By Barb Ziesman, PhD, Provincial Plant Disease Specialist, Regina
Clubroot of canola has been the focus of many discussions over that last year, in response to confirmation of the disease in commercial canola fields in 2017. To date, visible symptoms of clubroot have been confirmed in 37 canola fields. These fields include those identified through the Ministry-led clubroot survey, as well as those reported to the Ministry by agrologists and canola producers outside of the survey. The area where clubroot has been confirmed has expanded outside of crop districts AE, 9AW and 9B (2017 findings) to include crop districts 7B, 6B and 5B. These additional findings in 2018 indicate that the risk of clubroot in Saskatchewan has increased and, as a result, producers are encouraged to monitor their canola fields and implement management strategies to minimize the spread of clubroot and the impact of clubroot on canola yields by keeping pathogen levels low.
As part of the Ministry-led 2018 clubroot survey soil samples were collected from the field entrance for DNA-based testing to detect low levels of the clubroot pathogen. This testing allows for the detection of the clubroot pathogen even when visible symptoms are not present in the field. The testing is currently underway. When completed, the results as well as the location of the fields with visible clubroot symptoms, mentioned above, will be used to develop a clubroot distribution map which will be released by early 2019. This map will help to raise awareness of the distribution of clubroot in the province. To ensure that the map is as accurate and robust as possible, producers and agrologists are encouraged to report the location of all fields with clubroot to the ministry (township or rural municipality [RM] level, at the minimum). The clubroot distribution map will only include general location information (i.e., to an RM level) and specific locations will not be shared publicly.
Clubroot management and prevention
The key to clubroot management is keeping the pathogen levels as low as possible to allow canola production in a clubroot-infested field with minimum impact on yield. The earlier clubroot is detected, the easier it will be to manage. As a result, clubroot management can begin before symptoms are seen in a field and can even occur before the pathogen is present in a field by focusing efforts on preventing the introduction of clubroot-infested soil. Clubroot best management practices include:
The above practices are focused on managing the field in a manner that will reduce and keep pathogen levels low, thereby minimizing the impact of clubroot on canola yield and reducing the risk of selecting for strains of the pathogen that can overcome the resistance in commercially available clubroot-resistant varieties. The other important aspect of clubroot management is prevention. Clubroot is a soil-borne disease and the pathogen survives in the soil as small resting spores that cause infection of the host plant below ground. The resting spores are very small and can be moved in any way that soil can be moved, with activities that move large volumes of soil having a higher risk. Activities that minimize soil movement can be used to prevent the introduction of the clubroot pathogen to new areas and minimize the movement of clubroot-infested soil to new fields and the spread of the disease within the field. Some clubroot prevention strategies are as follows:
- Including canola in your crop rotation no more than once every three years. This allows for a two-year break from susceptible crops, including resistant varieties.
- Growing clubroot-resistant canola varieties in regions where clubroot has been identified. It is recommended that all producers in northern Saskatchewan, particularly in crop districts 5B, 6B, 7B, 9AE, 9AW and 9B, grow clubroot-resistant canola varieties.
- Not growing clubroot-resistant in short rotation. This will reduce selection pressure on the pathogen population to overcome the genetic resistance in the clubroot resistant canola variety.
- Controlling all canola volunteers and other susceptible weeds, including stinkweed, shepherd’s purse, flixweed and wild mustard.
- Carefully scouting canola crops, including both susceptible and resistant varieties. When scouting, pull up or carefully dig up plants and examine roots for the presence of swollen root tissue (galls). Near the end of the season, the galls will start to decompose and the infected roots may appear rotten. Focus your scouting efforts on field entrances and low areas or patches in the fields that are demonstrating above-ground symptoms of wilting, yellowing, stunting and premature ripening.
- In addition to scouting for visible symptoms, the presence of the clubroot pathogen in the field can be confirmed through a DNA-based test on soil collected from the field. This method of testing for the clubroot pathogen is advantageous, as it allows for the detection of the pathogen at levels lower than those required to cause disease symptoms in the field.
- Restrict the movement of potentially contaminated soil to non-contaminated areas. This can include:
- Practicing zero-till to reduce soil erosion. This will help prevent movement of contaminated soil via wind and soil erosion, but will also help to reduce the movement of the pathogen to new areas in a field if it is already present at low levels in an isolated patch within the field.
- Creating a separate exit as far as possible from the entrance to reduce the movement of the pathogen inoculum out of an infected field.
- Reduce the risk of spread of clubroot. The level of sanitation used should match the level of risk associated with field management. Sanitizing should include the removal of crop and soil debris when moving from field to field. The more soil that is moved with equipment, the higher the risk of introducing clubroot to a field. Washing with a pressure washer, using hot water or steam, and misting with a disinfectant such as two per cent bleach solution is also a good measure to take. Bleach is the most effective disinfectant. Virkon has been found to be only moderately effective in reducing the viability of the clubroot pathogen.
- Restrict the entry of vehicles unless they have been properly sanitized.
- Restrict unwanted vehicles from entering your field by posting “no trespassing” and “no hunting” signs.
For more information, contact the Agriculture Knowledge Centre at 1-866-457-2377 or firstname.lastname@example.org.