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Prevent Clubroot

Figure 1. Scouting for clubroot

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

Figure 2. Soil on tires and equipment

The best way to prevent clubroot in canola is to prevent the spread of the disease-causing soil-borne pathogen, Plasmodiophora brassicae, into an area where susceptible Brassica species will be grown. The pathogen has several stages in its lifecycle including hardy resting spores, which are released into the soil from infected plant root material, and due to their tiny size, can easily move with soil particles. Therefore, minimizing the risk of soil movement is critical to preventing the introduction of clubroot to a new area (ex. a new field or farm). Any individual who contacts agricultural soil should consider the risks of moving P. brassicae. As an added benefit, equipment sanitation will also help reduce the spread of other diseases, weeds and insects too.

Any vector that moves soil could move this disease, and generally the vectors that move the greatest amount of soil are the greatest risk. Farm equipment, machinery and vehicle traffic can move clubroot resting spores when they move soil, as can wind and water erosion. So by ensuring all incoming traffic is free of soil and ensuring farm operations minimize the movement of soil (ex. by minimizing tillage), the risk of clubroot will be reduced. Talk with stakeholders about sanitation expectations and limitations and agree on a sanitation plan that best manages risk, cost and logistics. Review the Managing Clubroot: Equipment Sanitation Guide for detailed information on sanitation options. Since soil can also be moved by means of exploration/construction equipment, recreational vehicles, livestock, manure (clubroot spores are reported to survive through the digestive tracts of livestock), hay or straw bales, seed potatoes (that have soil on them), common (uncleaned and untreated) seed, and even human footwear, these methods 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 (and therefore will require the greatest amount of sanitation). As a result, any soil transfer from an infested field should be viewed as a risk.

Effective clubroot prevention can also 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.

Farm operations that move soil can spread clubroot

 

 

 

 

 

 

 

 

Additional Prevention and Management Strategies

In addition to preventing the movement of soil which could contain clubroot resting spores, these strategies may also help prevent or manage clubroot.

  1. Planting clubroot-resistant varieties on fields with no history of this disease can be useful when clubroot is in your community. Although this won’t prevent clubroot from arriving to your field, 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..
  2. Choosing 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.
  3. Utilizing crop rotations. Rotating canola with other crops is a valuable tool that can prevent clubroot resting spores from multiplying quickly once they are in a field (but rotations will not prevent clubroot spores from arriving at a new field or new location). A minimum of a 2-year break between clubroot host crops (assuming no clubroot host plants are present) is needed to help reduce clubroot spore levels in soil. Again, longer breaks between susceptible hosts (crops and weeds) will not prevent clubroot from arriving at a field or new location, and it will not prevent this pathogen from being spread around a field or being moved to subsequent fields, but it will allow clubroot spores to break down more thoroughly before another host is grown.

Growers in clubroot-infested areas should grow clubroot-resistant canola varieties and have a break of AT LEAST 2 YEARS between canola crops, in order to prevent clubroot resting spore numbers from increasing in the soil if they arrive.

Cleaning farm equipment

        Field Entry Strategy

        Crop pests (ex. weed seeds, insects and pathogens causing plant diseases) can easily be transferred from field to field and cause significant consequences if no precautions are taken. Clubroot, for example, is a very serious soil-borne disease that can affect many agricultural stakeholders (ex. farmers, landowners and agronomists), oil and gas industry members, and anyone in rural landscapes if they contact contaminated soil or plant material (and potentially contaminated , runoff water). Therefore measures should be taken to prevent the unintended transfer of any crop pests into or out of any fields by an individual, their tools, implements or vehicles.

        The document below outlines the proper stewardship measures that should be followed by individuals who are requested or required to enter fields, in order to minimize the transmission of crop pests into or out of a specific field.

        Canola Council of Canada Field Entry Policy (PDF)

        Manage Clubroot

        Managing clubroot after plant 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 when used in a proper rotation. It is important to note, however, that various strains of the pathogen exist in field populations, so clubroot resistance may become less effective with continued use, especially when spore loads are high. Therefore, canola growers are encouraged to use multiple tools such as rotation, sanitation, biosecurity planning (including the use of separate field entrances and exits), restricting traffic to infected patches of field (and trying to ensure machinery work in the most-infested fields is only done when time for proper cleaning is permitted), soil amendments, etc., to manage clubroot, as this will give the best long-term control.

        Researchers continue to investigate new management tools and management strategies, such as liming and bait crops, as well.

        Utilizing Genetic Resistance

        Each time a susceptible host is grown in the presence of the disease, clubroot spores multiply, making management more difficult. Pathotype 3H, the predominant pathotype, is very well controlled by the genetics available in current varieties. Screening of field clubroot populations show that there are other pathotypes present as well. This diversity in pathotypes allows the pathogen to persist by favouring development of those pathotypes not controlled by resistant varieties. Managing new genetic tools properly through appropriate crop rotations and agronomic practices will be necessary to preserve their effectiveness against clubroot.

        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 amount of the seed that is susceptible.

        When scouting, if clubroot infection rates (frequency not severity) are greater than 5% in a random selection of seeded plants (ex. if 5% of the plants that are randomly selected have clubroot symptoms), or if a patch, or patches of clubroot-infected plants are noticed, this could be an indication that the clubroot resistance is no longer functional against the pathogen population in the field.

        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 a 2-year break 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.  

        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, but genetic markers (which will become another management tool) are being worked on for quick identification of these pathotypes.

        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 only create high soil spore loads and increase the probability for pathogen shifts which can rapidly defeat variety resistance (and the use of resistant varieties before clubroot arrives won’t reduce the effectiveness of the resistance if it does arrive).

        Visit www.clubroot.ca to learn more.


        These are the current clubroot-resistant varieties (as of April 2019):
         

        Brett Young
        6076 CR**
        6090 RR
        4187 RR (previously SY 4187)

         

        Canterra
        CS2000
        CS2600 CR-T

         

        DeKalb (Bayer)
        75-42 CR
        DKTF 94 CR  

         

        Invigor (BASF)
        L135C
        L241C
        L234PC**
        L255PC
        L258HPC 

         

        ** Contains a new clubroot resistance trait

        Pioneer/Brevant (Corteva)
        45H29
        45H33
        45CS40
        45CM36**
        45H37
        45CM39**
        D3155C
        1024 RR
        1026 RR
        1028 RR
        2028 CL
        B3010M**

         

        Proven (Nutrien)
        PV 581 GC
        PV 591 GCS
        PV 585 GC**

         

        Victory (Cargill)
        V 12-3
        V 14-1



        Crop Rotation

        Crop rotation is a valuable tool to help manage clubroot spore loads. A minimum 2-year break between canola crops (and other hosts) will help reduce clubroot spore levels in soil. A long rotation between susceptible hosts (crops and weeds) will not prevent a field from having clubroot resting spores introduced to a new field or location, but it will allow clubroot spore numbers to decline before the next host crop is grown.

        Control host weeds

        Controlling host weeds in all crops is very important to prevent the further increase in spores. Weeds should be controlled within 3 weeks of emergence so they aren’t able to produce new resting spores. Known clubroot hosts include: canola volunteers, Brassica weeds (ex. flixweed, stinkweed, shepherds purse, wild mustard), tame mustard, camelina, and other Brassica crops and vegetables.

        Reduce Tillage

        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 inconsistent 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. There are a number of factors involved with attempting to achieve desired pH, so understanding products, rates, timing, placement and other properties of the soil and lime products are critical.

        Bait Crops

        A new management technique that is being investigated is the use of bait crops. In a bait crop situation, plants that are susceptible to clubroot are allowed to grow for approximately four to five weeks to stimulate germination of the clubroot resting spores. The susceptible crop is then ploughed down and/or sprayed (the best method is still being investigated) 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 host crop. Although recent research in Alberta indicated that bait crops are not useful in severe infestations (the bait crops had no effect on clubroot severity in two highly infested field sites in Alberta), more research is needed on their impact under lighter infestations. One of the risks with bait crops is the unpredictability of weather conditions when the bait crop needs to be sprayed (at the proper spray timing), in order to effectively terminate the host plants before they are able to propagate clubroot.

        Patch Management

        Often clubroot appears in small patches and these patches increase in size with the movement of soil outward. If caught early, these patches can be isolated and treated separate from the field. Grassing these patches and even pulling infected plants from these areas will help diminish spore loads and keep soil from moving. Read more patch management ideas in this Canola Watch article.

        Clubroot Equipment 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.

        See more clubroot-related resources in the Contact and Resources section.

        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.

        Date modified: April 23, 2019