Linuron resistant common ragweed (Ambrosia artemisiifolia) populations in Quebec carrot fields: presence and distribution of target site and non-target site resistant biotypes.

Common ragweed is frequently observed in Québec carrot fields. Carrot growers essentially rely on the herbicide linuron to control this weed. Common ragweed plants resistant to this herbicide had been identified before at one location in Québec. Consequently, a survey was conducted in 2012 and 2013 to determine if other populations were present in Québec and what the mechanism of resistance was. Plants suspected to be resistant were collected in fields and seeds from these plants were planted in the greenhouse. The seedlings were sprayed with linuron and genetic tests were done. Common ragweed was the weed most frequently suspected to be resistant (95% of accounts) and 94% of these ragweed populations were diagnosed as resistant. A new mutation in the gene targeted by the herbicide was found in 37.5% of resistant populations tested. No mutations in this gene, known to confer resistance to linuron, were found in the other resistant populations. Except for two populations, resistant plants with the mutation in the target gene were located in the muck soil production area while the others were found in sandy fields located in a different area. To our knowledge, this is the first report of a particular mutation (Val219Ile in the psbA gene) conferring linuron resistance in common ragweed and of the presence of another mechanism of evolved resistance to linuron in this weed.