Resistance to Acetolactate Synthase (ALS) Inhibitors in the United States: History, Occurrence, Detection, and Management

Abstract

Acetolactate synthase (ALS) inhibitors are highly potent herbicides that include the sulfonylureas, imidazolinones, triazolopyrimidine sulfonamides and pyrimidyl salicylates. Currently there are ALS-inhibiting herbicides for use in all major crops including maize, soybeans, rice, and cereals. These herbicides kill plants by inhibiting the first enzyme in the biosynthetic pathway of the branched chain amino acids. Resistance to ALS inhibitors can be conferred by single mutations at multiple sites within the ALS gene. The first case of resistance to ALS inhibitors was discovered in 1987 in a wild lettuce (Lactuca serriola) population in Idaho, U. S. Resistance to ALS inhibitors in the U. S. is now found in populations of 26 species in all of the major crops. In all the cases studied, the mechanism of resistance has been due to selection of an altered form of the ALS enzyme. Detection of resistance to ALS inhibitors can be done through whole plant screening, in vitro and in vivo enzyme assays. ALS resistance can be managed through the by using ALS inhibitors in an integrated system which utilizes other herbicides as well as mechanical, cultural and biological weed control methods.