Occurrence and distribution of glyphosate-resistant giant ragweed (Ambrosia trifida L.) in southwestern Ontario

Vink, J. P., Soltani, N., Robinson, D. E., Tardif, F. J., Lawton, M. B. and Sikkema, P. H. 2012. Occurrence and distribution of glyphosate-resistant giant ragweed (Ambrosia trifida L.) in southwestern Ontario. Can. J. Plant Sci. 92: 533-539. Giant ragweed is the first confirmed glyphosate-resistant (GR) weed in Canada. A survey was conducted to document the distribution of GR giant ragweed in southwestern Ontario. Giant ragweed seed was collected from 102 sites in Essex (70), Chatham-Kent (21), Lambton (10) and Waterloo counties (1) during the autumn of 2009 and 2010 prior to soybean harvest. Plants were grown in a growth room, sprayed with glyphosate at 1800 g a.e. ha(-1) and classified as resistant or susceptible. GR giant ragweed has been confirmed at 47 new locations in three counties (Essex, Chatham-Kent and Lambton). The results from this survey indicate that GR giant ragweed biotypes occur across a greater area in southwestern Ontario than originally thought. This survey provides an important baseline for future surveys in the province.