Target and Non-target site Mechanisms Confer Resistance to Glyphosate in Canadian Accessions of Conyza canadensis


Page, E.R., Grainger, C.M., Laforest, M., Nurse, R.E., Rajcan, I., Bae, J., Tardif, F.J. (2018). Target and Non-target site Mechanisms Confer Resistance to Glyphosate in Canadian Accessions of Conyza canadensis. Weed Science, [online] 66(2), 234-245.

Plain language summary

Canada fleabane was the first weed in North America to no longer be controlled by the herbicide glyphosate, also known as resistance. Since the first report from Delaware in 2001, glyphosate resistant Canada fleabane has been reported in Ohio (2002), Pennsylvania (2003), Michigan (2007) and Ontario, Canada (2010), among others. In Canada, glyphosate resistant Canada fleabane has been observed with increasing frequency along a corridor stretching from the first reported case in the most southwesterly corner of Ontario to the northeastern border of the Province, just south of Ottawa. An experiment was conducted to examine how related 98 Canada fleabane collections were and to determine the mechanism(s) causing this resistance. Molecular markers were used to examine relatedness of these groups by comparing the DNA sequences for the gene that glyphosate acts upon within the plant, known as the target site. Results indicate that a majority of glyphosate resistant Canada fleabane collections from Ontario possessed a single change in the target site gene that decreases the ability of glyphosate to bind to it, thereby conferring resistance. Fleabane collections possessing this substitution were more resistance and were observed to form a subpopulation that was genetically distinct from other Ontario groupings. None of the previous cases of glyphosate resistant Canada fleabane from around the world have observed the target site change documented in this study and it remains unclear as to why this mechanism has been selected for in Ontario alone.


Glyphosate-resistant populations of Conyza canadensis have been spreading at a rapid rate in Ontario, Canada, since first being documented in 2010. Determining the genetic relationship among existing Ontario populations is necessary to understand the spread and selection of the resistant biotypes. The objectives of this study were to: (1) characterize the genetic variation of C. canadensis accessions from the province of Ontario using simple sequence repeat (SSR) markers and (2) investigate the molecular mechanism (s) conferring resistance in these accessions. Ninety-eight C. canadensis accessions were genotyped using 8 SSR markers. Germinable accessions were challenged with glyphosate to determine their dose response, and the sequences of 5-enolpyruvylshikimate-3-phosphate synthase genes 1 and 2 were obtained. Results indicate that a majority of glyphosate-resistant accessions from Ontario possessed a proline to serine substitution at position 106, which has previously been reported to confer glyphosate resistance in other crop and weed species. Accessions possessing this substitution demonstrated notably higher levels of resistance than non-target site resistant (NTSR) accessions from within or outside the growing region and were observed to form a subpopulation genetically distinct from geographically proximate glyphosate-susceptible and NTSR accessions. Although it is unclear whether other non-target site resistance mechanisms are contributing to the levels of resistance observed in target-site resistant accessions, these results indicate that, at a minimum, selection for Pro-106-Ser has occurred in addition to selection for non-target site resistance and has significantly enhanced the levels of resistance to glyphosate in C. canadensis accessions from Ontario.