Growing Brassica Crops on Soils Infected with Club Root Disease
Club root is caused by a primitive single-celled organism called Plasmodiophora brassicae which infests many acidic wet soils and is endemic throughout the temperate regions world-wide. It attacks many brassicaceous plants with varying degrees of virulence, though some plants are tolerant, while others are unaffected. Its main cause for concern is its effect on food and other crop plants of the cabbage family, including cabbage, cauliflower, sprouts and other brassicas, including oilseed rape. Ideally, it is best to avoid using contaminated soil for the cultivation of these plants, especially commercially, but many areas are affected, including allotments, where ignorance and neglect can result in severe infestation and raising of brassica crops to a successful harvest is nigh impossible and best avoided.
However, it is possible to cultivate brassicaceous crops on soils containing light infestation of club root disease (Plasmodiophora brassicae), though certain essential precautions need to be rigorously observed to prevent the spread and growth in incidence and effects of the disease.
Plasmodiophora brassicae has a life-cycle that is only partially pathogenic to brassicaceous crops. The life-cycle commences from an encysted spore residing in the soil of infected land. This spore is resistant to changes in the evironment and nature of its surroundings and remains viable for many years (various sources give conflicting estimates of between 15 – 25 years)
When suitable conditions arise (a rise in soil temperature, plentiful moisture, and an acid soil) the cyst will 'hatch', releasing a single-celled flagellate organism, which will propel itself by means of its two anterior flagellae through the soil moisture to the root of an adjacent plant. Though it 'prefers' roots of a brassicaceous plant it is not essential and it can infect the roots of grasses as well as other species of plants (weeds). This zoospore enters a cell of the root of its 'primary' host and loses its flagellae and undergoes cell division to produce many cells, the walls of which then break down to produce a multinucleate body or 'Plasmodium'. This phase is non-pathogenic. The plasmodium eventually reforms its cell walls to become many single celled flagellate zoocytes again, which break out of the primary host root and again swim through the soil water to infect a brassicaceous plant, or re-infect the original primary brassica host. This secondary zoocyte is the pathogenic form and again enters the secondary host through its root cells. Once established in the secondary host it repeats the process of cell division and formation of a plasmodium and causes the host plant to also undergo cell division to produce a swollen mass of cells or 'gall' which interferes with the uptake of water and nutrients through the root xylem, causing wilting, stunted growth and possibly eventual death of the host plant. The plasmodium, meanwhile forms single-celled cysts which are released back into the soil as the host root rots, and thus increases the infection within the soil.
There are phases within this life-cycle that renders the organism more vulnerable and thus less harmful to the crop plant.
Firstly, the soil water must be acidic (below pH6.5) for the cyst to 'hatch' and the zoospores fully survive. Liming the soil to increase the alkalinity (reduce the acidity) of the soil water reduces the survival rate of the 'hatched' cyst and the incidence of infection of a primary host. Secondly, it is also therefore desirable to have a completely weed-free soil to further reduce the survival of those zoospores which have hatched, as they cannot then complete this first phase of its life-cycle. Thus time must be allowed before planting the crop plant, which means the soil must be prepared early and kept weed-free until planting time, and thereafter. The soil should be dug in the autumn and limed, then allow the winter rains to wash the lime into the soil and given time to take effect. Alternatively, the prepared soil may be sown with a fast-growing 'trap crop' such as mustard, left to germinate and grow to attract the primary non-pathogenic infection and then cleared completely (including roots) and disposed of completely (preferably by drying and burning) away from the area to be cropped.
The soil should then be planted with the crop plant, which must have been raised in sterile compost, not directly sown into seed beds that may possibly be infected and then transplanted to the crop bed. Lighter, sandier, freely-draining soils hinder transmission by making it difficult for the zoospores to swim in the reduced soil water.
Once planted the crop plants must be regularly inspected and at any sign of wilting or leaf-yellowing, or stunted growth, must be rogued out of the crop and disposed of well away from the growing area. When the crop reaches maturity the whole plant, including roots, should be dug out and harvested. Cut the roots off and again dispose of completely, including attached soil, not composted. Do not leave the stump of the stem and roots remaining in the soil to rot down and disperse any encysted spores to re-infest the soil.
It is possible to transmit infection from affected areas to areas free of infection in soil attached to boots, shoes and tools and on the paws of pets. Strict hygiene should be used to avoid transmission by changing foot wear, washing tools and restricting pets, before entering from infected areas on to areas free of contamination. Affected crops should never be added to the compost heap, but disposed of by destruction away from the area.
Remember that brassicas are large plants that grow quickly, so it is important to give sufficient fertiliser and water to enable the plants to grow strongly and thrive. There are varieties which have been bred to have in-built resistance to club root and these should be chosen for areas of land suspected of contamination. However, 'resistance' does not imply total 'immunity', and it is still possible for resistant varieties to become infected, though they may 'tolerate' any infection more succesfully.
By strict observance of these precautions, the spread and build-up of the organism is severely restricted and infestation with viable encysted spores slowly reduced. Good crop rotation practices will also help reduce infestation over time. Although a harvestable crop can be produced it is unlikely that final yields will be as high as when grown on land free from contamination.
The amateur allotmenteer should not abandon all hope of growing brassicas, but should adopt a positive and pro-active approach to minimise the incidence and spread of the disease. A 'sloppy' observance of these methods of containing and minimising its incidence will only perpetuate and increase the contamination, both for yourself and those that follow.