By Barb Ziesman A.Ag, Provincial Specialist, Plant Disease
To date, clubroot has been confirmed in 43 commercial canola fields in Saskatchewan. These clubroot-infested fields were identified through increased surveillance but also as a result of increased awareness, monitoring, and reporting at the farm level. In 2018, the majority of clubroot-infested fields identified were actually reported to the Ministry by producers and agrologists outside of the ministry's survey. This provides valuable information about the distribution and severity of clubroot in Saskatchewan.
Distribution of clubroot
In response to clubroot findings in 2017, the Ministry partnered with SaskCanola and the Saskatchewan Association of Rural Municipalities (SARM) to conduct an extensive clubroot survey. The financial and in-kind support from SaskCanola and SARM, respectively, is greatly appreciated and was required for the successful completion of this survey. To fully understand the new clubroot reports and to interpret the findings of the 2018 clubroot survey, it is important to think about the objective of the clubroot survey and the geographic location of clubroot-infested fields. The purpose of the extensive 2018 clubroot survey was to gain a better understanding of the distribution of clubroot in Saskatchewan. The objective was not specifically to find clubroot-infested fields, as this would have required a much more targeted approach. The 2018 survey took a stratified, grid-sample approach where fields were surveyed without any production information or any information about the potential risk of clubroot in that field. Fields were distributed evenly across the survey area, with one field located as close to the centre of each township as possible. This sampling approach will provide insight on the geographic distribution of the disease, as well as how widespread it is across the survey, but was not designed to identify all clubroot-infested fields in the region. Approximately 1,500 fields were surveyed in 2018. In each surveyed field, plants were visually inspected for clubroot symptoms and soil samples were collected for DNA-based detection of the clubroot pathogen.
As part of this survey, two fields were identified to have visible clubroot symptoms and clubroot pathogen was detected at low levels in three fields (without the presence of clubroot symptoms). This means that clubroot and/or the soil-borne clubroot pathogen were only detected in less than one per cent of all fields surveyed. This indicates that although clubroot and/or the clubroot pathogen is known to be present in Saskatchewan, it is not present uniformly or widely distributed across all canola fields.
The confirmation of clubroot outside of the survey, through reports by producers and agrologists, indicates that the risk and distribution of clubroot in Saskatchewan is higher than previously thought. The good news is that clubroot has still been confirmed in only a small number of canola fields in the province, which indicates that we still have an opportunity to implement proactive prevention and management strategies to minimize the impact of clubroot in Saskatchewan. When determining your own clubroot risk, it is important to scout and monitor for clubroot symptoms in your own fields and start implementing proactive clubroot management strategies in regions with higher clubroot risk.
In addition to the distribution of clubroot in the province, we have learned a lot from the clubroot-infested fields that may help guide monitoring activities and management decisions.
Where to look for clubroot
When clubroot was first found in Alberta, researchers found that in 90 per cent of the cases, clubroot symptoms were found at the field entrance. This indicates that infested soil was most likely introduced into the field on equipment carrying clubroot-infested soil. In Saskatchewan, this rule of thumb held up for some fields, but not all. On closer examination of the clubroot-infested fields, we found that clubroot is not always present at (or not only at) the field entrance. Figure 1 is an illustration of other areas in the field where clubroot has been identified.
- Field entrance: 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).
- 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 the 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.
When monitoring for clubroot, producers are encouraged to scout and examine the roots of canola plants at all high-risk sites in the field (A to D in Figure 1). In areas of the province where clubroot has not yet been found, monitoring at the field entrance will still be the best strategy. In areas where clubroot has previously been reported, there is value in monitoring all high-risk areas of the field.
Pathogen levels in clubroot-infested fields
The level of the clubroot pathogen in the field is measured as the number of spores per gram of soil. Generally speaking, the higher the pathogen level, the higher the disease pressure. High pathogen levels will cause larger galls and more yield losses than when pathogen levels are low. In the fields identified in 2018, the spore levels ranged from 1,000 to 60 million spores per gram of soil. This represents a large range in spore concentration.
In all of the fields where clubroot and/or the clubroot pathogen is known to occur, crop rotations should be expanded to include a minimum of a three-year crop rotation to reduce or keep pathogen levels low and minimize the impact of clubroot on canola yields. Extended crop rotation will help reduce selection pressure on the pathogen population to overcome resistance in clubroot-resistant varieties. This will become increasingly important when pathogen populations are high. Higher populations of the pathogen will have a higher genetic diversity, which means that it is likely that there are already individuals in the pathogen population that can overcome resistance. In these fields, producers are encouraged to extend their rotations further or consider seeding the area to a perennial grass for a period of time to reduce pathogen levels. For more information on this approach and other clubroot management strategies, see The Saskatchewan Clubroot Management Plan.
Clubroot and soil pH
Previous research with the clubroot pathogen has found that disease development is favoured under low pH or acidic conditions, with pH of 5.0 to 6.5 being the most favourable for disease development. However, higher pH soil can still result in significant levels of clubroot, particularly under moist conditions and when pathogen levels are high.
Recently, there has been a lot of interest in using lime to increase soil pH as part of an integrated approach to clubroot management. To investigate whether or not this practice will have value in Saskatchewan soils, the soil pH was tested in a number of the clubroot-infested fields identified in 2018. In these fields, the pH ranged from 4.6 to 7.6.
This suggests that liming may have a fit as part of an integrated clubroot management plan in some fields. When you are considering using this tool, think about the size of the area and the level of the pathogen. This strategy will have the best fit when the clubroot-infested area is small, when pathogen levels are low, and when used with other management strategies such as a minimum of a three-year rotation and the use of clubroot-resistant varieties.