Soil Sampling for Clubroot

Soil testing is a useful tool to detect the clubroot pathogen at low levels, monitor the spore load, or detect the pathogen in the absence of a host crop. Based on its biology, the presence and quantity of the clubroot pathogen, Plasmodiophora brassicae, is known to be variable as it occurs in patches in a field. Due to this variability, a false negative may occur when a soil sample fails to detect the pathogen in a field where it is present at low levels or only in a small area of a field. To reduce the chance of false negatives, it is important to collect soil samples in a manner that increases the likelihood of detecting the pathogen in a field.
There are two recommended methods for representative sampling.  More accurate results are obtained using the individual soil sample method; however if only one sample is being taken per field, the composite sampling method is recommended.
How to take a representative soil sample
·         Reduce the risk of false negatives by focusing sampling on high clubroot risk areas in the field. Field experience indicates that the highest risk areas include field entrances, low spots, areas with low pH, natural water runs, and other high-traffic areas.
·         Ensure biosecurity protocols are followed to minimize contamination. Sanitize tools and equipment between fields if composite samples are being taken, or between samples for multiple samples within a field.
·         Brush off the top layer of crop residue to expose the bare soil. Sample the soil from the top five to 10 cm (two to four inches). Each sample should be approximately 475 to 710 ml (two to three cups) in volume.
·         Soil sampling for clubroot needs to be completed separate from soil fertility testing, as they require two different protocols and soil depths.
·         Ensure soil samples are allowed to dry if they are collected in damp conditions. Samples can be air dried by leaving the bag open in a dry, secure area. Avoid contamination between samples while drying.
·         Submit soil samples with all required information to a designated testing lab.
·         Collect soil samples in the fall when clubroot galls are degrading or in the spring the year after a canola crop is seeded. Spring soil sampling is recommended when a clubroot-resistant canola variety is grown.
Individual soil samples at each high-risk area
1.    At each site, take a composite sample of five locations within a small area. For example, at the field entrance collect five subsamples of approximately 120 ml (0.5 cups) of soil per subsample. Combine subsamples into one bag to be tested. Repeat for each risk area (example: low spots, natural water runs, or other high-traffic areas).
2.    This method has the lowest risk of false negatives, which can result from the dilution of infested clubroot soils in a composite sample.
3.    The method requires multiple tests per field and multiple test fees, which can be cost restrictive if many fields need to be tested.
Composite soil sample from multiple high-risk areas
1.    Focus only on the high-risk areas in the field.
2.    At each high risk area, collect 120 to 250 ml (0.5 to one cup) of soil at two to three sites and combine them into a single sampling bag. For example, collect two to three subsamples (120 ml or 0.5 cups each) at the field entrance and two to three subsamples (120 ml or 0.5 cups each) at a low spot or other high-risk area and combine them into one soil sampling bag.
3.    There is an increased risk of false negatives with this method compared to individual samples at each high-risk area. However, there is improved accuracy compared to testing only at a single site, with a similar testing cost.
Laboratories that Provided Analysis for Clubroot in Soil:
A&L Laboratories (London)
Address: 2136 Jetstream Rd,
London, ON  N5V 3P5
Phone: 519-457-2575 or toll free 1-855-837-8347
BioVision Seed Labs (Edmonton)
Address: 7225B Roper Road,
Edmonton, AB T6B 3J4
Phone: Toll free 1-800-952-5407
Discovery Seeds Lab (Saskatoon)
Address: 450 Melville St.,
Saskatoon SK S7J 4M2
Phone:  306-249-4484
Plant Surveillance Initiative Lab (Winnipeg)
Address: 5A-1325 Markham Road,
Winnipeg, MB R3T 4J6
20/20 Seed Labs (Nisku)
Address: 507 11th Ave,
Nisku AB T9E 7N5
Phone: Toll free 1-877-420-2099
For more information:
On soil test result interpretation: Interpreting a clubroot soil test result