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Studies into clubroot (Plasmodiophora brassicae) epidemiology and resistance

  • Author / Creator
    Botero, Andrea
  • Clubroot, caused by Plasmodiophora brassicae, is an important soilborne disease of canola (oilseed rape; Brassica napus). The effective and sustainable management of clubroot requires a deeper understanding of clubroot epidemiology and improved durability of host resistance. This research aimed to (i) develop a clubroot yield loss model for B. napus, (ii) characterize the spatial patterns associated with P. brassicae inoculum density and its relationship with soil pH and boron, calcium, and magnesium, and (iii) analyze the polygenic resistance harbored by a doubled haploid population obtained from a cross between ‘Aviso’ and ‘Montego’ (B. napus). The effect of clubroot on B. napus yield and yield-related parameters was evaluated under field and greenhouse conditions using the canola cultivars ‘45H31’ (susceptible), ‘45H29’ (1st generation resistance) and ‘CS2000’ (2nd generation resistance), following inoculation with different pathotypes of P. brassicae. The field experiment was conducted over 2 years in Edmonton, Alberta, in biosecure nurseries inoculated with low (5 × 104 resting spores per plant), intermediate (5 × 106 resting spores per plant) or high (5 × 108 resting spores per plant) concentrations of pathotype 5X or a mixture of pathotypes 5X and 3H. In the greenhouse experiment, the same cultivars were inoculated with pathotypes 5X, 3H or a 5X + 3H mixture at inoculum concentrations of 1 × 103, 1 × 104 and 1 × 106 resting spores per plant. In both the field and greenhouse, clubroot incidence and disease severity index (DSI) increased with increasing inoculum density; the highest levels of disease were observed in the susceptible cultivar, while the lowest were found in ‘45H29.’ Yield, pods per plant and 1000-grain weight decreased as the DSI increased in all cultivars. Yield was affected by DSI and canola cultivar, but not by pathotype. Regression analysis indicated that under greenhouse conditions, an increment of 1% in the DSI resulted in a decrease of 0.49% in yield; under field conditions, this percentage was reduced to 0.26%. While the rate of yield reduction was similar among cultivars, overall yield losses were lower in the clubroot resistant hosts, since clubroot was less severe. To study the spatial patterns of P. brassicae inoculum density and their relationship to different soil properties, four clubroot-infested fields in central Alberta were sampled in 2017 and 2019, and P. brassicae inoculum density, soil pH, and boron, calcium, and magnesium concentrations were quantified. Spatial autocorrelation of the inoculum density was estimated with the Moran’s I and semivariograms. A Bayesian hierarchical spatial approach was used to model the relationship between P. brassicae inoculum density and the soil parameters. Patchiness of the pathogen was detected, with most patches located at the field edges and adjacent to the entrance. Infested patches grew in size from 2017 to 2019, with an average increase in diameter of 221.3 m and with this growth determined by the maximum inoculum density and active dispersal methods such as movement by machinery and wind. Soil pH, boron, calcium, and magnesium concentrations were not found to have an important effect on the inoculum density of P. brassicae. Finally, the genetic control of DSI and resting spores per plant (RSP) was examined in a doubled haploid population consisting of 114 lines of winter oilseed rape, obtained from the cross‘Aviso’ × ‘Montego’ inoculated with P. brassicae isolate ‘eH’. Linkage analysis allowed the identification of three quantitative trait loci (QTLs) controlling DSI (pbBn_di_A02, PbBn_di_A04 and PbBn_di_C03). A significant decrease in DSI was observed when combining effects of the three resistance alleles at these QTLs. Only one QTL, PbBn_rsp_C03, was found to control RSP, reducing resting spore production by 40%. PbBn_di_C03 and PbBN_rsp_C03 partially overlapped, allowing the identification of a single region that controls both traits. Alignment of the genetic map with the reference genome of B. napus ‘Darmor-bzh’ indicated the presence of three genes related to disease resistance and defense in the overlapping region of PbBn_di_C03 and PbBN_rsp_C03. Consideration of both DSI and RSP in breeding for clubroot resistance is recommended for the long-term management of this disease.

  • Subjects / Keywords
  • Graduation date
    Fall 2021
  • Type of Item
    Thesis
  • Degree
    Doctor of Philosophy
  • DOI
    https://doi.org/10.7939/r3-jned-3q61
  • 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.