Brassicas are Britain's most valuable fresh vegetable. Production is concentrated in the South West, Kent, East Anglia, south Wales, Yorkshire, Lancashire, the Lothians and Fife. In the past decade clubroot disease has reached epidemic levels in all areas and is the largest single cause of crop loss. Even parts of Lincolnshire with soils above pH of 7.2 are badly infested. One pinch of infested soil contains tens of millions of the microbe's (Plasmodiophora brassicae) tiny spores.
The water and nutrient transport systems of infected plants are destroyed by the invading microbe. Crops turn bluish-green, wilt and massive tumours swell up in the roots, then growth stops and the plants die.
Reasons for the disease explosion
The reasons are four-fold:
- economic - highly intensive cropping of brassicas without rotational breaks in response to static or declining prices and a lack of uninfested land for rent;
- chemical - no agrochemicals are registered for use in Europe;
- environmental - mild, wet winters encouraging continuous growth of the microbe;
- genetic - resistance is limited and expensive to use.
It is not all doom and gloom, however. The good news is that clubroot can be controlled - and high-yielding, profitable crops can be harvested from infested land. Achieving control requires long-term strategic planning and disease monitoring. During the past 20 years scientists round the world have collaborated in formulating an "Action Plan for Clubroot". This uses elements of basic husbandry brought together into systematic steps. Control stages are increased in complexity as the level of risk posed by the amount of microbe in soil rises. The overall aim is encouraging natural control through physical, chemical and biological means.
Clean and low-risk soils
If clubroot is absent from your land, guard it well. Resist any temptation of "cleaning up" crop residues with imported sheep or cattle. They can bring the disease in fleeces, on hooves and in their guts, which may contain food residues from crops eaten on infested land.
Monitor contractors closely to prevent infection from being imported on dirty machinery coming from infected areas. All machines must be thoroughly washed before entering your land. Washings must not go into water courses supplying your crops.
Where economically feasible, incorporate non-brassica crops into rotations. Alliums, such as leeks, are especially helpful because their root exudates - substances that ooze out from plant pores - are apparently antagonistic to P. brassicae. This is because the exudates impede the swimming stage of the soil-resting spores of clubroot - the only time when the clubroot organism is vulnerable.
Other break crops, like cereals and legumes, prevent the microbe completing its life cycle. Swedish research has shown clubroot spores have a half-life of about five years. Ideally, rotations should mirror that. Weeds like charlock are "brassicas" allowing full reproduction of P. brassicae and must be eliminated.
Effective soil drainage is essential for both clean and mildly infected land. Clubroot thrives on waterlogged land. Deep subsoiling and chisel ploughing increases soil aeration and speeds water percolation down the profile. Water drainage physically moves spores away from the root zone. Adding farmyard manure enhances soil structure and stimulates the activities of benign microbes capable of destroying this pathogen. But the manure must come only from sources verifiably free from P. brassicae. If there is any doubt about this, do not use it.
Regular soil analyses are essential and should help maintain soil pH at 7.2. Apply coarsely ground limestone (calcium carbonate) in the autumn. One tonne per hectare of coarse lime should raise alkalinity by approximately 0.5 pH unit but takes four to six months to produce this effect. Be especially vigilant that contractors' lorries have not been used on infested land.
Grow crops using ridge or bed systems. This lifts developing roots away from pathogen spores and enhances further the quality of soil structure and water percolation. Research in Scotland showed that adding one to two per cent non-ionic wetter plus 15-20 parts per million boron (as Borax) into liquid starter fertilisers at transplanting reduced early infection. These chemicals reduce P. brassicae's ability to find host root hairs and grow inside them.
Low- to medium-risk soils
Calcium is a major part of integrated clubroot control. Both the chemical form and particle size affect its speed of action. Small particle sizes produce the fastest effects.
Calcium oxide, known as hot lime or Active Lime, produces the speediest changes to soil alkalinity and the effects disappear after about six months. This is a big advantage if potatoes follow brassica crops, as potato scab disease is encouraged by alkaline soils.
Mixing water with powdered calcium oxide produces the hydroxide form and dangerously large amounts of heat. Spreading hot lime is a task for registered contractors. The flaked calcium oxide product QuickCal can be spread with normal farm equipment, provided the standard safety measures are observed. Fine calcium carbonate is available from the sugar industry as LimeX. Both QuickCal and LimeX rapidly increase soil alkalinity and can be used in preparations for current-season crops.
Formulations of calcium and nitrogen offer efficient and economical combinations of liming value and nitrogen fertiliser. Calcium cyanamide (Perlka) fertiliser is a long-established and traditional source of both calcium and nitrogen. Research across the world has associated the use of Perlka with diminished incidence of clubroot. This is thought to result from the stimulation of soil microbes that are antagonistic to P. brassicae. Repeated application over several seasons builds soil fertility, encouraging biological control by bacteria like Pseudomonas.
Growing brassicas on ridges or beds allows banded applications of Perlka, reducing the amount used to about 0.5 tonnes per hectare.
Calcium nitrate is a highly soluble and readily available source of calcium and nitrogen also associated with diminishing clubroot disease. It is most useful as a post-planting topdressing. Calcium diminishes P. brassicae's movement towards root hairs and subsequent growth and encourages generalised host resistance.
Pyramiding clubroot controls
Combining husbandry elements yields effective clubroot control. Researchers in Australia and New Zealand achieved more by integrating several nutritional controls than was obtained using each alone. Adding products like QuickCal or LimeX with calcium cyanamide (Perlka) and calcium nitrate together is effective in reducing disease and cost efficient. Limes are applied first either during secondary cultivation, when ploughed land is broken down with tined or reciprocating harrows or as the planting beds are formed. Control is strengthened further by using boronated fertiliser.
Perlka is applied next into the ridges or beds as they are formed. This ensures proximity to roots emerging from the transplanting modules. At planting each module receives 75ml of liquid starter fertiliser supplemented with non-ionic wetter and boron. Finally, as the crop matures, calcium nitrate is used. Significant quantities of nitrogen and calcium are supplied to crops by both Perlka and calcium nitrate. These are debited against the normal nitrogen fertiliser needs of brassicas (usually around 250 units nitrogen per hectare). Scottish research has shown that building up soil calcium content substantially increased the activity of benign microbes, developing soils suppressive to P. brassicae.
Spores in soil
Integrated control demands knowledge of the amount of spores in the soil. Development of soil monitoring kits suitable for use in the field makes this approach more reliable.
Researchers at Warwick HRI have produced field kits for monitoring airborne diseases such as powdery mildew or light leaf spot. Now they are publicising kits capable of assessing the numbers of P. brassicae spores in soil. These kits use the principles of the ELIZA system, which apparently overcomes difficulties caused by the presence of soil organic matter in the test sample. Availability of a simple, cheap, field-robust test would be of much value for brassica growers round the world. Currently only a long-winded and less-than-accurate seedling bio-assay system is available. This can easily give falsely negative results depending on the environment used for the test and the skill of the technicians.
Measuring the effectiveness of integrated control systems where disease risk is low allows growers to determine their costs and benefits more precisely. Patenting and selling this test is a task for the biotechnology industry - hopefully to the ultimate benefit of the original investors, HDC's levy payers. Doubtless the University of Warwick already has negotiations in hand.
Including resistant cultivars into integrated control greatly raises its value. Recently, resistant cultivars of cabbage and cauliflower have come from Syngenta. These resistance genes were used previously in agricultural stubble turnips. P. brassicae contains physiological races that can tolerate this resistance. Preserving the resistance with integrated nutritional programmes is a priority.
Growing resistant cultivars on high-risk land is unwise. At high-risk sites the cultivars are exposed to substantial disease pressure and the speed of "breakdown" increases. Similar genes are present in some clubroot-resistant oilseed rape cultivars. Vegetable brassica growers using resistant cabbage and cauliflower cultivars should avoid land that has carried clubroot-resistant oilseed rape.