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Prevent Clubroot

Figure 1. Scouting for clubroot

Photo courtesy of S.E. Strelkov, University of Alberta

Figure 2. Cleaning farm equipment

Photo courtesy of Ron Howard, AAFC

Since the clubroot pathogen and primary disease symptoms all occur underground, it is crucial to scout fields throughout the season and pull up roots to look for symptoms (Figure 1). Clubroot is spread by the movement of soil containing soil-borne resting spores. Soil transport occurs mainly on farm machinery (see Figure 2). Clubroot surveys in Alberta have found that almost all new infestations begin near the field access, which indicates that contaminated equipment is the predominant spread mechanism. Any vector that moves soil will move this disease. Wind and water erosion, recreational vehicles, livestock, manure, hay, seed potatoes, common (uncleaned and untreated) seed, exploration/construction equipment, and even footwear may also move this pathogen. The amount of soil required to initiate infection in a new field depends on the number of spores in the soil being moved. Heavily infested soil requires significantly less soil to initiate infection than lightly infested soil. But in general, those vectors that move the greatest amount of soil are the greatest risk. Therefore, any soil transfer from an infested field should be viewed as a risk.

The best approach to managing clubroot is to be proactive. If you are a grower or landowner, ask those coming onto your land about the sanitation measures they use to prevent the spread of clubroot.

Recommended Prevention Strategies

Any individual who contacts agricultural soil should consider the risks of moving P. brassicae. These are some strategies that may help prevent the movement of this pathogen:

  1. Practice good sanitation to restrict the movement of possibly contaminated material (this approach will help reduce the spread of other diseases, weeds and insects too). The equipment cleaning procedure involves knocking or scraping off soil lumps and sweeping off loose soil. The level of sanitation should be based on the level of perceived risk. Equipment moving from infested fields to non-infested fields is high risk and should be sanitised to the highest degree.
  2. Restrict access to fields if risk of transfer of infested soil is perceived.
  3. Avoid common untreated seed. Earth-tag on seed from infested fields could introduce resting spores to clean fields.
  4. Avoid the use of straw bales and manure from infested or suspicious areas. Clubroot spores are reported to survive through the digestive tracts of livestock.
  5. Planting clubroot-resistant varieties on fields with no history of this disease can be useful when clubroot is in your community. This strategy relies on the genetic resistance to greatly reduce disease development/establishment compared to susceptible varieties if clubroot is inadvertently introduced to the field.
  6. Use direct seeding and other soil conservation practices to reduce erosion. Resting spores move readily in soil transported by wind or water erosion and overland flow.
  7. Rotations. While a long canola rotation will not prevent P. brassicae from being introduced to your farm, nor prevent this pathogen from being spread around the field and to other fields, long rotations will help prevent the build-up of clubroot resting spores. Growers in clubroot-infested areas should grow resistant varieties, and preferably only once every four years in order to reduce soil inoculum levels and preserve existing clubroot resistance for their farms for as long as possible.

        Manage Clubroot

        A patch of prematurely ripening canola

         

        Managing clubroot after establishment in a canola field is difficult. Commercial varieties with clubroot resistance are available from most seed companies and provide high levels of clubroot control. It is important to note, however, that various strains of the pathogen exist in field populations, and therefore clubroot resistance may become less effective with continued use. Therefore, canola growers are encouraged to use multiple tools such as rotation, sanitation, biosecurity planning (use of separate field entrances and exits, machinery work in most-infested field last, restricting traffic to infected patches of field), amendments, etc. to manage clubroot as this will give the best long-term control.

        Researchers are continuing to develop new management tools and management strategies, such as liming and bait crops, as well.

        Genetic Resistance

        Clubroot was present in Alberta prior to 2003, but it was not considered economically damaging in canola until that year when it was found in a severely infected canola crop near St. Albert. This virulent and economically damaging pathotype of clubroot was determined to be pathotype 3, Alberta’s most predominant pathotype. Screening of the clubroot populations shows that there are other pathotypes present as well. This diversity in pathotypes allows the pathogen to persist by favouring development of pathotypes not designed to be controlled by resistant varieties. The introduction of varieties with resistance to this predominant pathotypes may create selection pressure and increase other less common pathotypes able to infect our resistant varieties. Managing new genetic tools properly through appropriate crop rotations and agronomic practices will be necessary to preserve their effectiveness against clubroot.

        Understanding Clubroot Resistance and the Classification System

        Clubroot resistant (CR) canola varieties are key tools used to delay clubroot establishment and manage clubroot disease on the farm. However, to prevent rapid genetic shifts in clubroot populations and subsequent loss of effective resistance in CR varieties, this valuable resource must be used judiciously in an integrated management approach, which includes practicing a diverse crop rotation with at least two years between canola crops, effectively managing weeds, sanitizing equipment and minimizing soil movement.

        Resistance Classifications

        Clubroot resistance in a variety should be substantiated through standard testing procedures outlined in the Western Canada Canola/Rapeseed Recommending Committee (WCC/RRC) guidelines and protocols. Varieties are compared to the susceptible check variety for clubroot infection and are assigned either resistant (R), intermediate (I) or susceptible (S) ratings.

        Resistant (R) Intermediate (I) Susceptible (S)
        Classification Less than 30% infection compared to susceptible checks in disease tests. Between 30 and 50% infection compared to susceptible checks in disease tests. More than 50% infection compared to a susceptible check.
        What this means (R) varieties are not immune, but highly restrict the development of clubroot symptoms in fields with low to moderate disease pressure from resting spores in the soil. This (I) rating will mostly be used for adding rating labels to the base (R) label in multiple resistance gene varieties to specify moderate resistance against certain new strains. If there is no CR label on a variety, assume it is susceptible to clubroot. An (S) label could be added to a base (R) label to specify susceptibility to certain strains that aren’t common. 

        What to expect Under heavy pressure in severely infested fields, an (R) variety can show significant root galling, but may develop fewer and smaller galls than a susceptible variety. Under these heavy pressure situations and frequent use of CR varieties, clubroot populations rapidly evolve to strains that overcome the resistance.

        Although intermediate resistance may restrict the development of clubroot symptoms for the corresponding strains, the spore concentration in the soil will be increased. An extreme buildup of spores can occur very quickly when susceptible varieties are grown in short rotation on slightly infested fields. 
        Management tips To delay this shift in clubroot strains and loss of CR variety efficacy, CR varieties should not be grown in short rotations in infested fields. Varieties with additional (l) labels can provide marginally better disease protection on fields with presence of new corresponding strains, but should not be grown in fields where resistance to predominant strains has been widely defeated. Susceptible varieties should not be grown in clubroot-infested fields, or those at risk of becoming infested soon.

        Clubroot Pathotypes

        A base (R) resistance label requires that the variety is resistant to the predominant clubroot strains or pathotypes in Western Canada. Additional ratings can be appended to the base (R) label to describe resistance to specific uncommon or new pathotypes.  

        No CR varieties, including new ones with multiple resistance genes, are resistant to all of the clubroot pathotypes detected in Western Canada to date. As clubroot populations in infested fields become more diverse over time, and more CR resistance genes are bred into canola varieties, the usefulness of rotating CR varieties with different resistance will increase. Currently there are no tests commercially available for growers to distinguish or detect new virulent strains in their infested fields.

        Careful scouting to detect early infestations to alert growers and deploy resistant varieties is of utmost importance. Waiting to use (R) varieties until significant infestations have developed will create high soil spore loads and increase the probability for pathogen shifts which can rapidly defeat variety resistance.

        Visit www.clubroot.ca to learn more.

        These are the current clubroot-resistant varieties (as of September 2017):
         

        Pioneer Hi-Bred/Dupont
        45H29
        45H33
        D3155C
        45CS40
         

        Monsanto/DeKalb
        74-54 RR
        75-42 CR
         

        Bayer
        L135C
        L241C
         

        CPS
        PV 580 GC
        PV 581 GC
        PV 590 GCS
        VR 9562 GC
         

        Canterra
        CS2000

        Brett Young
        6056 CR
        6076 CR
        6086 CR
        4187 RR (previously SY 4187)
         

        Dow AgroSciences
        2020 CL
        1020 RR
        1024 RR
         

        Cargill
        V 12-3
        V 14-1
         

        Syngenta
        SY 4105


        Growers using clubroot-resistant varieties in clubroot-infested fields may experience some infected plants, which can be attributed to susceptible volunteers and off-types. Volunteer canola seed can germinate many years after it was last grown, and if this comes from a susceptible canola crop, then the volunteers will be susceptible. Off-types are a normal part of hybrid canola production – no canola hybrid is 100% pure, so there may be a small proportion (1 to 4%) of the seed that is susceptible. 

        When scouting, if one finds infection rates of greater than 10% of seeded plants (do not count volunteers) then that may indicate that the clubroot resistance is no longer functional against the pathogen population in the field. These infected plants may be restricted to a small patch which indicates a recent pathogen change. Indeed, in 2013, the first field confirmed to have clubroot resistance failure was due to the buildup of a strain/pathotype in the population capable of overcoming the clubroot resistance being used.

        Rotation

        Long canola rotations will help to prevent the build-up of clubroot resting spores and significantly reduce the number of viable spores in the field. Growers in clubroot-infested areas should reduce frequencing of growing canola in order to reduce soil inoculum levels and preserve existing clubroot resistance for their farms for as long as possible.

        It is also important to understand what a long canola rotation will not accomplish. A long canola rotation:

        • Will not eliminate P. brassicae from an infested field. 
        • Will not prevent clubroot from being introduced to a field. 
        • Will not prevent clubroot from being spread throughout a field.
        • Will not prevent clubroot from being spread to another field. 
        • May not increase your production, BUT, long canola rotations will reduce P. brassicae resting spores when used in conjunction with clubroot-resistant varieties and help preserve the effectiveness of resistance.

        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 within a field and between fields.

        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 on a field scale. 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 traces of clubroot on seed surfaces is currently being explored.

        Liming

        Liming acid soils to above pH 7.2 has shown erratic results for clubroot control in cole crops in British Columbia and eastern Canada. Other countries have had moderate to great success with liming lightly infested fields or liming prior to infection. Canadian researchers are continuing work with soil amendments.

        Bait Crops

        Recently completed research at two highly infested field sites in Alberta found that bait crops had no effect on clubroot severity. 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. Although the Alberta research indicates that bait crops are not useful in severe infestations, they might be useful in light infestations.


        Clubroot Management Resources

        Managing Clubroot: Equipment Sanitation Guide
        This pamphlet provides the steps needed to be taken when sanitizing field equipment.

        Recommended Guidelines for Disinfesting Farm Machinery and Equipment (PDF)
        This document outlines detailed steps that can be utilized to minimize the risk of clubroot spread via contaminated soil on field equipment. Any inquiries regarding this document should be directed to duke@gov.ab.ca.

        Recommended Guidelines for the Oil and Gas Industry
        The Canadian Association of Petroleum Producers adopted a set of best management practices to limit the spread of clubroot.

        Recommendations for Managing Clubroot Risks Associated with Field Research (MS Word)
        This document highlights the prevention measures that the agricultural research industry should practice to minimize the risk of clubroot movement. These guidelines were developed by the Saskatchewan Clubroot initiative for the larger agricultural industry.


        References

        Information adapted from Agri-Facts, Clubroot Disease of Canola and Mustard, Alberta Agriculture and Rural Development, March 2011 Revision and Clubroot of Brassica Crops, Manitoba Agriculture, Food and Rural Initiatives, May 2008.