazapyr) more resistant than the Bozeman accession, but cross-resistance levels did not differ appreciably between the Minot and Chester accessions. The Power accession was not cross-resistant to the four imidazolinone herbicides tested. The results demonstrate that degrees of ALS resistance and cross-resistance are highly variable among kochia populations: these differences may be due to the type of mutation in the gene encoding ALS.