Features Agronomy Diseases
Clubroot threatens canola and other Brassicas

Resistant canola varieties are expected in the next three to five years.

Clubroot infected plants in Ron Howard’s disease nursery. Left to right foregrounds yellow mustard, Argentine canola and Chinese cabbage. Photo courtesy of Ron Howard.

Clubroot may threaten canola and related crops, including mustards and cruciferous vegetables, across the prairies, not only in central Alberta, according to plant pathologists studying the disease. After the 2007 season, specialists concluded the disease is more widespread than they originally believed.

Since it was first found in Edmonton area canola fields in 2003, clubroot has been identified in many more fields, more than 250 by 2007, mainly in central Alberta, but also in the south, which was thought to be safe from the pathogen. In 2008, clubroot was found in a Lethbridge area dryland mustard field where canola had not been grown for 10 years, and in Red Deer area canola fields. “Now that we’re looking for it more intensively across the province, we’re finding more and more clubroot, some of it in what we thought were low risk fields,” says Alberta Agriculture and Rural Development plant pathologist, Ron Howard. “There’s been a lot of focus on scouting for sclerotinia and leaf diseases in canola in recent years, and unless you pull the plant and see the galls on the root, you could assume wilted or dying plants have an above-ground disease, but it might well be clubroot.”

Although central Alberta is most affected by the disease, pest risk models show canola-growing areas across the prairies may be at risk, especially in relatively wet years. The discovery of the disease in a dryland field in an area of low rainfall and high pH soils, all previously thought to lower clubroot risk, further expands the areas at risk.

Scientists are still working to understand the biology of clubroot, Plasmodiophora brassicae. The pathogen is a protist (originally considered a slime mould, it has been reclassified) that survives in soil as a resting spore. Moist soil conditions and root exudates from Brassica seedlings stimulate the spores to germinate and produce infective cells that invade roots through root hairs. Once inside the root, the pathogen promotes the development of galls, warty-looking growths. Inside the gall the pathogen increases and produces millions of spores that are released into the soil when the infected roots rot. Infected roots cannot function effectively and the plant has inadequate moisture and nutrients, appearing wilted, discolored and ripening prematurely. Seedling infection generally cuts yields by half. Later infection has less impact.

The pathogen that is attacking canola crops in Alberta is mainly pathotype 3, one of nine strains of clubroot, and different from the strain usually found in vegetable crops. It is particularly virulent on canola, both Argentine and Polish types, and on Asian vegetables such as Chinese cabbage. Yellow, oriental and brown mustards, along with Brassica juncea, all became infected in Howard’s clubroot disease nursery.

Clubroot spores have a half-life of about four years, but some spores may survive up to 20 years without a host. Infected weeds, particularly volunteer canola, shepherd’s purse and stinkweed act as sources of spores, maintaining inoculum levels.

The clubroot pathogen is carried to farm fields in contaminated soil, most likely on farm machinery. Soil eroded by wind or water, earth tag on seed, or in greenfeed or hay or manure from cattle or other animals grazing infected fields can also carry spores. Selection pressure in fields with frequent canola crops or other susceptible hosts favoured development of high populations of virulent spores.

There is no effective pesticide or other treatment against clubroot in canola. Canola breeders have incorporated resistance to the disease into the crop and expect to release commercial varieties in three to five years.

Alberta management plan
To minimize the spread of clubroot, Alberta has introduced a management plan and added clubroot to the list of declared pests under its Agricultural Pests Act. The listing gives municipalities the power to force farmers to take steps against this disease on their farms and to enter fields to assess their clubroot status at any time.

Agriculture Service Board field men, along with seed production companies, scientists and others, are surveying fields in every canola-producing county in the province for the presence of clubroot. Some hard-hit municipalities are checking every canola field for the disease. The provincial clubroot management plan lists best management practices to prevent establishment and minimize the spread of clubroot through long rotations and equipment sanitation.

For very light infestations, a three-year break from canola is considered sufficient to keep inoculum levels low. For moderate to severe infestations, breaks of five years or more are advised. Volunteer canola and cruciferous weeds should not be allowed to grow any more than six to eight weeks – the time it takes for the pathogen to complete its life cycle and produce spores.

Recommended sanitation includes cleaning soil and crop debris from equipment before leaving a field, which also minimizes transfer of weeds, insects and other diseases. Power-washing equipment, vehicle wheel wells and undersides of vehicles on a grassed area of each field offers greater safety. Misting with disinfectant (one to two percent bleach) after washing can inactivate clubroot spores. Howard is working on other sanitation systems.

Oil and gas workers are a major part of traffic among fields in some areas of the province, and the industry is at the forefront of action against clubroot. It has contributed money for research and developed sanitation procedures to ensure its vehicles and equipment don’t carry spores from field to field.

Higher sanitation standards are needed for walking in fields where clubroot is a high risk. Researchers and clubroot surveyors are parking on roads and using new disposable plastic booties and coveralls for every field. Wash your hands, soil sampling equipment, shovels and coveralls between fields.

Most clubroot is found within 150 feet of the field access, or wherever soil might be scraped off as you lift or lower equipment. In fields that are only lightly infected, or have infections only near the access point, moving the access can help cut down inoculum levels. Direct seeding cuts the risk of erosion by wind or water that could carry infected soil.

To recognize clubroot early, scout canola and mustard fields regularly and check for the cause of any wilting, stunting or yellowing. Clubroot can only be recognized by root galls, with confirmation by lab tests that take only a few days. “Clubroot’s been around for a while, but now we have more awareness, more eyes looking for it, we’re going to find more,” says Canola Council agronomist, John Mayko. “It’s like blackleg before we had resistant varieties. We have to take precautions to slow the spread of this disease until we get resistant varieties or other measures to fight it.”

Alberta’s research team working on clubroot, partly funded by the canola and oil and gas industries, includes molecular biologists working on rapid diagnostic techniques, plant pathologists working on effective seed and soil treatments and disinfectants for machinery and plant breeders developing
resistant varieties, as well as field monitoring groups.