Clubroot remains a serious threat to the long-term viability of canola in Western Canada. While clubroot-resistant varieties are a valuable new tool to reduce disease severity in clubroot-infested fields, crop rotation and other management measures are still important. Growers can’t let their guard down.
Tight rotations with canola every two or three years will threaten the long-term viability of the clubroot resistance trait.
Clubroot resistance likely comes from a single gene, and based on experience in other clubroot regions, single gene resistance does not seem durable. The pathogen adapts quickly. A winter canola variety called Mendel was released in the U.K. in 2003 and by 2007 resistance was failing. Australia has reported widespread failure of clubroot-resistant Chinese cabbage, a brassica relative to canola. And in North America in the 1960s, a clubroot-resistant cabbage variety called Bagger Shipper lost its resistance fairly quickly with tight rotations.
Bagger Shipper’s resistance trait could be somewhat restored if the variety was not grown for three or more years, which demonstrates the potential value of variety rotation. But that also means the pathogen population can shift quickly, adapting to single-gene resistance. Researchers are working to determine the genetic diversity of clubroot in Alberta. The more pathogen races found, the less durable single-gene resistance tends to be.
Clubroot here to stay
Despite the widespread use of resistant varieties in 2010, clubroot continued its spread through central Alberta. In the eight years since clubroot was first detected in canola around Edmonton, the pathogen has been found in canola in 18 Alberta counties, with three more having suspected cases.
The soil-borne pathogen is well adapted to the moist rich soils in Central Alberta, and the pathogen is more aggressive than first predicted. Based on this, we can assume that clubroot in canola is here to stay.
Growers outside the known clubroot regions should become familiar with the disease and its symptoms in anticipation of its arrival. The pathogen has been detected in Saskatchewan but no infected canola crops have been identified. The disease and pathogen were both found in Manitoba in 2007. It has not been found in Manitoba since that year, but an Agriculture and Agri-Food Canada ecoclimatic index map suggests that most of Manitoba’s agricultural region compares to central Alberta in its capacity to host the disease. (See the map below, courtesy Kelly Turkington, AAFC Lacombe.)
The pathogen infects the roots of canola and other brassica plants, creating galls that cut off supply of nutrients and water to the rest of the plant. Yield loss from clubroot can exceed 50%.
While the Canola Council of Canada and other industry partners continue to research management measures, here are the recommendations to date:
Clubroot management on land known to have clubroot
1. Use resistant varieties. Six clubroot-resistant hybrids are on the market for 2011. While these varieties can provide a significant drop in clubroot infection and severity, growers using them can expect some clubroot infection in the field. Host canola plants will most likely be susceptible volunteers in the field and susceptible off-types from the seed bag.
2. Use a minimum four-year rotation. Clubroot resting spores are long lived. Four years is considered a minimum rotation between canola in fields known to have the disease.
3. Rotate between resistant varieties. Because resistance likely comes from a single gene, that resistance can fail if used frequently on the same field. Rotating varieties may help, but we don’t know if current resistant hybrids have the same resistance gene.
4. Minimize traffic in and out of infested fields. Once clubroot is found in a field, the goal is to prevent the introduction of the long-lived resting spores into new fields. Minimize all equipment traffic into infested fields. Service and nurse trucks, for example, should remain on the road and field equipment should be brought to them. Equipment leaving an infested field should be thoroughly cleaned and disinfected. Click here for tips on how to clean machinery.
5. Limit tillage. 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. Reduced tillage or direct seeding also may help combat a clubroot infestation by reducing the movement of contaminated soil.
6. Control weeds. Volunteer canola and susceptible weeds (mustard family, dock, and hoary cress) must be controlled in the rotational crops. These volunteers and weeds host clubroot in non-canola years, reducing the effectiveness of rotation as a management tool.
7. Scout. When using resistant varieties, continue to scout for gall formation on plant roots. Low-level infection is normal for these hybrids, as noted in point No.1 above. Higher incidence levels could signal the start of a breakdown of resistance.
Some Group-14 fungicides are registered to control clubroot in other brassica vegetable crops, but they are not registered for use on canola. Do not use non-registered products on canola. Residues of unregistered products found on exported seed could jeopardize canola markets.
Clubroot management for non-infested land in an infested region
1. Use a resistant variety. This can keep undetected levels of clubroot from increasing to economically-significant levels.
2. Use a four-year rotation. This is the preferred rotation for disease management in general.
3. Practice good sanitation. Clubroot is spread mainly by movement of soil that contains clubroot spores. Almost all new infestations begin near the field access, according to Alberta surveys, indicating that equipment contaminated with clubroot-infested soil is the key mechanism for spread of the disease. Click here for tips on how to clean machinery.
4. Control weeds. This is a prevention strategy. If clubroot spores are introduced, these spores have nothing to infect if host weeds and canola volunteers are controlled. With nothing to infect, clubroot cannot reproduce and increase its inoculum levels.
5. Learn to identify the disease. Roots of infected plants become malformed due to increased cell division and growth. Clubroot galls tie up nutrients, and severely infected roots can’t transport adequate water and nutrients to aboveground plant tissues. Patches of prematurely ripening canola due to clubroot infection could be confused with other diseases such as sclerotinia or blackleg. The best time to scout for clubroot symptoms on roots is late in the season, approximately two weeks before swathing, when root galls should be easy to identify. Click here for more information on how to identify clubroot.
Clubroot prevention for non-infested regions
The key here is to be vigilant with machinery or field inspectors coming from known clubroot infested regions. Keep them out of your fields unless they sanitize thoroughly. Clubroot is soil-borne and spreads with the movement of infested soil.
Growers should also become familiar with the disease and what it looks like so they can take steps to limit its spread if it does arrive. Spend some time at www.clubroot.ca or take in a trade show presentation on clubroot.
Finally, growing clubroot resistant varieties is a risk management option for growers outside known clubroot zones. If the clubroot pathogen does arrive, resistant varieties could prevent it from becoming established in the field.
Published on Thursday, December 23, 2010