Background: Due to the accessibility of underlying technologies the 'Omics', in particular genomics, are becoming commonplace in several fields of research, including the study of agricultural pests. The weed community is starting to embrace these approaches; genome sequences have been made available in the past years, with several other sequencing projects underway, as promoted by the International Weed Genome Consortium. Chromosome-scale sequences are essential to fully exploit the power of genetics and genomics.Results: We report such an assembly for Conyza canadensis, an important agricultural weed. Third-generation sequencing technology was used to create a genome assembly of 426 megabases, of which nine chromosome-scale scaffolds cover more than 98% of the entire assembled sequence. As this weed was the first to be identified with glyphosate resistance, and since we do not have a firm handle on the genetic mechanisms responsible for several herbicide resistances in the species, the genome sequence was annotated with genes known to be associated with herbicide resistance. A high number of ABC-type transporters, cytochrome P450 and glycosyltransferases (159, 352 and 181, respectively) were identified among the list of ab initio predicted genes.Conclusion: As C. canadensis has a small genome that is syntenic with other Asteraceaes, has a short life cycle and is relatively easy to cross, it has the potential to become a model weed species and, with the chromosome-scale genome sequence, contribute to a paradigm shift in the way non-target site resistance is studied.
Double cropping is not presently a common practice in Canada. The long-term climate averages, however, suggest that the practice should be possible in the most southern portions of the country. The study described herein represents the first simultaneous evaluation of three crops—maize, soybean, and white bean—seeded at five seeding dates spanning late June through early August in the most southern region of Canada. Germplasm was chosen such that physiological maturity could theoretically be reached if seeded following winter wheat. Results indicate that, following summer seeding, development in all three crops was primarily driven by thermal requirements. Only the pod filling stage of soybean was influenced by the declining daylengths of autumn, and this effect was most pronounced in germplasm of longer relative maturity. Yields of white bean and maturity group 00 soybean were unaffected by seeding up to the third week of July, whereas yield of maize and higher maturity group soybeans declined from June onward. For the latter, declining yields were primarily attributable to the interaction of seeding date and relative maturity and their effect on season length. These results clearly demonstrate that with the appropriate selection of germplasm, there is a seeding date window where maize, soybean, or white bean can be expected to reach physiological maturity as a double crop in Canada.
Palmer amaranth is one of the most economically important and widespread weeds of arable land in the United States. Although no populations are currently known to exist in Canada, its distribution has expanded northward such that it is present in many of the States bordering Canada and multiple pathways exist for its introduction. In this short communication we report on the transport of viable Palmer amaranth seed on imported sweet potato slips. A reproductive pair of Palmer amaranth seedlings were identified from soil accompanying imported sweet potato slips in 2018. Identification was confirmed using species specific single nucleotide polymorphisms.
The full spectrum of herbicide resistance in a weed can vary according to the mechanistic basis and cannot be implied from the selective pressure. Common ragweed (Ambrosia atemisiifolia L.) is an important weed species of horticultural crops that has developed resistance to linuron based on either target site or non target site resistance mechanisms. The objective of the study is to characterize the cross resistance to metribuzin of linuron selected biotypes of A. artemisiifolia with target site and non target site resistance and determine its genetic basis. Crosses were made between two types of linuron resistant biotype and linuron susceptible biotypes and the progeny were further backcrossed with susceptible plants to third backcross generation to determine their responses to both herbicides compared to parental lines. The target site based linuron resistant biotype was cross resistant to metribuzin and resistance to both herbicides was maintained at the same level in the BC3 line. In contrast, the linuron selected biotype with non target site resistance mechanism was not cross resistant to metribuzin. In addition, the BC3 lines deriving from the non target site resistant parents had very low-level resistance. While the target site resistance trait is maintained through multiple crosses, non target site-based resistance would be lost over time when selection is absent or insufficient to retain all genes involved in resistance as a complex trait. This would imply A. artemisiifolia biotypes with different mechanisms would need to be managed differently over time.
Herbicide resistance is selected for within a weed population through the recurrent use of an herbicide. Once the use of the herbicide is reduced or discontinued, the trait may persist in the population unless resistance endows a fitness penalty. Few studies have examined the long-term persistence of an herbicide resistance trait in a weed population once the selection pressure that led to its prevalence has been removed. The objective of this research was to re-survey the locations described by A.J.J. Smisek in her 1995 study of a paraquat resistant biotype of C. canadensis on the grounds of the Harrow Research and Development Centre. Results indicate that, ∼20 yr after paraquat use was significantly reduced, a glyphosate resistant biotype had replaced the paraquat resistant biotype at all but one of the original locations surveyed by Smisek. The sole location where the paraquat resistant biotype was observed was an old, un-treed orchard row currently managed by mowing. This location not only served as a refuge for the paraquat resistant biotype, ensuring its persistence in the face of strong selection for glyphosate resistance at all other locations, but it facilitated the co-occurrence of glyphosate and paraquat resistant biotypes. The observation of a multiple resistant biotype at this location alone, with resistance to both paraquat and glyphosate, suggests a role for refugia in the stacking of herbicide resistance traits. Given that this stacking occurred in the absence of either herbicide, we hypothesize that the multiple resistant biotype arose through pollen mediated gene flow among biotypes.
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