A Survey and Characterization of Linuron-Resistant Amaranthus spp. in Southern Ontario Carrot Production.
Carrot is an important vegetable crop in southern Ontario, Canada. Carrot growers rely heavily on linuron for weed control due to its crop safety and wide spectrum. High reliance on this herbicide has selected for resistance in Amaranthus (pigweed) species, which was documented in Ontario in 1999. However, numerous reports from growers suggested continuous selection and spread of resistance throughout carrot growing areas. The aim of this thesis was to survey carrot fields in Ontario and collect samples of Amaranthus powellii and A. retroflexus in order to confirm resistance to linuron. Thirty-eight field locations were sampled in the summer of 2011 and 89% of these contained linuron resistant pigweeds. DNA was extracted from the plants in order to sequence the psbA gene so as to determine the molecular basis for resistance. Most resistant plants had the previously documented Val219Ile mutation that confers moderate resistance to most phenylureas. Some populations had an Ala251Val that is correlated with high resistance to metribuzin, but also confers resistance to linuron. A new mutation, Phe274Val, was observed in a number of populations. The resistance pattern associated with this mutation was similar to that of the Val219Ile mutation. Two populations had a double mutation, Val219Ile and Phe274Val. These plants had a much higher level of resistance than plants with either single mutation. The results suggest that resistance to linuron has progressed since its first documentation in Ontario, and under intense, continuous selection, a range of different mutations has been selected. This makes management of these weeds less predictable.