Manage Clubroot

Information adapted from Agri-Facts, Clubroot Disease of Canola and Mustard, Alberta Agriculture and Rural Development, May 2007 Revision and Clubroot of Brassica Crops, Manitoba Agriculture, Food and Rural Initiatives, May 2008.

Managing clubroot after establishment in a canola field is very difficult. Preventing the spread of the disease is critical. Although there are some management tools available in higher value crops like broccoli and cabbage, however these tools are either not registered for use on canola or are too expensive to be used economically (or both). The industry is putting a great deal of effort into finding additional management options, but crop rotation remains the most practical and effective strategy. This section outlines some of the management tools that are either available or under investigation.

Rotation

Long rotation out of canola is currently the only economically sound strategy to cope with clubroot infestations in canola fields although genetic tools are starting to come onstream. Canola, mustard, kale, and cole crops should not be grown for at least four years in slightly infested fields, and at least seven years in severely infested fields. These long rotations are necessary because of the longevity of the clubroot resting spores.

Volunteer canola and susceptible weeds (mustard family, dock, and hoary cress) must be controlled in the rotational crops. There is some evidence that a few non-cruciferous crops, such as orchardgrass and red clover, may be weak hosts for clubroot disease, but the rotational effect of such crops on clubroot incidence and severity is likely of little practical significance.

In combination with the rotation strategy, sanitize equipment and personnel (see Prevent Clubroot), and practice soil conservation measures to keep contaminated soil and infected crop debris from being transported from infested fields. Whenever practical, do not work infested fields when they are wet, because more mud will stick to equipment and could be transported to clean fields.

There has been one report from Norway of lower clubroot severity under reduced tillage. Reduced tillage or direct seeding also may help combat a clubroot infestation in Canada. Fewer tillage operations will help prevent movement of contaminated soil.

Interpretation of molecular test (PCR) results may eventually prove useful for quantifying the infestation that remains in a field after a certain number of years. For example, testing soil samples after three years have elapsed in an infested field could indicate whether the infestation has declined to low enough levels that canola could be re-seeded without economic yield losses.

Genetic Resistance

Clubroot was present in Alberta prior to 2003 but it was not considered economically damaging in canola until 2003 when it was found in a severely affected canola field in St. Albert. This virulent and economically damaging pathotype of clubroot was determined to be pathotype 3. Screening of the clubroot populations suggests that there are other pathotypes present in populations as well. This genetic diversity within the clubroot population requires caution as breeding and crop rotation strategies continue to be developed. The introduction of varieties with resistance to the predominant pathotypes may create selection pressure for other, rare pathotypes of clubroot with greater virulence against resistant varieties. Growing these resistant cultivars too often in rotation would increase this selection pressure and the more virulent pathotypes could quickly become predominant. Managing new genetic tools properly through appropriate crop rotations and agronomic practices will be necessary to preserve their effectiveness against clubroot.

Interim registration was granted to the clubroot resistant canola hybrid 45H29 in early 2009. This variety from Pioneer Hi-Bred possesses a high level of resistance to the most dominant pathotype in current Alberta clubroot populations, clubroot race 3, as well as resistance to other known, less frequent pathotypes in the Alberta populations. In 2010 there will be abundant seed supplies of 45H29, especially in the clubroot affected areas of western Canada. However, growers are reminded to not approach this technology as a magic bullet and to continue to practice smart canola agronomy and maintain proper rotational and sanitation practices.

Other industry players continue to explore genetic resistance options for the Western Canadian canola market and additional resistant varieties are expected to come on stream shortly.

Fungicides

Currently there are no registered fungicides for clubroot control or suppression in canola. Although there are fungicides registered for clubroot control in cole crops around the world, the relatively high cost and application method (transplant bed drench or broadcast incorporation) make them uneconomical for canola field production. The fungicides registered in Canada for use on vegetable crops for management of clubroot are listed below. All products are group 14.

These products are not registered for use on canola.

Product	PCP#	Crops
Quintozene (Terraclor®) 75%	07251	Broccoli, brussel sprouts; cabbage; and cauliflower
Adobe 75WP	28663	Broccoli, brussel Sprouts; cabbage; and cauliflower
Crusoe 75WP	28238	Broccoli, brussel sprouts; cabbage; and cauliflower
Quintozene 75WP	27416	Broccoli, brussel sprouts; cabbage; and cauliflower
Quintozene 75WP	11425	Cole Crops

Calcium cyanamide, an old form of nitrogen fertilizer with fungicidal properties, has shown promise for reducing clubroot in cole crops. However, high application rates, significant cost, and limited availability make it a poor option for canola.

The effectiveness of seed treatments for managing clubroot is currently being explored.

Liming

Liming acid soils to above pH 7.2 has shown poor or erratic results for clubroot control in cole crops in British Columbia and eastern Canada. Given the inconsistency and high cost of the practice, liming is not a reliable option for clubroot control in canola.

Bait Crops

Research is currently under way to determine if using bait crops could be an effective tool in reducing clubroot spores in the soil. In a bait crop situation, plants that are sensitive to clubroot are allowed to grow for approximately four to five weeks to stimulate germination of the clubroot resting spores. The sensitive crop is ploughed down before the clubroot pathogen completes its lifecycle, which prevents the addition of more resting spores to the soil. This strategy helps draw down the population of resting spores in that field, which may shorten the time needed between plantings of a commercial canola crop.

Minimizing Spread

Once clubroot is found in a field, the goal is to prevent the introduction of the long-lived resting spores into new fields. Try to prevent the movement of spores from one area to another by means of soil carried on equipment and personnel. Minimize all equipment traffic into infested fields. For example, service and nurse trucks should remain on the road and field equipment should be brought to them.

More information on minimizing the spread of clubroot can be found at Prevent Clubroot.