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.