Crops: Cole crops
Why is it a problem? Clubroot is a disease that deforms the roots of Cole crops preventing and distorting their development and making the plant susceptible to secondary infection. It is difficult to control, and spores can survive many years in the soil. High spore loads in the soil can make it unsuitable for growing cole crops for many years. Careful management is needed to prevent infections, and to prevent spore build up in the soil where it already occurs.
Where and when is it a problem? All Cole crop growing areas are likely to be susceptible. It continues to spread through Bhutan. Even in areas where it already occurs, it may currently only be affecting individual farms.
The disease results in striking clubroot, an abnormal enlargement of the root system with clubs often thickest at the center, tapering spindle-like towards the ends. Infection results in plants wilting, even under the slightest water stress. Initial foliar wilting can be followed by reddening of the leaves which become chlorotic and eventually necrotic. Plants may become stunted, and flowering accelerated.
Confusion with other pests: The club root is unmistakable.
Lifecycle: Clubroot is a soil-borne pathogen that infects plants through their root hairs. Once in the plant it stimulates abnormal growth of the tissue. Infection is favoured by excess soil moisture and low pH. Once infected resistant spores are produced within the clubbed tissue. When the tissue decays spores are released in the soil in which they can survive for at least ten years.
Dispersal: Spores can be moved long distances in infected soil and seedlings. Locally, they can be moved by water, soil and infected plants.
When can damage be expected? Incidence rates increase with spore loads in the soil. Problems can therefore be expected to steadily increase if management practices are not implanted, and if Cole crops and mustard are grown continually.
Hosts: Clubroot can attack a wide range of species in the genus Brassica. In Bhutan, it is becoming a very serious pest of Cole crops. However, it is also likely to be able to infect other Brassica crops such as mustard and turnip.
The best solution for managing is clubroot is prevention, both to prevent movement onto the farm and to slow its spread once on the farm. This can be achieved through regular monitoring and strict hygiene practices. It is very difficult to manage once the disease is well established. Management will require ongoing effort, and investment is needed to keep the spore loads down if the land is to remain suitable for growing Cole crops.
Careful, regular monitoring that will allow the early detection of clubroot is critical. If your farm is free of clubroot then become familiar with the symptoms and be aware of its presence in the surrounding area. If it is already on the farm then carefully monitor where it is to help prevent its further movement. Remove and destroy infected plants as they are found. In heavily infected fields monitor its prevalence through time to help inform management decisions such as crop rotation.
Effect of variety
Some resistant cultivars are available. However, plant resistance has not been very useful in clubroot control because of rapid development of new races of the pathogen.
At the border
- There are many different pathotypes in the world. The entry of new pathotypes into Bhutan should be prevented through strict quarantine practices, including only sourcing seed from accredited sources.
Keeping your farm, or part of the farm, free of clubroot
- Once clubroot is established within a field it is there to stay, and will therefore need to be managed on an ongoing basis. Strict quarantine and surveillance practices are therefore needed to keep it free of clubroot, especially if it is known to occur in the area
- Carefully wash all shared tools of soil and plant material prior to moving them on-farm, or from infected to uninfected areas. If feasible start any farm activities in areas that are known to be clean
- Avoid unnecessary movement on farms, by both people and animals, to prevent the movement of spores onto and within the farm.
- Only source seedlings that can be guaranteed to be free of clubroot. Ensure transplanting equipment are completely free of clubroot.
- If infected areas are found then minimize the risk of them becoming a source of infection. Fence off if necessary. Manage water run-off if possible. Stop growing cole crops and mustard, and prevent “volunteer” mustard plants. Do not use manure from that area to fertilise other parts of the farm.
- Remove and destroy infected plants as they become apparent, including wild hosts by burning. do not feed infected plant to cattle.
- Extension staff and researchers visiting club root infested areas should rinse their shoes/boots before leaving the farm. Alternately, soak shoes by immersing half of the shoe sole in bleach solution (robin fabric whitener) for at least 15 minutes. Clumps of soil must be removed from the shoe soles before soaking.
Keeping spore loads low
- Once a field is infected management needs to focus on preventing spore loads from building up to a point where Cole crops can no longer be grown. A long term strategy, together with ongoing monitoring, is necessary to achieve this.
- Crop rotation is critical for suppressing spore loads in the soil. Recent studies elsewhere suggest that two years with resistant or non-hosts can result in big reductions in spore loads.
- Ensure infected land does not have alternative hosts (like wild mustard) between crops.
- Avoid irrigating healthy fields with water running from infected fields
- Never use infected plants for composting as they will be a source of spores
- Raising the soil pH above 7.0 is a traditional method of control through application of lime to maintain alkalinity. However, studies elsewhere in the world often produce conflicting results. Note, this will not be sufficient on its own if other conditions are suitable (moisture, temperature and host). Also, raising the pH too high will have deleterious effects on the crop.
There are currently no chemicals available for use against clubroot in Bhutan.
Image acknowledgements: NPPC