Control Clubroot
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Information adapted from Agri-Facts, Clubroot Disease of Canola and Mustard, Alberta Agriculture and Rural Development, March 2011 Revision.
Prevention of clubroot requires all participants in the canola industry to be vigilant. Stopping the introduction of clubroot to clean fields is the best method to control clubroot because once clubroot is established in the field, it is nearly impossible to eradicate this disease. Since wide-spread clubroot infestations are still relatively localized to central Alberta, take precautionary measures to curb the spread of this disease outside known infested areas.
Clubroot is spread mainly by the movement of soil containing soil-borne resting spores. Soil transport can occur by wind and water erosion, on field machinery (see Figure 2), in manure from animals fed contaminated feed, and in soil attached to seeds (earth tag). The amount of soil required to initiate infection in a new field is unknown. Therefore, any soil transfer from an infested field should be viewed as a risk. 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.
Growers, custom operators and all land users (including oil and gas industry, recreationalists) must be diligent in removing potentially contaminated soil from equipment prior to leaving fields.
The best approach to managing clubroot is to be proactive. If you are a grower or landowner, ask agronomists, custom operators, researchers, and anyone else coming onto your land (oil and gas crews) about the sanitation measures they use to prevent the spread of clubroot.
Recommended Prevention Strategies
The following have been adapted from the Agri-Facts, Clubroot Disease of Canola and Mustard, Alberta Agriculture and Rural Development, March 2011 Revision:
1. Use long rotations – do not grow canola more frequently than once every four years (i.e. three years out of canola). Although this practice will not prevent the introduction of clubroot to clean fields, it will restrict this and other canola disease development within the field and probably avert a severe infestation.
2. Planting clubroot-resistant varieties on fields with no history of this disease can be useful when clubroot is present nearby. 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.
3. 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 resting spores are most likely to spread via contaminated soil and infected canola plant parts. Thus, producers should follow the practice of cleaning soil and crop debris from field equipment before entering or leaving all fields. The equipment cleaning procedure involves knocking or scraping off soil lumps and sweeping off loose soil.
For risk-adverse producers, the following additional cleaning steps may provide some extra benefit but involve considerably more work and expense:
a. after removal of soil lumps, wash off equipment with a power washer, preferably with hot water or steam
b. finish by misting equipment with weak disinfectant (1-2% household bleach solution)
4. 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.
5. Scout canola fields regularly and carefully. Identify causes of wilting, stunting, yellowing and premature ripening – do not assume anything!
6. 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.
7. Avoid common untreated seed (including canola, cereals and pulses). Earth-tag on seed from infested fields could introduce resting spores to clean fields. Certain seed treatment fungicides may control spores on contaminated seed, but this observation needs further research to confirm.
Recommended Guidelines for Field Entry Activities
This document was developed to provide guidance on field entry procedures in both clubroot and clubroot suspect areas.
Recommended Guidelines for Field Entry Activities
Clubroot Field Entry Checklist
This checklist can be used as a simple way to document that appropriate steps were taken during a field visit in both clubroot and clubroot suspect areas.
Clubroot Field Entry Checklist
Recommended Guidelines for Disinfesting Farm Machinery and Equipment
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 Disinfesting Farm Machinery and Equipment
Recommended Guidelines for Canola Field Research Activities
This document was developed by members of the canola research community in Canada to provide coordinated guidance that will help ensure that research activities are executed in a manner which will help prevent the spread of clubroot.
Recommended Guidelines for Canola Field Research Activities
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. View it here.
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.
Managing clubroot after establishment in a canola field is difficult. However, with the introduction of clubroot resistant canola varieties, growers now have a new tool to manage this disease. Commercial varieties with clubroot resistance are available from most seed companies, and are reported to provide high levels of clubroot control in clubroot-infested fields. It is important to note, however, that clubroot resistance is not expected to be durable (meaning it will break down over time). Therefore, growers should not rely solely on clubroot resistance to manage this disease once it is established in a field. Canola growers are encouraged to use multiple tools (rotation, sanitation etc) to manage clubroot as this will give the best long-term control.
Researchers are continuing to develop new management tools as well. Currently there are some management tools available in higher value brassica 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).
This section outlines some of the management tools that are either available or under investigation.
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. Screening of the clubroot populations shows that there are other pathotypes present as well. This diversity in pathotypes indicates that clubroot resistance may break down or erode over time and, therefore, the canola industry will need to develop strategies to keep ahead of this disease. 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.
Current canola varieties with resistance available to growers are:
· 45H29 – from Pioneer Hi-Bred full registration in 2010
· 9558C – from Viterra – full registration in 2010
· D3152 – from DuPont – full registration in 2010
· 73-67 RR – from Monsanto – full registration in 2011
· 73-77 RR – from Monsanto – full registration in 2011
· 1960 – from Canterra – full registration in 2011
· L135C – from Bayer interim registration in 2011
· DL Seeds – 2 RR lines recommended for interim in 2011
All canola seed companies continue to develop clubroot resistant varieties and expect to introduce new varieties in the next few years. Researchers for these organizations also state that they will introduce new sources of resistance (different resistance genes) to follow the existing resistance genes on the market today.
These current varieties are not immune. Growers using clubroot resistant varieties in clubroot-infested fields should expect 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.
Rotation
Long rotation break out of canola is an agronomic strategy that can complement the use of resistant varieties to cope with clubroot infested canola fields. 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, which are suspected to survive in soil up to 20 years.
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 within a field and between fields.
Molecular tests (namely quantitative PCR) 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.
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.
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Product
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PCP#
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Crops
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Quintozene (Terraclor®) 75%
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07251
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Broccoli, brussel sprouts; cabbage; and cauliflower
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Adobe 75WP
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28663
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Broccoli, brussel Sprouts; cabbage; and cauliflower
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Crusoe 75WP
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28238
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Broccoli, brussel sprouts; cabbage; and cauliflower
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|
Quintozene 75WP
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27416
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Broccoli, brussel sprouts; cabbage; and cauliflower
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|
Quintozene 75WP
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11425
|
Cole Crops
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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 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
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.
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 via 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[G1] . Resting spores of the clubroot pathogen have been detected on canola, pea and wheat seeds, as well as potato tubers, harvested from clubroot-infested fields, suggesting that the seedborne dissemination of clubroot is possible. However, the levels of infestation found were generally lower than that required to consistent symptoms in greenhouse plant bioassays, and practices such as commercial cleaning and seed treatments appear to be sufficient to mitigate this risk.
More information on minimizing the spread of clubroot can be found at Prevent Clubroot.
Stewardship
Now that clubroot resistance is available for canola growers, we need to think about strategies to ensure that this management tool will last a long time. Clubroot resistance is not expected to be durable, which means that it’s expected that it will breakdown or erode over time. The clubroot pathogen has been observed to overcome resistance in Sweden in two crop rotations. For how many crop rotations resistance in Canada can be expected to be durable is not known.
Clubroot resistance may be the best tool we have to control this disease, but this tool needs to be used correctly if we want it last a long time. Researchers continue to improve our understanding of this disease and build stronger and more durable sources of resistance, but we need to be careful in using resistance in these introductory years.
Suggestions for using clubroot resistant varieties:
- Rotation. Do not use clubroot resistant varieties in more than a one 1 in 4 year rotation. This includes fields that are known to be infested with clubroot and those that do not have the disease. Using these varieties more frequently than a 1 in 4 rotation will increase the selection pressure on the clubroot pathogen to change and to eventually overcome the resistance.
- Scouting. Growers using clubroot resistant varieties should scout their field regularly to see if there is any evidence of the resistance breaking down. If the field is known to be infested with clubroot, then scouting should be very thorough at the end of the season. ”Thorough” means examining the whole field to measure if there is an indication of reistance failing which will have a large impact on the future use of canola on that field.. If the field is not known to be infested with clubroot, then scouting near the field entry points should occur once near the end of the growing season.
- Manage weeds. Control of canola volunteers and other weeds that act as hosts for clubroot will be essential to keep the selection pressure low by not allowing the disease to build-up in the non-canola years.
- Sanitation. Preventing spread of the disease is important in the long term control of this disease even with clubroot resistant varieties. Sanitation practices are essential for fields known to be infested with clubroot, and important to ensure that areas without clubroot are not infested as well. In general, all the prevention strategies should still be employed even with clubroot resistant varieties.
To find out more about specific stewardship policies, please visit your canola seed company for more details.