Canadian Canola Clubroot Cluster Pillar 1: Integrated disease management

Project Summary

Background

Canola, one of the most important crops in the prairie region, 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.

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.

Objectives

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.

Potential impact

The primary beneficiary of this research will be the canola production industry. The project investigates the etiology of novel clubroot strains that can overcome resistance, explore and refine methods of soil modification and fertilization to prevent clubroot establishment at field entrances (where the disease typically establishes itself first), model the relationship between yield loss and clubroot severity, monitor spore buildup in the soil by resistance-defeating clubroot pathotypes, and develop strategies to rotate clubroot resistant host genotypes to avoid resistance breakdown. Information on soil properties will help to predict and develop methods to reduce the rate of dissemination of both the original strains and of new pathotypes of clubroot.

This project aims to:

• Provide the information necessary for the successful and sustainable integrated management of clubroot in canola.
• Provide canola producers with practical approaches to managing clubroot in the new reality where resistance may not necessarily be effective.
• Develop clubroot management tools to complement genetic resistance, ensuring its continued durability in areas where disease pressure is high.
• Lead to the development of an integrated set of management tools to minimize the effects of clubroot and the spread of new clubroot pathotypes.
• Develop secure disease nurseries to investigate resistance against novel clubroot pathotypes.
• Lead to the training of students and future leaders in the canola industry.
• Transfer results to industrial sponsors, producers and other stakeholders through information bulletins, news releases, presentations at meetings and the 2018 International Clubroot Workshop; field tours at disease nurseries.

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)

Soil will be taken from selected fields where resistance has broken down and the clubroot pathotype population structure examined within each.  Soil samples will be subjected to analysis in order to define their characteristics; measures of texture, pH, organic matter content, cation exchange capacity (CEC), and nutrients (potassium, magnesium, etc.), microbiota, will be recorded.

Field pre-treatment and soil amendment techniques (led by Hwang and Strelkov)

Treatments to increase soil pH, such as calcium or lime formulations, will be evaluated in conjunction with inoculum density and the other soil variables mentioned above, with the goal of establishing a pre-infestation protocol for field entrances.

Integrated clubroot management methods (led by Hwang, Strelkov, Gossen, and McDonald)

Infested soils will be limed at optimal rates and then planted to susceptible and moderately resistant canola cultivars. One moderately resistant cultivar has some resistance not only to pathotype 3, but also to all of the pathotypes that have caused resistance breakdown.

An additional study will be conducted in “hot spots” where clubroot resistance has broken down in two or more commercial fields near Edmonton. The sites will be treated and monitored over several years to assess the impact and interaction of selected IPM treatments on resting spore concentration.

Development of yield loss model related to clubroot severity (led by Hwang and Strelkov)

Measure the effect of different inoculum densities of various P. brassicae pathotypes on canola plant growth parameters, including height and biomass accumulation, and on yield and yield-related parameters, including pod number and seed number and weight.

Effect of the soil environment on clubroot development (led by Hwang and Strelkov)

Clubroot spores will be inoculated at several concentrations into soils with nitrogen/calcium content, texture, pH, EC and origin (black, grey wooded, brown soil) as variables. Canola growth, yield parameters, and disease severity will be recorded for both inoculated and non-inoculated soils.

Effect of cultivar rotation on clubroot pathotype structure (led by Strelkov and Hwang)

The impact that planting susceptible or resistant cultivar(s) has on the relative predominance of pathotypes 5x and 3 will be determined.  The pathotypes will be inoculated into the soil as a 50:50 mix and the pathotype composition compared after four cropping cycles.

Disease nursery and resistance screening (led by Hwang and Fredua-Agyeman)

Spores of some of the key novel pathotypes will be incorporated into soil in securely fenced areas to establish disease nurseries at CDC North. The area will be used as an ongoing resource to screen for new resistant lines of canola, and will be used as a demonstration area for clubroot extension activities and will be showcased in the upcoming International Clubroot Workshop in 2018.

2018 International Clubroot Workshop (Entire Team)

This major international event was held at CDC North in 2018, where field trials will be showcased to international researchers, canola producers and industry leaders.

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.