Influence of pH on the clubroot pathogen: are there pH-insensitive strains?

Term: 3 years, ending September 2023
Status: Complete
Researcher(s): Stephen Strelkov, Sheau-Fang Hwang, Yoann Aigu, U of A
SaskCanola Investment: $97,750
Total Project Cost: $383,250
Funding Partners: ACPC, MCGA

Grower Benefits

• Clubroot isolates show differential sensitivity to liming based on disease severity index and resting spore production in a susceptible host.  Repeated exposure to limed soil can increase or decrease the efficacy of liming, depending on the isolate, and this efficacy is also affected by the initial inoculum level in the soil.

• While liming may serve as an effective tool to complement host resistance, its potential utility is likely to be affected by field-specific conditions that need to be considered before growers invest in this approach.

Project Summary

Clubroot, caused by Plasmodiophora brassicae, is an important soilborne disease of canola managed mainly by planting resistant cultivars. Unfortunately, the widespread cultivation of resistant hosts has resulted in the emergence of new P. brassicae pathotypes that are able to overcome this resistance, highlighting the need for an integrated approach to clubroot control. The application of lime to increase soil pH represents a potential tool to complement resistance, since clubroot favors acidic soils.

In this project, we addressed two questions: (1) Does the application of lime have varying effects on different P. brassicae isolates? and (2) Can liming inadvertently select for pH-insensitive strains of the pathogen?

The experiments were conducted under greenhouse conditions, and two resting spore concentrations were evaluated, 5 × 104 and 5 × 105 resting spores per g of soil mix.  Hydrated lime was applied to adjust the pH of the soil mixture.

At the higher spore concentration, the effect of lime application on disease severity index (DSI) was minimal. In contrast, at the lower spore concentration, liming was effective in reducing the DSI by an average of 27 points across all nine isolates.  The extent of this reduction, however, was highly variable and depended on the isolate tested.  Resting spore quantification indicated that liming also reduced spore production per plant for all isolates, although there was no significant correlation with DSI.  Over three cycles of infection in the absence of liming, DSI tended to increase in each cycle if it was not already at the maximum. On the other hand, DSI in the liming treatment generally decreased over the cycles. In most cases, the DSI continued to decrease until it reached values comparable with a resistant variety. However, for one isolate, an increase in DSI was detected between the second and third cycles. Trends similar to those observed for DSI were found with respect to resting spore production per plant over the multiple cycles of infection.

Full Report PDF: Influence of pH on the clubroot pathogen: are there pH-insensitive strains?