Managing small patches of clubroot infestation in canola fields

Project Summary

Overview

Genetic resistance is effective against clubroot but can break down quickly. Plant breeders are also falling behind in developing cultivars resistant to the many newer clubroot pathotypes. As a result, farmers need effective ways to manage clubroot in situations where genetic resistance is not yet available or where high spore numbers increase the risk of a breakdown in resistance.

This research aimed to provide farmers with practical approaches to minimize the risk of clubroot spreading from small patches in newly infested fields and to manage hot spots of new pathotypes in fields where the pathogen is already established. It included lab studies of rotation crops and grass cover crops, field studies of the efficacy of liming and grasses, and identification of improved techniques for estimating resting spore numbers in soil. The field-site experiments 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.

Results

One of the first outputs of the study was an improved method to quantify the numbers of resting spores in soil using a molecular approach known as digital drop PCR (ddPCR). This approach was less expensive, more accurate and more robust than previous methods, so it was used throughout the study.

In short, the treatments (liming, grassing, gypsum) did not consistently have a measurable effect on spore concentrations relative to the bare soil control.

• Short-term dense planting of grass and cereal crops produced
  a small reduction in resting spore concentration in infested soil. However, this effect was not detectable at field sites, regardless of soil type or pH.
• Application of lime did not reduce spore numbers relative to the untreated control. The target pH for liming treatments was 7.5, but it was difficult to maintain that pH at sites with highly acidic soil.
• Long-term perennial grass cover crop on clubroot patches did not, on its own, reduce spores but did hold soil in place. This minimizes movement of infested soil by wind and water erosion as well as the risk of contaminating farm machinery and other vehicles driven across the infested patch.

Conclusions

This study strongly supports the recommendation that growers extend crop rotations in clubroot-infested fields to at least two to three years (longer is better) between susceptible crops. This allows the spore concentration in infested areas to fall, which reduces the risk of spreading spores and breakdown of resistance.

The outcomes also confirm that growers should always select resistant cultivars for use in infested fields, because high numbers of clubroot spores were almost always still present after five years of non-host crops.