Acetyl-CoA carboxylase herbicide resistance and fitness of spring milletgrass (Milium vernale) populations infesting winter cereal monoculture in Greece

Herbicide resistance constitutes a production threat in cereals monoculture. Two putative resistant (R1, R2) spring milletgrass (Milium vernale M. Bieb.) populations, originating from winter cereal monoculture fields in northwestern Greece, were studied to elucidate the levels and underlying mechanisms of resistance to acetyl-CoA carboxylase (ACCase)- and acetolactate synthase (ALS)-inhibiting post-emergence herbicides. Whole-plant rate-response assays showed that the R1 and R2 populations were highly cross-resistant to aryloxyphenoxypropionate ACCase-inhibitors clodinafop-propargyl and diclofop-methyl, but they were susceptible to the cyclohexanedione ACCase-inhibitor clethodim and the ALS-inhibitors mesosulfuron-methyl + iodosulfuron methyl-sodium and pyroxsulam. The analysis of the ACCase gene sequence revealed a point mutation (ATT to ACT, GTT, GCT) at 2041 position resulting in an amino acid substitution from isoleucine (Ile) to threonine (Thr), valine (Val) or alanine (Ala). By contrast, all sequenced plants of the S population were found with the wild-type allele encoding Ile-2041. The R1 population grown in the absence of competition produced more fresh weight and tillers than the S population, while both R1 and S populations grown in competition with winter wheat or barley produced similar fresh weight and tillers. Winter wheat or barley grown in competition with either R1 or S population produced similar fresh weight, tillers, and ears. Conclusively, two spring milletgrass populations have evolved target-site mediated cross-resistance to aryloxyphenoxypropionate herbicides, but the competitive ability of the R1 was similar to that of S population, suggesting lack of association between ACCase mutation and weed fitness.