Resistance to clethodim in Italian ryegrass (Lolium perenne ssp. multiflorum) from Mississippi and North Carolina

• Published in: Pest management science 2020-04, Vol.76 (4), p.1378-1385
• Main Authors: Nandula, Vijay K, Giacomini, Darci A, Lawrence, Benjamin H, Molin, William T, Bond, Jason A
• Format: Article
• Language: English
• Subjects: ACCase; Acetyl-CoA Carboxylase; Biotypes; Chemical control; clethodim; Cyclohexanones; Glyphosate; herbicide; Herbicide Resistance; Herbicides; Italian ryegrass; Lolium; Lolium perenne; Lolium perenne ssp. multiflorum; Metabolism; Mississippi; Mutation; North Carolina; Organic chemistry; Piperonyl butoxide; Plant growth; Populations; Public domain; resistance; Sethoxydim; Weight reduction
• ISSN: 1526-498X; 1526-4998
• DOI: 10.1002/ps.5650

BACKGROUND Clethodim, an acetyl‐CoA carboxylase (ACCase)‐inhibiting herbicide, is one of the few postemergence chemical control options available to growers of Mississippi to manage glyphosate and/or other herbicide resistant Italian ryegrass populations. Recently, clethodim failed to adequately control Italian ryegrass populations across Mississippi. A sethoxydim, also an ACCase inhibitor, ‐resistant Italian ryegrass population from North Carolina was cross‐resistant to clethodim. This research characterized the magnitude and mechanisms of clethodim resistance in the Mississippi and North Carolina Italian ryegrass populations via whole‐plant herbicide dose response, cross resistance, and metabolism studies, and molecular analysis. RESULTS Two clethodim‐resistant biotypes from Mississippi, MS24 and MS37, were 10‐ and 4‐fold resistant, respectively, relative to a susceptible (SUS1) biotype. A North Carolina biotype, NC21, was 40‐fold resistant to clethodim compared to SUS1. Two additional biotypes from North Carolina, NC22 and NC 23, recorded shoot dry weight reduction of only 17–30% of nontreated at the highest clethodim dose of 2.17 kg ha−1, (8×). The NC22 biotype was cross‐resistant to sethoxydim, fluazifop, quizalofop, and pinoxaden. Metabolic inhibitors such as piperonyl butoxide and 4‐chloro‐7‐nitrobenzofurazan did not affect resistance of MS37, MS51, and NC22 biotypes to fenoxaprop, clethodim, or pinoxaden. The MS37 biotype had three target site mutations, I2041N, C2088R, and G2096A. Another clethodim‐resistant biotype from Mississippi, MS51, had only the C2088R substitution. The NC22 and NC23 biotypes had I1781L, I2041N, and D2078G replacements. CONCLUSION This study shows that the mechanism of resistance to clethodim in Italian ryegrass from Mississippi and North Carolina is due to target site modifications in the ACCase gene leading to broad cross‐resistance to other ACCase‐inhibiting herbicides. Published 2019. This article is a U.S. Government work and is in the public domain in the USA. This research characterized the magnitude and mechanisms of clethodim resistance in the Mississippi and North Carolina Italian ryegrass populations via whole‐plant herbicide dose response, cross resistance, and metabolism studies, and molecular analysis.