Marla Roth | ALES Graduate Seminar

A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES MSc Final Exam Seminar by Marla Roth. This seminar is open to the general public to attend.

Thesis Topic: Evaluation of Plasmodiophora brassicae for the occurrence of pH insensitive isolates

MSc with Drs. Stephen Strelkov and Sheau-Fang Hwang.

Seminar Abstract:

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.