RESPONSE OF A CHLORSULFURON-RESISTANT BIOTYPE OF KOCHIA-SCOPARIA TO SULFONYLUREA AND ALTERNATIVE HERBICIDES

Kochia growing on an industrial site where chlorsulfuron was applied r epeatedly over several seasons was confirmed to be resistant to chlors ulfuron and several other acetolactate synthase (ALS) -inhibiting herb icides. In growth room experiments, resistant (R) plants were 2 to >18 0 times more resistant to five sulfonylurea herbicides and one imidazo linone herbicide (imazethapyr) than susceptible (S) plants, as measure d by the ratio of dosages required to inhibit shoot dry matter accumul ation by 50% (GR50 R/S). Similarly, in vitro assays of ALS activity in dicated that from 3 to 30 times more herbicide was required to inhibit the enzyme from R plants than from S plants. Results of ALS enzyme as says indicated that R kochia was approximately equally resistant to me tsulfuron, triasulfuron, and thifensulfuron, and 2.5 times more resist ant to tribenuron than thifensulfuron. However, the response of R koch ia growing in a spring wheat crop in the field was not consistent with results of the ALS enzyme assays. In field experiments, thifensulfuro n at 32 g ai ha-1 had little effect on R kochia. In contrast, metsulfu ron, triasulfuron, and tribenuron at 8 g ha-1 did not reduce R kochia seedling densities, but caused severe stunting such that 2 mo after tr eatment the shoot biomass of plants in untreated plots was four times greater than in sprayed plots. Herbicides with alternative modes of ac tion including fluroxypyr, bromoxynil/MCPA ester, dichlorprop/2,4-D es ter, and 2,4-D ester provided good control of R kochia in the field. Q uinclorac did not reduce kochia densities, but surviving plants were s tunted. To delay or avoid development of ALS inhibitor-resistant kochi a populations, these alternative herbicides applied alone or in tank m ixtures could be incorporated into a herbicide rotation.