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

Authors: Nandula, Vijay; GIACOMINI, DARCI - University Of Illinois; LAWRENCE, BENJAMIN - Delta Research & Extension Center; Molin, William; BOND, JASON - Delta Research & Extension Center

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/2019
Publication Date: 3/2/2020
Citation: Nandula, V.K., Giacomini, D.A., Lawrence, B.H., Molin, W.T., Bond, J.A. 2020. Resistance to clethodim in Italian ryegrass (Lolium perenne ssp. multiflorum) from Mississippi and North Carolina. Pest Management Science. 76:1378-1385.

Interpretive Summary: Clethodim 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-resistant Italian ryegrass population from North Carolina was not controlled by clethodim. Scientists from the Crop Production Systems Research Unit of USDA-ARS, Mississippi State University, and University of Illinois characterized magnitude and mechanisms of clethodim resistance in the Mississippi and North Carolina Italian ryegrass populations. 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 to 30% of nontreated at the highest clethodim dose of 2.17 kg ha-1, (8X). The NC22 biotype was cross-resistant to sethoxydim, fluazifop, quizalofop, and 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. 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. The above results indicate the necessity of developing clethodim-resistant Italian ryegrass management strategies that include chemical, cultural, and mechanical tools.

Technical Abstract: 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 was cross-resistant to clethodim. This research characterized magnitude and mechanisms of clethodim resistance in the Mississippi and North Carolina Italian ryegrass populations. 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 to 30% of nontreated at the highest clethodim dose of 2.17 kg ha-1, (8X). The NC22 biotype was cross-resistant to sethoxydim, fluazifop, quizalofop, and 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. 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.