Glyphosate Resistant Weed Update

Kristen Callow, Weed Management Program Lead – Horticulture
From ONvegetables in The Grower, December 2012

There has been a lot of work conducted by Dr. Peter Sikkema, Dr. Francois Tardif and many graduate students (Joe Vink, Holly Byker, Joanna Follings) at the University of Guelph to determine the extent of glyphosate resistant weeds in Ontario and potential management options.

Currently, there are three species with resistance to glyphosate in Ontario, giant ragweed since 2008 (71 locations), Canada fleabane since 2010 (84 total locations) and common ragweed (1 location).

  • Glyphosate resistant giant ragweed has been confirmed in Essex, Kent, Lambton and Lennox Addington counties.  Three of these sites have confirmed multiple resistance to Group 9 (glyphosate) and Group 2 (cloransulam).
  • Glyphosate resistant Canada fleabane has been confirmed in Essex, Kent, Lambton, Elgin and Niagara counties.  Five of these sites have confirmed multiple resistance to Group 9 (glyphosate) and Group 2 (cloransulam).
  • Glyphosate resistant common ragweed has been confirmed in Essex county.
Sites With Glyphosate Resistant Giant Ragweed, 2008-11
Sites With Glyphosate Resistant Giant Ragweed, 2008-11
Sites With Glyphosate Resistant Canada Fleabane,  2010-11
Sites With Glyphosate Resistant Canada Fleabane, 2010-11

What does this mean for horticulture producers?

Half of the 24 glyphosate resistant weeds worldwide have been found in perennial cropping environments, specifically orchards.  The glyphosate resistant weeds currently in Ontario could very easily become problems in horticulture crops.  If you suspect resistance get your weeds tested, do not wait years to verify.  Confirmation testing is confidential and free of charge: www.plant.uoguelph.ca/resistant-weeds/  or contact kristen.callow@ontario.ca and please, please, please do not let suspect weeds go to seed.  Plan your herbicide program based on weed species and suspected resistance.

Giant Ragweed in Soybeans
Giant Ragweed in Soybeans
Mature Canada Fleabane in Soybean
Mature Canada Fleabane in Soybean
The Future: Fields With More Than One Glyphosate Resistant Weed Species (Giant Ragweed and Canada Fleabane)
The Future: Fields With More Than One Glyphosate Resistant Weed Species (Giant Ragweed and Canada Fleabane)

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