Abstract
Sulfonylurea herbicide resistant Lactuca serriola L. (prickly lettuce) plants were discovered near Lewiston, Idaho in April 1987. This was the first confirmed occurrence of herbicide resistance resulting from the use of sulfonylurea. Kochia scoparia (L.) Schrad (kochia) plants from Liberal, Kansas were confirmed as resistant to chlorsulfuron and metsulfuron-methyl in 1988. Since then, sulfonylurea resistant K. scoparia has been identified in eight other States and one Canadian Province. Salsola iberica Sennen & Pau (russian thistle) plants from Kansas, Montana, North Dakota and Washington and Stellaria media (L.) Vill. (common chickweed) plants from Alberta, Canada have also been found to be resistant to sulfonylurea herbicides. Most resistant plants were collected from fields where dryland winter wheat (Triticum aestivum L.) was grown either continuously or in rotation with summer fallow and where chlorsulfuron or chlorsulfuron plus metsulfuron-methyl had been applied at 7 to 14 month intervals for 3 to 5 years. Total sulfonylurea herbicide used ranged from 52 to 200 g a.i.ha-1. Resistant K. scoparia plants have also been collected from noncrop areas where sulfometuron-methyl was applied annually for 3 to 4 years. Total sulfometuron-methyl used ranged from 96 to 425 g ai ha-1. The mechanism of resistance is an altered site of action, acetohydroxyacid synthase enzyme, which is inhibited less in resistant than in susceptible biotypes by sulfonylurea and imidazolinone herbicides. Resistance is not due to differences in herbicide absorption, translocation, or metabolism. Chlorsulfuron-resistant K. scoparia and L. serriola are resistant to several other sulfonylurea and imidazolinone herbicides. However, resistant and susceptible biotypes are usually controlled equally by herbicides with different modes of action. Resistance in L. serriola is controlled by one nuclear gene with incomplete dominance.
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