Evaluation of Plasmodiophora brassicae for the occurrence of pH insensitive isolates

• Author / Creator

  Roth, Marla N.

• Clubroot (Plasmodiophora brassicae) is a serious soilborne disease of canola (Brassica napus), negatively affecting the Canadian agricultural sector. Since clubroot development is favored in acidic soils, the application of lime to increase soil pH to ≥ 7.2 is recommended as a disease management strategy. It is unknown, however, whether there is differential sensitivity to pH in P. brassicae isolates. A replicated greenhouse experiment was conducted in which a clubroot-susceptible canola genotype was grown at pH 6.3, 7.2 and 8.0 in a soil mix inoculated with each of five field isolates representing various pathotypes of P. brassicae. While clubroot symptoms were severe across isolates at pH 6.3 under both medium and high inoculum densities, results at pH 7.2 were more variable, with milder disease severity occasionally observed. One isolate (L-G2) appeared particularly sensitive to pH 7.2, causing lower levels of disease relative to the other isolates. Only trace symptoms of clubroot developed at pH 8.0, and only at the high inoculum density. In a follow-up experiment, three of the previous five isolates were tested on the same clubroot-susceptible canola genotype at pH 6.3, 7.0, 7.3, 7.6 and 7.9. Clubroot was severe at pH 6.3, 7.0 and 7.3, dropping significantly at pH 7.6 and again at pH 7.9, regardless of the isolate. An in vitro study of P. brassicae resting spore germination at pH 6.0, 6.5, 7.0, 7.5 and 8.0 indicated that germination rates were generally similar at pH 6.0 to 7.5, but very low at pH 8.0, suggesting that some of the reduction in clubroot observed in the greenhouse studies at pH 8.0 could reflect reduced germination. Collectively, the results indicate that there is some variability in the pH sensitivity of P. brassicae isolates, and that targeting a pH ≥ 7.2 may not always be sufficient for clubroot management.

• Subjects / Keywords

  • clubroot
  • lime
  • soil pH
  • Plasmodiophora brassicae

• Graduation date

  Fall 2022

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-1yh9-cj49

• License

  This thesis is made available by the University of Alberta Library 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.