Cross-resistance Patterns to ACCase-inhibiting Herbicides Conferred by Mutant ACCase Isoforms in Alopecurus Myosuroides Huds. (black-grass), Re-examined at the Recommended Herbicide Field Rate

Background: Target-site-based resistance to acetyl-CoA carboxylase (ACCase) inhibitors in Alopecurus myosuroides Huds. is essentially due to five substitutions (Isoleucine-1781-Leucine, Tryptophan-2027-Cysteine, Isoleucine-2041-Asparagine, Aspartate-2078-Glycine, Glycine-2096-Alanine). Recent studies suggested that cross-resistance patterns associated with each mutation using a seed-based bioassay may not accurately reflect field resistance. The authors aimed to connect the presence of mutant ACCase isoform(s) in A. myosuroides with resistance to five ACCase inhibitors (fenoxaprop, clodinafop, haloxyfop, cycloxydim, clethodim) sprayed at the recommended field rate.

Results: Results from spraying experiments and from seed-based bioassays were consistent for all mutant isoforms except the most widespread, Leucine-1781. In spraying experiments, Leucine-1781 ACCase conferred resistance to clodinafop and haloxyfop. Some plants containing Leucine-1781 or Alanine-2096 ACCase, but not all, were also resistant to clethodim.

Conclusion: Leucine-1781, Cysteine-2027, Asparagine-2041 and Alanine-2096 ACCases confer resistance to fenoxaprop, clodinafop and haloxyfop at field rates. Leucine-1781 ACCase also confers resistance to cycloxydim at field rate. Glycine-2078 ACCase confers resistance to all five herbicides at field rates. Only Glycine-2078 ACCase confers clethodim resistance under optimal application conditions. It may be that Leucine-1781 and Alanine-2096 ACCases may also confer resistance to clethodim in the field if the conditions are not optimal for herbicide efficacy, or at reduced clethodim field rates.