Cross‐resistance profile of mesosulfuron‐methyl‐resistant Italian ryegrass in the southern United States

Diclofop‐resistant Lolium species (ryegrass) is a major weed problem in wheat production worldwide. This study was conducted to determine the resistance pattern of diclofop‐resistant ryegrass accessions from the southern United States to mesosulfuron‐methyl, a recently commercialized herbicide for ryegrass control in wheat; to determine the cross‐resistance pattern of a Lolium multiflorum Lam. (Italian ryegrass) accession, 03‐1, to acetolactate synthase (ALS) and acetyl‐CoA carboxylase (ACCase) inhibitors; and to determine the resistance mechanism of Italian ryegrass to mesosulfuron‐methyl. Seventeen ryegrass accessions from Arkansas and Louisiana, including standard resistant and susceptible accessions, were used in this experiment. Fourteen of the 17 accessions were more resistant (four‐ to > 308‐fold) to diclofop than the standard susceptible biotype. One accession, 03‐1, was resistant to mesosulfuron‐methyl as well as to other ALS inhibitor herbicides such as chlorsulfuron, imazamox and sulfometuron. Accession 03‐1, however, did not show multiple resistance to the ACCase inhibitor herbicides diclofop, fluazifop, clethodim, sethoxydim and pinoxaden, nor to glyphosate. The in vivo ALS activity of the 03‐1 biotype was less affected by mesosulfuron‐methyl than the susceptible biotype. This indicates that the resistance mechanism of Italian ryegrass to mesosulfuron‐methyl is partly due to an alteration in the target enzyme, ALS. It is concluded that diclofop‐resistant ryegrass in the southern United States can be generally controlled by mesosulfuron‐methyl. However, mesosulfuron‐methyl must be used with caution because not all ryegrass populations are susceptible to it. There is a need for more thorough profiling of ryegrass resistance to herbicides. Copyright © 2007 Society of Chemical Industry