By Barbara Ziesman AAg, Provincial Specialist, Plant disease and Autumn Lawson, summer student
Clubroot is a disease that affects plants in the brassica family. In Saskatchewan, visible symptoms of clubroot have been confirmed in 43 commercial canola fields to date. Clubroot is characterized by the presence of galls (swollen root tissue) on the roots on infected plants. These galls contain millions to billions of spores that will be released back into the soil at the end of the growing season. The larger the gall, the more spores that it will contain. The size of the clubroot gall is also directly linked to the yield loss due to the disease. The larger the clubroot gall, the bigger the impact on yield.
The earlier that clubroot is detected in a field, the easier it is to manage. The best way to detect clubroot early is to scout for the presence of the disease and/or the pathogen. The presence of the disease can be detected by looking for the presence of clubroot galls. DNA-based soil testing can also be used to detect the clubroot pathogen at levels as low as 1,000 spores per gram of soil. This will allow for detection of the pathogen at levels lower than would typically cause visible symptoms under field conditions. Typically, under field conditions, visible symptoms are visible at levels above 80,000 to 100,000 spores per gram of soil, but can occur at levels as low as 10,000 spores per gram of soil. Clubroot severity and potential yield loss increases as spore levels increase. If the presence of the clubroot pathogen is detected at low levels, proactive management strategies can be used to keep spore levels low and prevent or minimize yield losses due to the disease.
When collecting soil for clubroot testing, it is important to collect the soil from the areas of highest clubroot risk. This will help to prevent false negatives, which can occur when a clubroot soil test fails to detect the presence of the clubroot pathogen in a field where it is present at low levels or only in a small area of the field. For more information on how to collect a clubroot soil sample watch our #SKcropwalk on the subject or view the fact sheet "Clubroot soil testing on the prairies".
When looking for visible symptoms, it is important to visit fields and uproot plants to look for clubroot galls. The best time to scout for clubroot is later in the season at or after swathing. Focus scouting on the areas of highest clubroot risk in the field. These areas include:
- Field entrance(s): Will still be the primary site when clubroot is introduced to a field by equipment carrying clubroot-infested soil.
- Low spots: If the clubroot pathogen is present in the field, the highest disease severity will likely occur in low areas of the field where environmental conditions are most favourable for disease development (wet conditions and low soil pH).
- Natural water runs: Clubroot is a soil-borne disease that can be moved in any way that soil can be moved. This includes movement via water erosion. Water runs and their associated soil movement can result in the spread of clubroot-infested soil between fields. Also, due to increased moisture, these areas will have favourable conditions for disease development and may have higher disease severity than other areas of the field.
- Old yard sites, grain storage areas or other high-traffic areas of the field. This is tied to movement of infested soil on equipment.
If your field is in an area where clubroot is not known to occur, scouting should still be focused around the field entrance. Scouting in the other high-risk areas of the field is important when you see suspicious patches (plants that are yellowing, wilting or ripening prematurely or have poor plant growth) or if the field is located in a region where clubroot and/or the clubroot pathogen are known to occur. It is important to scout all canola fields, including those with clubroot-resistant varieties.
When clubroot and/or the clubroot pathogen is known to occur or when the field is known to occur in a clubroot-infested area, proactive clubroot management strategies can be used to keep pathogen levels low and prevent or minimize yield losses. Clubroot is best managed through an integrated approach that includes both extended rotations (minimum of a three-year rotation) and the use of resistant varieties. Neither crop rotation nor the use of clubroot-resistant varieties alone will provide effective control of clubroot, but when used together they will effectively keep pathogen levels low and minimize yield losses. Using clubroot-resistant varieties in short rotations will increase the risk of resistance breakdown that occurs when there is a shift in the pathogen population to favour pathotype or strains of the pathogen that are capable of causing disease in the resistant variety. The pathogen population is known to be diverse, and there are often multiple pathotypes present in a single field and even in a single gall. As a result, there will be a small portion of the population that may be able to cause disease in a resistant variety. When the variety is grown in short rotation, the levels of these aggressive strains will increase until that pathotype becomes the dominant pathotype in the population. When this occurs, the resistant variety can be severely infected and can suffer yield loss, as it will no longer be effective against the pathogen population. When pathogen populations are high, this shift in the pathogen population and resistance erosion can occur quite quickly. Figure 1 shows how resistance erosion can occur when a clubroot variety is grown in a short rotation.
In Alberta, the first field with resistance erosion was identified only four years after the first clubroot-resistant variety was released. To prevent this from occurring in Saskatchewan, we need to only grow clubroot-resistant varieties in extended rotations. The extended rotation will allow spore populations to decline and reduce the selection pressure on the pathogen population. To further improve clubroot management, the following strategies can be included in a clubroot management plan.
- Create a separate entrance and exit (as far as possible from the clubroot-infested area) and knock soil off your boots and equipment before leaving the field to prevent spreading clubroot to new fields or areas.
- Notify easement holders and other groups working on your land that clubroot has been confirmed to ensure that they are aware of the increased risk and can ensure that they take biosecurity measures to prevent spreading the disease.
- Use clubroot-resistant varieties and extended crop rotations in all fields on the farm (not only in the field confirmed to have clubroot).
- Use soil conservation practices such as minimum tillage to prevent the spread of clubroot to other fields but also within the infested field.
- Control canola volunteers and other brassica weeds to prevent increasing spore levels throughout the rotation.
- Grass the clubroot patch to prevent movement of the infected soil but also to help further reduce spore levels.
We also strongly encourage producers and agronomists to report new clubroot findings to the ministry. This will allow us to ensure that the Saskatchewan clubroot distribution map is as robust and accurate as possible. If you would like to report a new clubroot finding, please contact your
Regional Crop Extension Specialist.