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Factors influencing Plasmodiophora brassicae resting spore loads in soil and clubroot disease severity in canola (Brassica napus) Open Access


Other title
Plasmodiophora brassicae
Type of item
Degree grantor
University of Alberta
Author or creator
Tabori, Mirko
Supervisor and department
Dr. Stephen Strelkov and Dr. Sheau-Fang Hwang
Examining committee member and department
Dr. Stephen Strelkov (Agricultural, Food and Nutrition Science)
Dr. Guillermo Hernandez (Renewable Resources)
Dr. Scott Chang (Renewable Resources)
Dr. Sheau-Fang Hwang (Government of Alberta)
Department of Agricultural, Food, and Nutritional Science
Plant Science
Date accepted
Graduation date
Master of Science
Degree level
Clubroot, caused by Plasmodiophora brassicae, is an important soilborne disease of Brassica napus canola. Studies were conducted to increase understanding of the impact of various soil parameters and crop rotation regimes on P. brassicae inoculum levels and clubroot severity, respectively. In one study, the concentration of pathogen resting spores was assessed in 284 soil samples, collected from three clubroot-infested fields (Bassano, Edmonton, and Parkland County, Alberta), by quantitative PCR analysis. The samples also were analyzed for soil organic matter content, electrical conductivity, pH, and boron and calcium concentrations. Data analysis based on the P. brassicae-positive samples pooled from the three fields indicated that resting spore concentration was positively correlated with soil organic matter (r2 =0.51, P <0.0001, N = 127), calcium (r2 =0.44, P <0.0001, N = 127) and boron concentration (r2 =0.11, P <0.0001, N = 127), but negatively correlated with soil pH (r2 =0.09, P = 0.0004, N = 127) and electrical conductivity (r2 =0.23, P <0.0001, N = 127), respectively. These correlations, while significant, were generally weak and suggest that other factors, such as cropping history, may affect resting spore concentrations in the soil. In a second study, the effects of six crop rotation regimes on clubroot severity were evaluated under greenhouse conditions. Rotations included: T1) continuous cropping of a clubroot-susceptible canola cultivar (S); T2) S –fallow (F) - S – F - S; T3) S – barley (B) - S – B – S; T4) S – resistant canola cultivar (R) - S – R – S; T5) continuous cropping of the same resistant cultivar; and T6) continuous cropping of different clubroot resistant canola cultivars. Index of disease (ID) was recorded along with other plant growth parameters for the canola at the end of each cycle of a rotation. When galled root tissue collected from a treatment was incorporated back into the soil, the average ID of the susceptible canola cultivar included in T1 to T4 increased significantly (p ≤ 0.05) from 93.1 % (after the first crop cycle) to 99.6 % (after the fifth crop cycle). The ID of the resistant canola cultivar included in T5 also increased significantly from 12.5 % to 22.3 %. In contrast, when different clubroot resistant canola cultivars were grown in succession in T6, the average ID decreased significantly from 12.5 % to 9.4 % (after the fifth cycle). The data suggest some erosion of resistance after continuous cropping of the same resistant cultivar, which was not observed when different resistant cultivars were rotated.
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