An excerpt from a study on production techniques to where a site has a history of clubroot disease (Plasmodiophora brassicae):
Crop rotation is essential, brassicaceae should not be grown in the same place each year - a minimum 3 year cycle is recommended. The optimum alkalinity level to help suppress clubroot is at pH7.2 to 7.5. Test pH levels and in the example of a soil of pH6.0, apply lime once in autumn at a rate of 550/m2 in the area. Repeat liming in each brassica area every year, and check pH level every 3 years. Identify and remove any weed hosts such as hairy bittercress and shepherds purse, also ornamentals like wallflowers.
Don’t sow the brassica seed direct into the soil; instead sow away from the production area in fresh sterile soil. before sowing, sterilise this propagation area using steam, maintaining a minimum 90 degrees celsius for 30 minutes to a depth of 20cm. Prick out the plants into 12cm pots when the plants have 2 or 3 true leaves, burying the stem as deep as you can without the leaves touching the soil. Only ever water from the bottom, and trim off any roots emerging from the bottom of the pot. This will encourage them to form a good, strong fibrous root system.
Grow on in the pots until they have a stem diameter of around 6mm and are around 20cm high. Before planting out onto the site, ensure that there is good drainage. The spores of the disease prefer wet conditions, and can be killed by heat. Lighting fires on the area to be used for brassicas may help, also repeated cultivation in warm dry weather. Don’t dispose of brassica waste by composting; burn it.
