Clubroot is a soil-borne disease caused by the microbe Plasmodiophora brassicae. Clubroot affects the roots of cruciferous field crops such as:
- Oilseed radish; and
It also affects cruciferous vegetables such as:
- Brussels sprouts;
- Chinese cabbage,;
- Rutabaga; and
Cruciferous weeds (e.g. stinkweed, shepherd's purse, wild mustard) can also serve as hosts.
Symptoms of clubroot
The invasion of the interior of host roots alters hormone balance and leads to increased cell division and growth, resulting in clubroot galls. These deformed roots have a reduced ability to absorb water and nutrients leading to stunting, wilting, yellowing, premature ripening and shrivelling of seeds. The cause of these above-ground symptoms can be confirmed by digging up suspected plants to check roots for gall formation.
Clubroot affects canola yield and quality to a similar degree as other diseases affecting water and nutrient uptake. Its impact depends on soil conditions and the growth stage of the crop when infection occurs. Early infection of seedlings tends to result in great yield losses. Spore germination in Plasmodiophora, infection and disease development are favoured by warm soils, high soil moisture and low soil pH.
A canola disease survey is conducted annually in the province by a collaboration of plant pathologists, agronomists and crop specialists. They are from the Saskatchewan Ministry of Agriculture, Agriculture and Agri-Food Canada and private industry. The objective of the canola disease survey is to monitor the presence and severity of common canola diseases and to detect the appearance of new diseases such as clubroot.
Where clubroot has been found
Clubroot affects crucifers worldwide. It was first identified in Europe in the thirteenth century. In Canada, clubroot is primarily established in the vegetable growing regions of:
- British Columbia;
- Ontario; and
- the Atlantic provinces.
It has also been found in canola in Quebec since 1997. After 45 years of large scale production of canola in Western Canada, the disease was reported for the first time in this crop near Edmonton, Alberta. Since then clubroot has been confirmed in more than 1,000 fields in Alberta and was added as a declared pest to Alberta's Agricultural Pests Act in 2007.
Clubroot symptoms have not been observed in any of the Saskatchewan canola crops randomly selected for inclusion in the annual canola disease surveys (2,063 crops surveyed between 2008 and 2013). In 2008, 30 soil samples were tested using DNA diagnostics to detect Plasmodiophora brassicae and a bioassay in which canola plants were grown in a sample of soil and observed for clubroot symptoms after six weeks. One soil sample from west-central Saskatchewan was found to be positive for clubroot using these tests, despite the absence of symptoms in the crop. Clubroot was not detected in any additional soil surveys in 837 fields between 2009 and 2016.
In 2011, clubroot was identified in two canola fields in north-central Saskatchewan in private canola industry research sites. In 2012, one soil sample from west-central Saskatchewan out of the 91 tested across the province was found to be positive for clubroot. Nevertheless, symptoms were absent in the crops.
The spread of clubroot
Infected roots will eventually disintegrate and release resting spores into the soil. These are then transported by:
- Water erosion;
- Vehicles/tires; or
- An earth tag on agricultural or industrial field equipment.
Resting spore numbers will decline over time when non-host crops are grown, but a small proportion can survive in the soil for up to 20 years. Clubroot is primarily a soil-borne disease; it does not infect seed but it may be found in soil attached to seed or other plant parts. Clubroot does not present any legal phytosanitary issues for trading.
The oil and gas industry and clubroot
The Canadian Association of Petroleum Producers has developed a set of best-management practices. They are designed to promote the development of effective and achievable procedures to minimize the spread of clubroot pathogen spores where susceptible crops are grown.
What growers can do about clubroot
Producers growing susceptible host crops should follow the recommended best-management practices. These include proper crop rotation and sanitation for prevention and management of clubroot. Producers are advised to scout susceptible crops or weeds diligently, and if clubroot is suspected to contact the Saskatchewan Ministry of Agriculture. The Ministry will provide assistance with diagnosis and clubroot management.
Fungicides are not a practical solution for clubroot in canola and there are no foliar products or seed treatments registered for control of clubroot in canola in Canada. Many Canadian canola varieties are susceptible and currently, available resistant varieties are not effective against all pathotypes of clubroot.
Research on clubroot
Producers are also funding clubroot research through their canola levy. Saskatchewan researchers at Agriculture and Agri-Food Canada in Saskatoon are working in collaboration with the University of Alberta (U of A), Alberta Agriculture and Rural Development (AARD), the University of Guelph, and Ibaraki University in Japan. They are working to isolate, screen and discover indigenous microorganisms for biological control of clubroot in canola.
This research is part of an integrated disease management approach supported by provincial canola development commissions, grower associations and the Canola Council of Canada. Researchers at the U of A and AARD have also been studying pathogen and control options. Both public and private research programs have been screening Brassica germplasm and have been developing clubroot resistant (or tolerant) canola lines for Western Canada.
Canola industry organizations and clubroot
Industry organizations are assisting producers through education and awareness for the prevention and the spread of clubroot in Saskatchewan. The organizations help direct the canola levy to appropriate research initiatives such as the development of clubroot tolerant and resistant canola varieties. Canola industry organizations are also assisting the Saskatchewan Ministry of Agriculture through the Saskatchewan Clubroot Initiative.
As part of the provincial clubroot management plan, the Saskatchewan Clubroot Initiative was established to promote awareness and identify priorities for clubroot prevention and management. In June 2009, the Saskatchewan Ministry of Agriculture declared clubroot a pest, giving municipalities powers to control clubroot under The Pest Control Act. These powers include:
- The appointment of Pest Control Officers to enforce, enter land, perform inspections, collect specimens or issue orders to any person;
- The authority to pass bylaws to prevent, control or destroy clubroot; and
- The ability to require individuals to take actions to control or destroy clubroot on the land they own, occupy or control.
Education and awareness continue to be a priority to help producers and industry members prevent the spread of clubroot into and within Saskatchewan. The Clubroot Management Objective aims to promote awareness and minimize the risk of clubroot in Saskatchewan.