Prevalence of cross- or multiple resistance to the acetyl-coenzyme A carboxylase inhibitors fenoxaprop, clodinafop and pinoxaden in black-grass (Alopecurus myosuroides Huds.) in France

BACKGROUND: Repeated use of acetyl-CoA carboxylase (ACCase) inhibitors, especially fenoxaprop and clodinafop, since the late 1980s has selected for resistance in Alopecurus myosuroides Huds. (black-grass) in France. We investigated whether resistance to pinoxaden, a phenylpyrazoline ACCase inhibitor to be marketed in France, was present in French black-grass populations. We investigated pinoxaclen resistance conferred by five mutant ACCase isoforms. Using 84 French black-grass field samples, we also compared the frequencies of other mechanisms endowing resistance to fenoxaprop, clodinafop or pinoxaclen. RESULTS: ACCase mutant isoforms Leu-1781, Gly-2078 and, likely, Cys-2027 conferred cross-resistance to pinoxaclen, while isoform Asn-2041 possibly conferred moderate resistance. Other mechanisms of resistance to fenoxaprop, clodinafop and pinoxaclen were detected in 99, 68 and 64% of the samples investigated, respectively. Cross- or multiple resistance to fenoxaprop or clodinafop and pinoxaclen was not systematically observed, suggesting a diversity of mechanisms exist. CONCLUSION: Pinoxaden resistance was observed before pinoxaclen release in France. Only a fraction of the mechanisms endowing fenoxaprop or clodinafop resistance also confer pinoxaclen resistance. Pinoxaden resistance was likely mostly selected for by ACCase inhibitors, and, in some cases, possibly by herbicides with other modes of action. This illustrates the necessity to use metabolisable herbicides cautiously where black-grass has evolved non-target-site-based resistance. (C) 2009 Society of Chemical Industry