Canadian Canola Clubroot Cluster Pillar 1: Integrated disease management

Project Summary

The goal of this project is to develop management practices to reduce clubroot spore populations and prevent their buildup in at-risk areas. These practices are necessary to protect genetic resistance in canola varieties.

Specific project objectives are to:

1. Characterize soil properties and pathotypes in clusters where resistance has been defeated.
2. Test field pre-treatment and amendment techniques, including liming under varying spore concentrations and liming field entrances prior to clubroot introduction.
3. Quantify yield loss in relation to disease severity.
4. Assess the effect of cultivar rotation on clubroot pathotype structure.
5. Screen clubroot-resistance canola varieties against novel clubroot pathotypes.

Research activities

This research project is made up of multiple studies or research activities, which are led by different researchers, as provided below.

• Characterization of soils and pathotypes in clusters where resistance breakdown has occurred (led by Hwang and Strelkov)
• Field pre-treatment and soil amendment techniques (led by Hwang and Strelkov)
• Integrated clubroot management methods (led by Hwang, Strelkov, Gossen, and McDonald)
• Development of yield loss model related to clubroot severity (led by Hwang and Strelkov)
• Effect of the soil environment on clubroot development (led by Hwang and Strelkov)
• Effect of cultivar rotation on clubroot pathotype structure (led by Strelkov and Hwang)
• Disease nursery and resistance screening (led by Hwang and Fredua-Agyeman)
• 2018 International Clubroot Workshop (Entire Team)

Outcomes

The utility of genetic resistance as a clubroot management tool is at risk. The entire zone infested by clubroot is susceptible to resistance breakdown and every commercially available clubroot-resistant genotype of canola is potentially susceptible to novel clubroot strains. Deployment of resistant canola varieties must be combined with other clubroot management strategies. This project explored various strategies, including lime, canola cultivar rotation and perennial grasses.

Hydrated lime (not limestone) can work

Researchers conducted field and greenhouse trials to study the effects of lime amendments to soil. For the field trials (at the Crop Diversification Centre North, Edmonton), researchers spread three rates of hydrated lime and seeded a susceptible canola cultivar. Rates were low (1.9 tonnes per acre), medium (3.2 t/ac.) and high (4.6 t/ac.).

At the first field site, untreated control blocks had a pH of 5.6. The highest lime treatment increased pH to 7.8 at the time of seeding. This high rate reduced the clubroot disease severity index by 91 per cent at eight weeks after planting and by 71 per cent at harvest time. The high rate increased yield by 13 per cent. At the second field site, untreated control blocks had a pH of 5.5. The highest lime rate increased soil to a pH of 7.7 at the time of seeding. The high rate reduced the clubroot disease severity index by 45 per cent at eight weeks after planting and 50 per cent at harvest time. Yield increased 343 per cent.

For greenhouse trials, researchers inoculated potting soil with four different populations of P. brassicae spores, then treated the soil with “zero grind” limestone or hydrated lime at rates equivalent to 4.7, 8.1, 11.4 and 14.8 tonnes per hectare of lime. These rates adjusted the pH to 6.0, 6.5, 7.0 and 7.5, respectively. Eight weeks after inoculation, disease severity indices were 92-100 per cent and nine-13 per cent, respectively, in the susceptible and resistant controls (no lime) treatments. The index of disease decreased to zero per cent in both the susceptible and resistant cultivars following treatment with any of four tested rates of hydrated lime. In contrast, the application of limestone resulted in a modest decrease in clubroot severity and only at the two lowest inoculum levels evaluated.

Researchers also note that when clubroot resistance worked on the clubroot pathotypes present in a field, lime didn’t help yield.

Cultivar rotation

Researchers used a greenhouse study to compare continuous cropping of three treatments: the same susceptible canola cultivar, the same resistant canola cultivar and alternating resistant canola cultivars.

At the end of the first cycle of each rotation, the susceptible cultivar had a disease severity index of highest 91.5 per cent, the resistant cultivar had an index of 78.0 and the resistant rotation had an index of 69.1. Back to back canola increases the risk of clubroot, and CR only helps marginally and for only a short time. After subsequent rotation cycles, disease levels were not significantly different.

Wheat a good rotation crop

Researchers conducted growth room studies to assess the effect of dense seedlings of perennial grasses and conventional rotation crops (wheat, barley, pea and soybean) on resting spores of P. brassicae.

They showed that resting spore concentration declined more quickly in the presence of grass and wheat seedlings than in bare soil. Wheat was as good at reducing spores in soil as perennial ryegrass, but the effect of barley was less consistent, and soybean did not result in any measurable reduction relative to bare soil.

Background on clubroot

Canola, one of the most important crops in the Prairies, contributes over $25 billion to the Canadian economy each year. By 2016, clubroot infestations were confirmed in 2443 canola fields in Alberta, up from just 12 fields when surveys started in 2003. Clubroot-resistant hybrids were released in response to the rapid spread of the disease and dramatically reduced the incidence and severity of clubroot.

However, in 2013, clubroot reappeared in a field planted to a resistant hybrid variety and since then has been identified in 64 fields planted to these hybrids. This indicates that the utility of genetic resistance as a clubroot management tool is at risk. The entire zone infested by clubroot is susceptible to resistance breakdown and every commercially available clubroot-resistant genotype of canola is susceptible to the novel clubroot strains. Resistance breakdown also jeopardizes years of research and development that the seed industry has invested in breeding for clubroot resistance in canola. It is becoming clear that the deployment of resistant canola varieties must be combined with other clubroot management strategies.  

Preliminary studies have shown that the soil environment plays a major role in clubroot infection and that soil amendments and treatments have some potential to mitigate clubroot. Preventative measures such as soil nutritional modification also hold some promise and will be explored. Furthermore, the effects of soil properties (such as structure, organic matter, and ionic content) on clubroot development will be investigated. The effectiveness of clubroot mitigation strategies can be more fully evaluated using models that relate crop yield losses to disease severity, since these provide a measure of the disease impact that is more relevant to farmers from a practical and economic perspective. Finally, changes in pathotype composition resulting from the rotation of resistant canola varieties in various combinations will also be assessed, in order to understand how resistance sources are impacting strains of the pathogen.