Field testing rutabaga cultivars for resistance to cabbage maggot
Project Code: PRR12-110
Peggy Dixon - Agriculture and Agri-Food Canada
Field tests to compare resistance to cabbage maggot in rutabaga between selected lines and the commercial cultivar Laurentian
Summary of Results
Brassicaceae crops in all production regions in Canada suffer from losses attributed to the cabbage maggot, Delia radicum L. (Diptera: Anthomyiidae) and closely-related species. Its management relies primarily on applications of chemical insecticides. Three active ingredients are currently registered for use against this pest, including cypermethrin (in British Columbia only), cyantraniliprole, and chlorpyrifos. When this project was initiated, growers relied heavily on chlorpyrifos; however, research in 2013 confirmed that 75% of populations tested in British Columbia (BC) were chlorpyrifos-resistant. Additional studies are currently ongoing to document the extent of resistance to chlorpyrifos in Brassica growing regions nationwide. Reduced risk alternatives are required to replace conventional insecticides and to become part of routine integrated management of cabbage maggot in these crops.
The project aimed to field-test new rutabaga lines developed by the University of Guelph under Risk Reduction project PRR10-140 (Development of rutabaga cultivars resistant to Cabbage maggot). These lines contain a gene of resistance to cabbage maggot that originated from a cruciferous weed, Sinapis alba. The gene was first introduced into canola and from there into Laurentian rutabaga by means of backcrossing cycles. The genetic makeup of the final progeny is 93.5% rutabaga and 6.5% canola. These lines were field-tested in Ontario as part of the breeding project and were found to exhibit significantly lower levels of root maggot damage as compared with the unsprayed Laurentian check. Horticulturally, the resistant lines resemble the Laurentian rutabaga in terms of root shape, size, colour, and flavour.
In the current project the resistant lines were field-tested in rutabaga growing regions outside of Ontario. The lines available in 2012 were the second backcross generation (BC2). They contained the resistant gene but only 87% rutabaga genome overall. Trials with 4 of these lines were conducted that year at Agriculture and Agri-Food Canada in St. John’s, Newfoundland (NL), Bouctouche, New Brunswick (NB), and Abbotsford, BC, and at the University of Saskatchewan in Saskatoon, Saskatchewan. In 2014, trials with 7 new lines that contained 93.5% rutabaga genome (BC3) were conducted in the same locations, with the exception of Bouctouche, NB, which was not available for the trials. In 2016, trials were conducted at St. John’s, NL only, with 2 new lines and one line from 2014. Tests were set in a Latinized Block design with one treatment per block and four blocks per site. Data collected include damage ratings, as well as weight and marketability of the harvested crop. Infestation indices were calculated from collected data.
The lines tested in 2012 did not differ from the untreated Laurentian with respect to cabbage maggot damage, as measured by the infestation index and number of rutabagas harvested with visible damage. Due to the insufficient percentage of rutabaga genome in those early lines (87%), some did not show the desired horticultural traits that would make them marketable. This included roots that were too small, too large, had multiple roots, or showed signs of rot and/or other diseases. Line 49, for example, had disfigured roots characterised by side roots and an undetermined rot. Line 26 appeared to be more susceptible to club root (although the pathogen was not confirmed) and produced rutabagas of a more conical shape as compared with the spherical Laurentian.
Most of the new lines tested in 2014, which contained 93.5% rutabaga genome, displayed the desired horticultural characteristics in terms of seedling vigor and root size, shape, and colour. Rating of cabbage maggot damage was not possible at the Saskatchewan site due to extensive rot – likely a bacterial soft rot – triggered by root cracking. This phenomenon is typical under conditions of erratic temperatures and soil moisture, which lead to excessively rapid growth of the roots. The results from NL and BC showed that as measured by infestation index and % control, line #20 performed significantly better than all other lines, including untreated Laurentian. The performance of this line was similar to treated Laurentian. In 2016, however, infestation index and % control were similar across all lines, therefore the experimental lines were not different from untreated Laurentian.
Considering the results from Ontario, where a significant difference was recorded between the resistant lines and the commercial Laurentian, the picture arising suggests regional differences in the performance of the resistant lines. This may be attributed to various factors, including, but not limited to, the species of Delia flies that are prevalent in the different areas, the cultural practices of growing rutabaga that vary between regions, and environmental conditions. Outside the scope of this project, trials are currently being conducted under controlled conditions in Newfoundland to allow for a better estimate regarding the potential of these lines to become part of the Integrated Pest Management against the cabbage maggot in that area.
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