All brassicas including ornamentals such as wallflower are susceptible to clubroot disease.
Minute soil-borne microbe (Plasmodiophora brassicae) present as resting spores that are characterised by their extreme longevity (more than 20 years) and high concentrations (more than one million spores/gm).
These release mobile spores that fix onto and invade root hairs. From there the pathogen invades more deeply, disrupting cellular growth and functioning. Multiplication by the microbe produces another generation of many millions of resting spores that are released back into the soil.
Infected plants wilt, growth fails, the leaves turn reddish/purple and quality is totally destroyed. Normal root growth ceases and is replaced by characteristic development of galls (pictured). Plants become incapable of taking up water or nutrients and eventually rot and die.
Clubroot disease is encouraged by warm, moist conditions.
Waterlogged, poorly drained and badly structured acidic soils (pH less than 6.5) favour disease development. In some areas alkaline soils (pH greater than 7.0) containing large numbers of resting spores are now too risky for vegetable growing, especially where previously cropped with oil seed rape.
Integrated disease management
Clubroot disease can be controlled only by using systematic crop management aiming at reducing the concentration of resting spores present in the soil, stimulating crop health and building populations of benign microbes that are antagonistic against the pathogen.
Soil care Retain cereal stubbles overwinter or drill ryegrass cover crops, which builds soil organic matter content, microbial activity and soil structure. Increase soil drainage using pan-breaking tines and deep ploughing.
Farmyard manure Clubroot spores resist acidic degradation in the guts of cattle and sheep and are expelled into farmyard manure. Caution should be exercised ensuring that beasts have not been fed on clubroot-infested swedes or turnips. Nor should they have been folded on clubroot-infested land prior to being finished on lowland vegetable land because mud containing resting spores can be transported in their hooves and wool. There is evidence that clubroot spores can be transported in infested soil attached to seed potatoes and similar vegetative propagules.
Liming Use regular soil analyses as guidance for regular liming applications that maintain a soil pH greater than 7.0. Calcium carbonate limes raise pH but it may take several months before a treatment is effective. Quicker efficacy is obtained with calcium oxide and hydroxide-based products applied by approved contractors.
Specialist products such as sugar beet lime (LimeX) are also valuable treatments.
Integrating non-brassicas into the cropping programme helps to reduce the numbers of soil-borne resting spores, but this is a long-term process requiring breaks of three-to-five years. Allium, legume and cereal break crops are particularly valuable.
Drilling or transplanting into ridges is associated with reductions in clubroot, possibly because of improved soil aeration and drainage, which encourages more vigorous and healthy root systems.
Several fertiliser products are associated with increasing crop health and quality as well as diminishing the impact of clubroot disease.
Calcium cyanamide (Perlka) This supplies calcium and nitrogen for healthy crop growth and encourages populations of benign soil microbes that help to diminish the impact of clubroot disease. This fertiliser has long-lasting efficacy and contains compounds that diminish soil nitrification and consequential nitrate leaching into groundwaters. It is therefore especially valuable in nitrate-sensitive zones. In-the-row applications increase the cost-beneficial efficiency of this product.
Calcium nitrate (YARA Tropicote) This is the most rapidly available form of calcium and nitrogen for root uptake. Its use is also associated with improvements in crop health and quality as well as reductions in clubroot intensity. Normally it is applied as a top dressing as crops are approaching maturity.
Boron (Solubor and boronated compound fertilisers such as Nitrabor) Applications of boron, especially in association with non-ionic wetters applied at transplanting, diminish root invasion by the pathogen for periods of up to 14 days, allowing crop establishment and initial growth.
The use of boronated compound fertilisers has longer-term beneficial effects, reducing clubroot impact.
Rigorous cleanliness and hygiene should be observed by plant propagators. Before reuse, recycled cell trays and stillages should be thoroughly cleansed of field soil and debris that might convey clubroot spores.
These are available from some plant breeders in crops such as cabbage, cauliflower and Chinese cabbage. Similar mono-genes are being deployed in each of these crops and in clubroot-resistant oil seed rape cultivars. The resistance therefore requires management, otherwise tolerant strains of the pathogen will emerge and negate this form of control.
Resistance should not be used as a single strategy and should instead be part of the package of integrated disease control. Land that has grown clubroot-resistant oil seed rape should be avoided and there should be a break of two-to-three years between vegetable cultivars that contain resistance genes. Managing resistance will slow but not stop the evolution of tolerant strains of P. brassicae.
Agrochemicals No chemicals active against P. brassicae are registered for use within Europe. Elsewhere, some products are used but it would be illegal for them to be applied to any vegetables that are destined for the European market.
For use on all fresh produce. Growers of vegetable crops must, in advance of use, ensure that a particular commercial agrochemical formulation is legally acceptable for their particular crop/husbandry regime and also accepted by the intended purchaser's crop-quality standards specification, as agreed with the relevant crop technologist.