HORSEWEED (Conyza canadensis) with GROUP G/9 resistance: (INHIBITION OF ENOLPYRUVYL SHIKIMATE PHOSPHATE SYNTHASE ) Inhibition of EPSP synthase
MUTATION: PROLINE 106 to SERINE
Horseweed (Conyza canadensis) is a dicot plant in the asteraceae family. A single amino acid substitution from Proline 106 to Serine has led to resistance to Inhibition of Enolpyruvyl Shikimate Phosphate Synthase as indicated in the table below.		Horseweed

Chemical Family	Example Herbicide	Resistance Level
Glycines	glyphosate	Intermediate Resistance < 10 fold

REFERENCES

Page, E., C. Granger, F. Tardif, I. Rajcan, M. Laforest, and R.E. Nurse. 2017. A chrono-geographical assessment of Canadian populations of Conyza canadensis. WSSA Proceedings 57 : 259 - .

In the decade since the introduction of herbicide tolerant (HT) crops, an increasing number of weed species have been identified with resistance to the widely utilized herbicide, glyphosate. In the USA, there are now 14 weed species with confirmed resistance to the non-selective herbicides utilized in these systems. Similarly in Canada, there are 5 weed species that exhibit glyphosate resistance (GR); two of which were documented in the last year alone. Conyza canadensis has developed GR in both countries and its capacity for long-distance dispersal raises the potential of cross-border movement of herbicide resistant traits. The objective of this research was to examine the origins of GR C. canadensis populations from SW Ontario and explore their mechanism(s) of resistance. New and historic populations of C. canadensis collected throughout the counties comprising the most SW portion of the province of Ontario, as well as from bordering states of the USA. Eight microsatellite markers were used to characterize the relatedness of these populations and the target site gene for glyphosate (EPSPS2) was also sequenced. Dose-response studies were carried out to ascertain or confirm the resistance levels of the various populations. Results from the microsatellite and dose response studies indicate that the majority of Canadian GR populations are distinct from the American populations tested and exhibit notably higher resistance. Sequencing of EPSPS2 confirmed that, while GR in American populations is mediated by non-target site mechanisms, the predominate mechanism of resistance in Canadian populations of C. canadensis is target site mediated. These results represent the first report of target site mediate resistance to glyphosate in C. canadensis and clearly expose geographically distinct mechanisms of resistance to a given active ingredient on a regional scale within a single highly mobile species..

This case was entered by Todd Gaines Email: todd.gaines@colostate.edu

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