Clubroot is a soil-borne disease that affects the roots of cruciferous vegetables and crops such as cabbage, cauliflower, broccoli, brussel sprout, turnip, rutabaga, radish, canola and mustard.  Cruciferous weeds (e.g. stinkweed, shepherd's purse, wild mustard) can also serve as hosts to the disease.

Clubroot is caused by a fungal pathogen, Plasmodiophora brassicae.  The pathogen overwinters in the soil as resting spores.  In the spring, exudates (secretions) from the roots of host plants stimulate the resting spores to germinate and produce another spore type (zoospores) that swim in soil water until they contact the host roots.  Infection occurs through root hairs or wounds.  Once inside the plant, the pathogen produces secondary zoospores that re-infect the roots of the initial host or nearby plants.  The pathogen invades the interior of the host root, which alters the normal hormonal balance and leads to the characteristic distorted shapes that give this disease the name "club" root.

Clubroot is mainly spread by soil movement containing soil-borne spores.  Soil transport can occur by wind, water erosion, on field machinery, in manure from animals fed contaminated feed, and in soil attached to seeds (earth tag).

Clubroot is a disease that affects crucifers worldwide.  In Canada, clubroot is primarily established in the vegetable growing regions of British Columbia, Quebec, Ontario and the Atlantic provinces.

In 2003, clubroot was identified in canola in the Edmonton region of Alberta, the first report of clubroot in canola in Canada.  Since 2003, clubroot has been found increasingly in canola crops in the Edmonton region of Alberta.  In 2007, clubroot was confirmed in a total of 58 fields in 11 counties in Alberta, including Sturgeon, Leduc, Parkland, Northeast Edmonton, Wetaskiwin, Strathcona, Westlock, Camrose, Flagstaff, Barrhead, Lac Ste. Anne and Newell.  While clubroot has been mainly limited to the Edmonton region, the observation in Newell county, which is in Southern Alberta, causes concern.

No, clubroot has not been identified in canola or cruciferous vegetables in Saskatchewan.  Annual canola disease surveys are conducted across the main canola-production regions of the province, and clubroot has never been observed during these surveys.  Furthermore, clubroot symptoms have not been reported, nor have any samples been submitted to the provincial lab.  However, there has not been an intensive survey conducted in Saskatchewan to look specifically for clubroot, nor to test soil for the presence of clubroot spores.

It is possible for clubroot to occur in Saskatchewan, and that is why producers must be aware of this disease and be proactive in helping prevent its establishment and spread.  However, it is predicted that the development and spread within Saskatchewan would be much slower because our climatic conditions are less ideal, and we do not have a history of cruciferous producing vegetables in canola production regions.

Roots of infected plants become malformed due to increased cell division and growth, which leads to the development of gall formations.  The deformed roots have a reduced ability to absorb water and nutrients.  As a consequence, the above ground symptoms of clubroot are similar to the symptoms of drought stress and/or nutrient shortage (i.e. stunting, wilting and yellowing).  The actual cause of the problem can be determined by digging up affected plants to check for gall formation on the roots.  Plants infected late in the season may not show obvious symptoms except that the plants ripen prematurely and produce few seeds that have low oil content.

The infected roots will eventually disintegrate, releasing resting spores back into the soil.  These resting spores have a half-life of about four years, but can survive for up to 20 years.  The longevity of the resting spores is a key factor contributing to the seriousness of the disease.

For symptoms, refer to Alberta Agriculture and Rural Development's factsheet on Clubroot in Canola and Mustard at: http://www1.agric.gov.ab.ca/department/deptdocs.nsf/all/agdex8593

As a general rule of thumb, yield losses due to clubroot are about half of the percentage of infected stems.  For example, if almost all plants in a field are infected, about 50 per cent yield loss is anticipated, while infestations of 10 to 20 percent of the stems may lead to 5 to 10 per cent yield loss.  (Note: this yield loss estimate is similar to sclerotinia stem rot infection of canola, a disease which also affects the uptake of water and nutrients).

There is no known clubroot resistance in current Canadian canola varieties.  There are no registered fungicides for clubroot control or suppression in canola.  Although there are registered products and soil amendments for clubroot control in vegetable cole crops, the cost of these treatments makes them unsuitable for treatment of canola fields.

Warm soil temperatures (20°-24°C), high soil moisture and acidic soil (pH less than 6.5) favour spore germination, infection and disease development.  High soil moisture areas of the field (low spots, clay zones, pivot points) typically have the most severe clubroot problems.  Irrigation may also favour clubroot infection.  While the majority of soils in Saskatchewan have a pH greater than 6.5, it is still possible for the disease to develop.

Clubroot was added as a declared pest to Alberta's Agricultural Pests Act in April 2007.  Under this Act, Agricultural Fieldmen are empowered to inspect land for clubroot and to collect samples. The owner or occupant of the land has the responsibility for taking measures to prevent the establishment and spread of clubroot.  Alberta's Clubroot Management Plan outlines management and sanitation measures to prevent the spread of clubroot from infested areas.  Failure to meet these recommendations may result in action being taken by the Agricultural Fieldmen to ensure compliance.

For information on Alberta's Clubroot Management Plan, go to: http://www1.agric.gov.ab.ca/$Department/newslett.nsf/all/agnw13068

It is important for Saskatchewan producers to take precautionary measures to prevent the introduction of clubroot disease.  There are no economically viable options for controlling clubroot once it is established in a canola field, so prevention is essential.

Recommended preventative measures include the following (adapted from Alberta's Clubroot Management Plan):

1. Use long rotations between canola crops (one canola crop every four years) to prevent a severe infestation from developing.
2. Control volunteer canola and cruciferous weeds.
3. Practice good sanitation by restricting movement of potentially contaminated soil to non-contaminated regions.
   Note: For Saskatchewan producers, this means not bringing in seed, vehicles, field machinery or oilrig equipment with earth tag from infected regions.  Ask questions about where the equipment or seed is from, and ask what sanitation measures have been used.
   Cleaning steps may include: removal of crop debris and soil, washing of equipment with power washer using hot water or steam, misting with disinfectant (one to two per cent bleach solution).
4. Use direct seeding and other soil conservation practices to reduce soil movement via wind or water erosion, as well as soil movement on tillage equipment.
5. Scout canola fields regularly and carefully. Identify causes of wilting, stunting, yellowing and premature ripening.
6. Avoid use of straw, hay or greenfeed, silage and manure from infested or suspicious areas.
7. Avoid common untreated seed (including canola, cereals and pulses). Make sure seed is free of earth tag (the effect of seed treatments is unknown).

To restrict movement of contaminated soil to non-contaminated regions, equipment operators should use preventative measures similar to sanitation practices recommended for producers:

1. Machinery with earth tag from infected regions pose a risk of spreading clubroot resting spores and contaminating the soil in clean fields.
2. Before entering or leaving fields, remove soil and crop debris from field equipment, tools and vehicles. This may involve sweeping, knocking, or scraping loose soil and soil clumps off the equipment.
3. Depending on resources available, consider an additional step of washing and/or disinfecting machinery. Hot water or steam may be delivered with a pressure washer. Machinery may be misted with a weak disinfecting agent such as a one to two per cent bleach solution.
4. Whenever practical, avoid working in areas of infested fields that are wet, as more mud will stick to equipment and then be transported to clean fields.

The Canola Council of Canada's site on clubroot:
http://www.clubroot.ca/ (coming soon)

Alberta Agriculture and Rural Development:
Clubroot of Canola and Mustard:
http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex8593

Alberta Clubroot Management Plan:
http://www1.agric.gov.ab.ca/$Department/newslett.nsf/all/agnw13068

Saskatchewan Ministry of Agriculture
Agriculture Knowledge Centre 1-866-457-2377