Author(s): R. A. SALAS, N. R. BURGOS, A. MAUROMOUSTAKOS, R. B. LASSITER, R. C. SCOTT AND E. A. ALCOBER

Abstract: Lolium perenne ssp. multiflorum (Italian ryegrass) is a major weed problem in wheat production. Being an obligate outcrossing species, it has a high propensity to evolve resistance to herbicides. This study was conducted to determine the level of resistance and resistance patterns of Italian ryegrass populations in the United States to ACCase- (diclofop and pinoxaden) and ALS (imazamox, mesosulfuron, and pyroxsulam) inhibitors. Dose-response bioassays were conducted on 47 populations from suspect herbicide-resistant fields in the southern United States (Arkansas, Mississippi, Georgia, North Carolina, South Carolina,Virginia), Kansas, and Washington collected between 2008 and 2011 from fields suspected of resistance to ALS inhibitors. Eighty-seven percent of the populations were resistant to diclofop. Eight of the diclofopresistant populations (25%) were also resistant to pinoxaden. Thirty-eight diclofop-resistant populations (81%) were resistant to at least one ALS inhibitor. Thirty-nine populations (83%) were resistant to mesosulfuron, 38 of which were also resistant to pyroxsulam. All mesosulfuron-resistant populations tested were cross-resistant to imazamox and pyroxsulam. Cross-resistance patterns to ALS inhibitors differed. Of 45 populations with resistance to ACCase or ALS inhibitors, 35 (78%) had multiple resistance to both modes of action. The majority of diclofop-resistant L. perenne populations can be controlled with pinoxaden, but widespread resistance to pinoxaden can evolve soon if it is intensively used. A comprehensive weed management approach prior to planting will be critical in managing L. perenne in wheat and preventing or delaying resistance evolution.

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