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Canada Fleabane (Conyza canadensis) is an economically important, resistant weed species. The objective of the study was to characterize paraquat and diquat resistance in geographically isolated, evolutionarily unique biotypes of C. canadensis, and evaluate inheritance using segregating F2 families. Resistant (R) biotypes were crossed to a common susceptible (S), and amongst themselves. Discriminating and parental eliminating doses of paraquat and diquat were identified through dose responses. Results indicated F2 families deviated from expected 3:1 ratio for resistance to paraquat, indicating that resistance in C. canadensis was not attributable to single gene inheritance. Evaluated digenic models suggested an 11:5 ratio may best describe paraquat resistance. These results are some of the first to evaluate impacts of herbicide resistance on expression of transgressive segregation. F2 progeny from the paraquat RxS cross demonstrated transgressive segregation at discriminating and parental eliminating doses. Transgressive segregation was not observed in progeny from a paraquat RxR cross.

Reports of herbicide-resistant weeds continue to rise at an increasing rate, despite current efforts to mitigate this issue. The repeated use of glyphosate resulted in glyphosate-resistant Canada fleabane [Conyza canadensis (L.) Cronq.] populations in Delaware, USA, in 2001, and Ontario, Canada, in 2010. Research has reported rye (Secale cereale L.) cover crops consistently reduced the density and height of Canada fleabane. This research examined if fall-seeded rye can suppress Canada fleabane seedling density and growth, then the interaction(s) of multiple selection pressures should enhance Canada fleabane control which aids farmers in developing integrated weed management strategies. In 2018 and 2019, field studies were conducted to evaluate the interactions between fall-seeded rye cover crops, shallow fall tillage, and spring-applied herbicides. Interactions between selection pressures were observed to have an additive effect on improving weed control; environmental variables, however, had a direct influence on these interactions. The Ontario Ministry of Agriculture, Food and Rural Affairs, Grain Farmers of Ontario, The Canadian Agricultural Partnership

Farmers in southern Ontario face a significant threat from glyphosate-resistant weeds such as Canada fleabane and giant ragweed. Left uncontrolled, glyphosate-resistant weed interference in soybean can result in substantial yield loss. The three objectives of this study were: 1) to determine the biologically effective rate of isoxaflutole, metribuzin, and glyphosate on glyphosate-resistant Canada fleabane and giant ragweed biotypes, 2) to assess the response of isoxaflutole and glyphosate applied as a tank-mix on glyphosate-resistant and glyphosate-susceptible biotypes of both species, and 3) to ascertain the response of isoxaflutole, metribuzin and glyphosate tank-mix on the glyphosate-resistant biotypes of both species. In growth room experiments, antagonism is observed at low rates of glyphosate plus isoxaflutole in glyphosate-susceptible giant ragweed and at field rates of the same tank-mix in glyphosate-resistant Canada fleabane. The addition of metribuzin to the tank-mix resulted in an additive or synergistic response across all rates in the glyphosate-resistant biotypes of both species tested. Field experiments confirmed the additive and synergistic response of the three-way tank-mix in both species tested. An estimated 631 (126+504) and 420 (84+336) g a.i. ha-1 of isoxaflutole plus metribuzin in a 1:4 ratio are required for control of glyphosate-resistant giant ragweed and Canada fleabane respectively. Bayer CropScience