Managing small patches of clubroot infestation in canola fields

Term: : 4 years ending March 2024
Status: Complete
Researcher(s): Bruce Gossen, Tim Dumonceaux, AAFC; Mary Ruth McDonald, University of Guelph
SaskCanola Investment: $146,400
Total Project Cost: $344,000
Funding Partners: MCGA, DL Seeds, Manitoba Ag, Graymont

Grower Benefits

• Grass and cereal crops produced a small reduction in resting spore concentration in infested soil, but the effects on spore survival were too small to be detectable in the field. However, spore numbers dropped dramatically at all sites over 3-4 years, which supports the value of extended rotations in infested fields.

• This study strongly supports the recommendation that growers extend crop rotations in clubroot-infested fields to at least 2-3 years between susceptible crops because that allows the spore concentration in infested areas to fall, which reduces the risk of spreading spores and breakdown of resistance.

Project Summary

Genetic resistance is highly effective against clubroot but can quickly break down. Also, many newer pathotypes have been identified and plant breeders are falling behind in the development of new clubroot-resistant cultivars. As a result, producers need options for dealing with the disease in situations where genetic resistance is not yet available or where high spore numbers (hundreds of millions or billions of spores per gram of soil) increase the risk of a breakdown in resistance.

The objective of this research was to provide canola producers with information on practical approaches to managing small patches of clubroot to minimize the risk of spread of clubroot from small patches in newly infested fields. However, the results are also applicable to managing hot spots of new pathotypes in fields where the pathogen is already established.

An improved method for estimating spore numbers from soil was developed under controlled conditions using a relatively new molecular technique known as ddPCR. Then the effect of dense plantings of several grass and crop seedlings on resting spore survival was assessed over 8 weeks under controlled conditions. Finally, the effect of liming and grass cover crops was assessed at infested sites in commercial fields over 3-4 years.

The field sites were conducted on a wide range of soil types (clay, loam, sand) with a range of soil pH (5.5-7.0) at sites in Alberta, Saskatchewan, and Manitoba. These sites provide a robust assessment of the range of conditions across a broad area of the Canadian prairie region. The target pH for liming treatments was 7.5 but it was difficult to maintain that pH at sites with highly acidic soil.

Liming can reduce clubroot severity and there were strong indications that at least one grass crop stimulated germination of resting spores, which would reduce inoculum availability over time. The study examined the effect of liming and grass cover crops on survival of resting spores of P. brassicae in short-term studies under controlled conditions and in commercial fields over several years. The lab studies showed that grass and cereal crops quickly produced a small reduction in spore numbers. However, this effect was not detectable at field sites, regardless of soil type or pH. However, the concentration of resting spores declined rapidly over time. This result strongly supports the recommendation for a minimum 2-3 year break between canola crops (with the use of resistant cultivars) wherever clubroot is present. Growing a perennial grass cover crop on clubroot patches did not reduce spores but minimizes movement of spores to new sites within the field.

Full Report PDF: Managing small patches of clubroot infestation in canola fields