Infection and susceptibility
Ideal conditions for clubroot infection are:
- acidic soils
- high soil moisture
- warm temperatures (20-25°C)
- presence of susceptible brassica host.
Infection can occur in soils where the ideal conditions are not met however the disease will be less severe.
Infection in the plant’s root hairs takes place soon after brassica plants contact infested soil. Further infective spores are rapidly produced in the plant roots and the characteristic swelling of the roots occurs. The spores can infect adjacent roots and nearby brassica plants. Plants are susceptible to the clubroot pathogen at all growth stages.
Brassica crops vary in their susceptibility to clubroot. Chinese cabbage is very sensitive to the disease. Cauliﬂower is less susceptible but more susceptible than broccoli and head cabbage. There is also different tolerance to clubroot between varieties within each species.
As there are several strains, crop tolerance to one strain of clubroot may not mean the crop is tolerant to other strains of this disease.
Farm hygiene is very important in preventing the spread of the clubroot pathogen to new properties and reducing the risk of the disease spreading to non-infested areas on infested properties. The disease can be transported in soil on machinery, farm implements, footwear, livestock or vehicles. Clubroot spores can also be spread in water.
Good on-farm hygiene practices include:
- Thoroughly clean all equipment that enters and leaves a farm, using high pressure washing, to ensure that soil is not being transported.
- Work from the least infested areas of the farm to those that are the most infested.
- Purchase high quality seedlings from a reputable source and avoid placing racks and trays on the ground.
- Where possible, irrigation dams should not receive run-off from infested paddocks. Where this is not possible, irrigation water should be taken from the surface, in the stillest part of the dam. Clubroot spores will settle to the bottom, so avoid taking the muddy water from near the bottom of a dam.
- Erect signs asking visitors to respect your farm hygiene. Visitors and workers should report to a designated area, such as the office, prior to entering the farm and be aware of your farm hygiene protocols.
Clubroot spores can persist in soil and water for up to 20 years even in the absence of host plants. There is no known method to eradicate spores from infested land. However, clubroot can be managed, allowing brassica crops to grow on infested land. It is important to use several management techniques together to ensure the best control possible.
Some factors increase the risk for clubroot infection and include:
- acid soils
- short rotation time between brassica crops
- the presence of brassica weeds
- poor drainage
- clay soils
- planting during the warmer months
- severe infection on the area when last cropped.
Assessment should be made of these factors prior to planting the crop. Clubroot can be managed by reducing the risk associated with these factors.
Soil pH and liming
Clubroot is less virulent when the soil pH is at 7.0 or above as the germination of the spores is inhibited. Paddocks that have acidic soil should be limed. Agricultural lime should be applied at least three months before crop transplanting to allow the lime sufficient time to influence soil acidity. Incorporate the lime into the soil and lightly irrigate after application to start the liming process. Look for lime with a high neutralising value and fine particle size when considering purchasing.
Care must be taken not to raise soil pH too much, otherwise nutrient deficiencies may occur or diseases of other crops may become more prominent, for example powdery scab on potatoes.
Some soils are unresponsive to lime. Do not keep adding lime if the expected increase in soil pH does not occur. Check to see if other soils in the area are unresponsive.
Light clubroot infections and those on acid soils are more likely to respond to lime than severe infections or those on alkaline soils.
Calcium and boron
Calcium and boron can help protect young seedlings from clubroot infection. Calcium is applied when soils are limed but extra calcium and boron should be applied in the first three weeks after transplanting. Calcium and boron are more effective at reducing the severity of clubroot gall formation when the soil pH is 6.5-7.5. Calcium nitrate with added boron may be used.
Crop rotation and weeds
The number of clubroot spores in the soil can increase rapidly in the presence of host crops or weeds. A short time between brassica crops will cause a rapid build-up of spores in the soil leading to severe crop symptoms and possible total crop loss. Lowering the number of spores in the soil is important as it reduces the rate of infection leading to a healthier crop. Low spore numbers make symptoms difficult to detect.
The following management practices do not guarantee that infection will not occur, but will help to reduce the impact of the disease:
- Avoid double cropping brassicas, but rotate brassica crops with non-brassica crops such as potatoes, carrots, sweetcorn or pasture.
- Control brassica weeds, particularly when non-brassica crops are being grown.
- Make the time between brassica crops as long as possible.
- After a brassica crop has been harvested, the remaining plant matter should be ploughed in to avoid plants re-shooting and remaining as live hosts.
Soil and weather
Clubroot symptoms are more prevalent in warm, poorly-drained soils. These conditions favour infection and disease development. Avoid planting in infested paddocks during warmer months. Even with mild infection, a crop is less likely to survive if the temperature is warm.
Chemical treatments should be combined with other management strategies such as crop rotation, liming and applying additional calcium and boron to improve control.
The registration and availability of chemicals for pest, disease and weed control changes regularly. Consult a trained and experienced horticultural agronomist or the Australian Pesticides and Veterinary Medicines Authority (APVMA) website for chemicals which are currently registered or have a permit for use on this crop. The information on the label or permit for a chemical must be followed, including the directions for use, critical use comments, withholding period and maximum residue limit. Quality assurance (QA) schemes for horticultural crop production require producers to have current information on chemical registrations readily available.
This information was originally authored by Rachel Lancaster.