Clubroot disease overview*
Clubroot is a serious soil-borne disease of canola, mustard and other crops in the cabbage family. Cole crop vegetables like broccoli, Brussels sprouts, cabbage, cauliflower, Chinese cabbage, kale, kohlrabi, radish, rutabaga and turnip, are susceptible to clubroot, as are many cruciferous weeds, for example, wild mustard, stinkweed and shepherd's purse.
Appearance
As the name of this disease suggests, roots of infected plants may exhibit a club-like appearance. However, overall symptoms will vary depending on the growth stage of the crop when it becomes infected. Infection at the seedling stage can result in wilting, stunting and yellowing symptoms by the late rosette to early podding stage, while premature ripening or death can be observed in canola or mustard plants nearing maturity. Plants infected at later growth stages may not show wilting, stunting or yellowing, but may still ripen prematurely, and seeds may shrivel, thus reducing yield and quality (oil content).
Commonly confused diseases or disorders
Above-ground symptoms of clubroot may be confused with drought, nutrient deficiencies or other diseases, so suspect plants should be carefully dug from the soil to check for typical clubroot galls on the roots. Swellings of unknown origin called hybridization nodules are occasionally seen on canola roots and can be confused with young clubroot galls. These nodules are more spherical and firmer than clubroot galls and do not decay when mature as clubroot galls do. Exposure to phenoxy herbicides can also result in swellings on lower stems and roots of canola, mustard and cole crop vegetable plants, but these malformations usually lack the large size and lobed appearance of typical clubroot galls.
Causes
Clubroot is caused by a microscopic, soil-borne plant pathogen called Plasmodiophora brassicae. The clubroot pathogen is classified as a "Protist", a group of organisms with characteristics of plants, fungi and protozoans. The life cycle of the clubroot pathogen is illustrated at Clubroot disease of canola and mustard.
First found in Alberta
Clubroot was first reported on broccoli, cabbage and cauliflower in a few home gardens in the Edmonton area in the mid-1970s. The first economically important infestation in Alberta was observed on Chinese cabbage in a market garden near Edmonton in 2001. Clubroot was first detected in canola in Alberta in Sturgeon County northwest of Edmonton in 2003.
Effects
Most varieties of canola, mustard and cole crop vegetables currently being grown in Alberta are highly susceptible to clubroot. This disease is capable of significantly reducing yield and quality, and may destroy a crop if infestation levels are high. Swedish researchers found that infestations in canola fields nearing 100% affected plants caused about 50 to 80% yield loss, while infestations of 10 to 20% led to 5 to 10% yield loss. These results are similar to sclerotinia stem rot infection in canola, where a general rule of thumb is that estimated yield loss is half of the percentage of infected stems. A few cases of total crop loss (that is, not worth combining) have been reported in central Alberta.
Soil life
The resting spores of Plasmodiophora brassicae are extremely long-lived and may survive in soil for up to 20 years, according to Swedish research. Similar persistence is being reported in Alberta. Resting spore longevity is a key factor contributing to the seriousness of the clubroot disease, especially under short crop rotations. Clubroot is not a phytosanitary issue affecting international trade of canola or mustard.
Methods of spread
In Alberta, clubroot is being spread mainly through soil infested with resting spores. Infested soil can be carried from field to field by farm machinery, especially tillage equipment, and can also be moved by wind and water erosion. Seed of various crops, as well as hay and straw, can also become contaminated with resting spores via dust or earth tag when they are grown in clubroot-infested fields.
Efforts
In spring 2007, clubroot was added as a declared pest to Alberta's Agricultural Pests Act (APA). This act is the legislative authority for the enforcement of control measures for declared pests in Alberta. Annual surveys of canola, mustard and/or cole crop vegetables have been carried out to determine the location of infested fields in the main production areas for these crops, see the map in the next section. Researchers from many agencies, including the University of Alberta, Alberta Agriculture and Forestry and Agriculture and Agri-Food Canada, have many active research projects on clubroot. Private breeding programs have released clubroot-resistant canola varieties for western Canada.
Current state of clubroot in Alberta
By the end of 2018, clubroot was present in 42 municipalities in Alberta, mainly in central Alberta as shown in the 2003-2018 map showing infested municipalities: Cumulative clubroot infestations in Alberta. Clubroot has the potential to spread to and become established in many of the traditional canola-growing areas of western Canada.
In 2014, the first Alberta case of a pathogen shift to overcome current variety resistance was confirmed from diseased areas of a field planted to a resistant variety (observed in 2013). By 2018, there were hundreds of fields with new virulent pathotypes that could overcome our current genetically resistant cultivars.
Clubroot Management Plan
The Clubroot Management Plan's objective is to minimize yield losses due to clubroot and reduce the further spread and buildup of clubroot in canola, mustard and market garden vegetable fields in Alberta.
Regulatory status
Alberta's APA is the legislative authority for the enforcement of control measures for declared pests.
The Minister of Alberta Agriculture and Forestry is responsible for this act; however, enforcement is the responsibility of provincial municipalities. Agricultural Fieldmen are responsible for enforcing pest control measures in their respective municipalities.
Clubroot was added as a declared pest to the APA in April 2007.
Pest inspectors may be appointed by the local municipality or by the Minister of Agriculture and Forestry. Agricultural Fieldmen are pest inspectors under the APA. Inspectors have the power to enter land at a reasonable hour, without permission, to inspect for pests and collect samples. Check the Association of Alberta Agricultural Fieldmen website for a contact list of Agricultural Fieldmen and assistants in Alberta.
The owner or occupant of land has the responsibility of taking measures to prevent the establishment of any pest on land, property and livestock and to control or destroy all pests in the land or property.
Control measures for clubroot are specified in this management plan. It is important to understand that these control measures represent an acceptable standard that is to be applied in all municipalities across the province. Municipalities can enhance the standard within their own jurisdictions.
Factors favouring clubroot's spread in Alberta
Resting spores can be spread from field to field via contaminated soil on agricultural, petroleum industry and construction equipment and machinery. Soil tillage equipment represents the greatest risk of spreading the disease as soil is frequently carried on shovels, discs, openers, frames and tires. Clubroot surveys in Alberta have found that most new infestations begin at or near the field access, which indicates that contaminated equipment is the predominant spread mechanism.
Other secondary methods of spread could include movement of soil with water or wind and as soil attached to seed (earth tag), hay, straw or greenfeed.
Resting spores are extremely long-lived. It was traditionally understood that the spores had a half-life of about 4 years, but may survive in soil for up to 20 years. Recent studies have suggested that rather than a half-life there may be a more rapid decline in spore viability in the first 2 years without a host, followed by a slow decline for up to 20 years. Regardless of the kinetics of spore decline, the longevity of the resting spores is a key factor contributing to the seriousness of the disease, especially under tight canola rotations.
All land users, including growers, custom agricultural services, oil and gas industry operators, construction and transportation companies, recreational vehicle users, etcetera, need to continue their diligence in removing potentially contaminated soil from vehicles, machines and equipment prior to leaving fields. The removal is crucial to prevent the movement and introduction of clubroot to clean fields and to reduce the widespread distribution of spores within infested fields. Widespread resting spores and frequent exposure to resistant varieties will accelerate changes in the pathogen populations to strains that are not controlled by resistance in current clubroot-resistant canola varieties.
Management plan rationale
Clubroot in Alberta is managed through a proactive program that utilizes and prolongs the durability of clubroot-resistant canola varieties in combination with continuing efforts to prevent the further spread of this pathogen in the province. The program includes both an industry/public awareness program and a disease management plan.
The long-term goal of this management plan is to minimize canola yields losses through the judicious use of resistant varieties and to reduce the further spread of clubroot in Alberta.
Best management practices
- Use clubroot-resistant varieties when growing canola in areas where the disease is established. Alternate growing clubroot-resistant varieties with different sources of resistance when they become available.
- Although crop rotation will not prevent introduction of clubroot to clean fields, the practice will lower subsequent disease buildup and severity and reduce other diseases, such as blackleg. Crop rotation will not eradicate the clubroot pathogen from the soil. A minimum of a 2-year break from all clubroot-susceptible hosts is recommended for all producers. A longer break may be required if clubroot is well-established, or a Notice to Control is issued by the local authority. Canola growers in high-risk situations (confirmed clubroot in the field or area) should follow traditional canola rotation recommendations (one canola crop every 4 years) using clubroot resistant varieties. The one- in 4-year rotation recommendation using resistant varieties is designed to slow down pathogen population shifts. There have been numerous reports in Alberta of pathogen population shifts to a strains not controlled by clubroot-resistant canola. This has occurred many times in other parts of the world in canola and cole crops.
- Growing a clubroot-resistant variety in fields without known clubroot but in areas where the disease is prevalent can help slow the establishment of the disease. Since there would be low spore numbers when clubroot does get introduced to the field, this approach should not significantly induce changes in the strains to those that are not controlled by the variety resistance. The greatest pressure to alter the pathogen strains is frequent exposure (rotation length) of the same resistance to high soil spore populations (distinct clubroot patches have occurred in the field).
- Volunteer canola and cruciferous weeds must be controlled in infested fields to prevent more than 3 weeks of growth, to avoid the production of new resting spores on these host plants.
- Practice good sanitation (cleaning and disinfection) of machinery and equipment to restrict the movement of potentially contaminated soil. This approach will also help reduce the spread of other diseases, insects and weed seeds. Resting spores can be spread via contaminated soil. Moderate to high infestations will leave high spore concentrations in soil on field machinery, thus sanitation is very important in these situations. All producers should follow the practice of cleaning soil and crop debris from field equipment before transport from all fields. The most critical step in cleaning equipment is physical dirt removal – knocking or scraping off soil lumps and sweeping off loose soil.
- For risk-averse producers or with heavy infestations, there are additional cleaning steps, with diminishing returns on investment, that will slightly decrease the risk of spread, but will involve considerably more work and expense:
- After removal of soil lumps, wash equipment with a power washer, finish by misting equipment with disinfectant. Recommended products include 2% active ingredient bleach solution, Spray Nine, Adhere, Premise and AES 2500. The use of a disinfectant without first removing soil is not recommended because soil inactivates most disinfectants. A 20 to 30-minute wet period is recommended for good efficacy. For more information on disinfecting equipment for clubroot prevention see the fact sheet Preventing Clubroot: Agricultural Sanitization.
- Disinfectant footbaths can be an effective first line of defence in a biosecurity program. However, footbaths are not able to completely eliminate biosecurity risks in all situations. Disposable foot coverings should be utilized where possible and in combination with a foot bath to more fully minimize biosecurity risks associated with soil-borne diseases like clubroot. For more information on how to develop a comprehensive biosecurity plan for a farm or business, see the Canadian Food Inspection Agency’s National Voluntary Farm-Level Biosecurity Standard for the Grains and Oilseeds Industry.
- For risk-averse producers or with heavy infestations, there are additional cleaning steps, with diminishing returns on investment, that will slightly decrease the risk of spread, but will involve considerably more work and expense:
- Seed and establish an area with grass near the field exit. A well-sodded grass will retain soil removed during equipment cleaning without creating a mud hole after washing and thus will reduce the re-introduction of infested mud to wheels when moving from this area to the exit. The grass area will not be susceptible to clubroot if volunteer canola and mustard weed species are controlled.
- Use direct seeding and other soil conservation practices to reduce erosion. Resting spores can also readily move in soil transported by wind or water erosion. Reducing the amount of tillage on any given field will reduce the spread of the organism within the field and to other fields.
- Minimize vehicle and equipment traffic to and from fields.
- In situations where fields are lightly infested only near the current access, create a new exit at another distant edge of the field if possible.
- Scout canola fields regularly and carefully. Identify causes of wilting, stunting, yellowing and premature ripening – do not assume anything!
- Avoid the use of straw, hay or greenfeed, silage and manure from infested or suspicious areas. Clubroot spores may survive through the digestive tracts of livestock.
- Avoid common untreated seed (including canola, cereals and pulses). Earth tag on seed from infested fields could introduce resting spores to clean fields. The effect of current seed treatment fungicides on resting spore viability on seed is currently being studied.
*The content of this webpage was obtained from the Alberta Government's Alberta clubroot management plan webpage, to visit the webpage, please select the following link:
Visit the Alberta Clubroot Management Plan Webpage