Pathotypes of Plasmodiophora brassicae from clubroot resistant canola and assessment of amisulbrom for clubroot control

  • Author / Creator
    Hollman, Keisha
  • Clubroot, caused by Plasmodiophora brassicae Woronin, is an important soilborne disease of canola (oilseed rape; Brassica napus L.). In Canada, clubroot management relies heavily on the planting of resistant cultivars, but since 2013, resistance has been broken in an increasing number of fields. Prior to the introduction of resistance, P. brassicae pathotype 3H, as defined on the Canadian Clubroot Differential (CCD) set, was predominant in Alberta. In testing of pathogen collections from 2014-2016, however, pathotype 3A was most common, indicating rapid shifts in the pathogen population. Up-to-date knowledge of pathotype composition is important for effective resistance breeding and stewardship. Furthermore, strategies to supplement resistance, such as the application of fungicides, may also contribute to sustainable clubroot management. In this thesis, isolates of P. brassicae were obtained from 166 canola crops in Alberta, Saskatchewan and Manitoba, and evaluated for pathotype designation on the CCD set and the differentials of Somé et al. Seventeen pathotypes were detected on the CCD set, including the previously reported pathotypes 3A, 3D, 3H, 5L, 5X, 8E, 8N and 8P, plus the novel pathotypes 2C, 6D, 8D, 9A, 9B, 9C, 11A, 13A and 13B. Five pathotypes were identified on the hosts of Somé et al. including P1, P2, P3, P4 and P5, with P4 and P5 reported here from Canada for the first time. The majority of the isolates, representing 39 fields in 2017 and 92 fields in 2018, could overcome genetic resistance. In a second study, the fungicide amisulbrom was tested for its efficacy in controlling clubroot in field trials conducted in 2018 and 2019 with the clubroot resistant canola cultivar ‘CS2000’ and the susceptible cultivar ‘45H31’. The results in 2018 indicated a significant effect of cultivar on clubroot severity, but the application of different rates of amisulbrom did not result in significant differences in disease level or plant growth parameters. Flooding of many of the plots in 2019 precluded the acquisition of meaningful results in that year. The field data were supplemented with results from a greenhouse study conducted using the susceptible canola cultivar ‘45H31’ at low (1 × 104 resting spores/g soil mix) and high (1 × 107 resting spores/g soil mix) P. brassicae inoculum levels. Treatment of the potting mix with three rates of amisulbrom (500 g active ingredient (ai)/ha, 1000 g ai/ha, and 1500 g ai/ha) resulted in significant declines in clubroot severity, and increases in plant height and aboveground weight, relative to the untreated control at both inoculum levels. Collectively, the results from this thesis suggest significant diversity in the virulence of P. brassicae populations and an increasing prevalence of resistance-breaking P. brassicae strains, as well as some potential for amisulbrom to reduce clubroot severity, at least under greenhouse conditions.

  • Subjects / Keywords
  • Graduation date
    Spring 2021
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.