Clubroot disease devastates British brassica crops. Growers well know that once land is infested with the soil-borne microbe Plasmodiphora brassicae, clubroot persists for decades. This disease's symptoms of cancerous root galling are easily recognised. Infected crops lose vigour and the leaves show an unhealthy, dull green colour, wilting easily.
Crop quality, sequencing and yield are lost. The microbe moves in soil moisture films from the long-lived, resting spores into host root hairs. There it penetrates, multiplies and moves deeper into the main roots. Host cells multiply chaotically as their hormone controls are lost. Another generation of resting spores forms in the galls and is discharged back into the soil. Tens of millions of spores are liberated from each gall.
Careful husbandry using integrated disease management planning can control clubroot. That requires freely draining soils and high fertility achieved from continual liming and using specialist nutrients. Holding soil pH at 7.2-7.4 and using fertilisers such as calcium cyanamide (Perlka), calcium nitrate (YaraLiva Tropicote) and boron are essential components of disease-control strategies. They encourage natural biocontrol, which slowly reduces the soil's bank of resting spores.
Global issue
Clubroot is a big problem on vegetable brassicas worldwide. But that does not gain it serious research funding. However, a new outbreak in Canada has changed that. Canola (Canadian oil seed rape, Brassica napus) is their dominant farm crop. Clubroot was first found in canola in 2003.
Canadian canola is a world-traded commodity crop on a par with cereals and soya. Business is booming with increasing demand for its health-giving polyunsaturated vegetable fats and demand for biofuels is further driving sales. Countries such as Japan rely almost entirely on Canada for rape oil.
The Canadian prairie provinces of Alberta, Manitoba and Saskatchewan grow seven million hectares of canola, yielding 16 million tonnes. It is their growers' biggest income source, with a farm-gate value of Canadian $3bn. The Canola Council of Canada estimates canola's farm-level economic impact at $10.5bn, providing 194,000 jobs.
The whole supply chain through to consumers benefits the total Canadian economy by $15.4bn, providing 228,000 jobs. Threats to this industry gain serious political attention.
Clubroot was found first in the Edmonton area of Alberta in 2003 on three farms. Over the following 10 years, 1,064 outbreaks have been confirmed. The disease is now affecting 689,000ha in 24 counties of Alberta. Resultant crop losses reach 80-90 per cent in heavily infested fields. Canola quality falls, 1,000-seed weights drop by 10-30 per cent and the all-important oil quality is lost.
As a first step towards control, Alberta's farmers were banned from growing canola on infested land for five years. Major public and private partnership funding came on stream for research very quickly. That has produced the largest clubroot research programme ever seen.
At the same time, China revealed the scale of its clubroot problem in rape and Chinese cabbage (B. rapa). China is the world's biggest grower of both crops used solely for home consumption. Clubroot studies now top China's arable research programme. Canada and China now collaborate in research and that will ultimately benefit Britain's vegetable growers. Early research and development progress was publicised at the recent three-day workshop in Edmonton organised by the Canola Council of Canada.
Spread and diagnosis
Any new disease immediately requires information on how it is spreading and finding means for rapid diagnosis. From the outset, clubroot disease in Canada's canola spread remarkably fast. Between 2007 and 2012, the infected area increased 90-fold. Spore trapping identified that this rapid spread was primarily caused by the minute resting spores being windblown in the dust clouds common in the hot, dry prairie summers. One gram of dust easily carries 100,000 spores.
Machinery travelling around and between infested farms is the second major means of spread. Tractors can carry 150kg and cultivators 50kg of infested soil. Thirdly, a new and unexpected means of spread was found on farms' cereal and canola seed and potato tubers. In future potato tubers and possibly other vegetative "seed" like ornamental bulbs grown on land with a history of brassicas should be tested for the P. brassicae.
Diagnosis of P. brassicae using conventional bio-assay systems takes six-to-eight weeks and is error prone. However, rapid molecular biological tests have been perfected. Diagnosis by commercial Canadian laboratories is now routine. Consistently, these tests detect P. brassicae in soil and plant samples coming from farmers, municipalities and other industries.
The P. brassicae DNA is detected down to 1,000 resting spores per gram of soil. Research now allows the measurement of resting spore numbers as routine. The efficacy of control strategies and potential risks from machinery movements as well as from seed and potato tubers is now measured with confidence.
Rapid, reliable and routine measurement of resting spore numbers considerably raises the efficacy of integrated disease control. This approach was pioneered for Australian vegetable brassica growers some years ago. Where risk is measurable, then the use of items in the control toolkit of soil drainage and fertility, liming, the use of specialist fertilisers and rotational growing may be increased or decreased as needed. Additionally, vegetable growers who rent land for brassicas gain a firmer grasp of land values.
Ideally, disease control should be simple, straightforward, effective and environmentally beneficial. Resistant cultivars fit those requirements most closely. Unfortunately, easily useable resistance genes controlling P. brassicae in brassicas are scarce. In addition, resistance in vegetable brassicas must be placed in cultivars that have the highly specialised quality characteristics demanded by the retail market.
Resistant vegetable cultivars lacking quality traits are valueless. Recently, Syngenta and its predecessors produced some valuable clubroot-resistant cabbage and cauliflower cultivars, which are now gaining market acceptance. At about the same time, European oil seed rape breeders produced clubroot-resistant cultivars. Both breeding programmes used similar resistance genes from turnip.
Resistance is now the major lifeline for canola growers. Very speedily, Canadian private and public plant breeders have commercialised several clubroot-resistant lines. These are now being cropped in Alberta. Growers whose land is infested by P. brassicae may resume production using these resistant selections following a period of three-to-four years growing non-host break crops. Resistance is rated as effective where infection falls to between 0.2 and nine per cent, compared with a 30 per cent loss in yield in susceptible lines.
Pathogen variations
The presence of slight infection in resistant lines indicates the presence of variants in pathogen populations capable of causing disease. This is a normal biological event and gives all plant breeders a perpetual problem. Fortunately, there is sufficient funding for research and development in Canada allowing widely based and detailed searching for genes conferring broader resistance from across the whole brassica family. Eventually, this will produce packets of genes whose strength lies in combining several resistance mechanisms. Ultimately, that will provide big benefits for vegetable growers.
Rotation with cereals and legumes is the sole husbandry option for canola growers. The land areas involved are too huge for other measures. Consequently, it is canola growers' second weapon. Rotation builds up populations of microbes that feed on resting spores of P. brassicae in the soil. Research in Sweden estimates that brassicas must be absent for 20 years before the content of resting spores in soil approaches zero. Achieving that is almost impossible because of the presence of susceptible weeds. Also, Canadian research indicates that P. brassicae can grow more extensively than previously thought in plants outside the brassica family.
Combinations of rotation and resistance are vital for canola growers. Crop rotation is more difficult with brassica vegetables. The sheer intensity of production forced on the vegetable industry by their markets normally makes growing alternative crops uneconomic. Where rotation is feasible then alliums, especially leeks, are very effective at reducing the subsequent incidence of clubroot.
Biocontrol
Biocontrol is potentially an attractive option for clubroot. Antagonistic microbes either kill Plasmodiphora brassicae directly or out-compete it for water or nutrients. Some biocontrolling microbes, for example, produce compounds that seize all available iron depriving the pathogen of a vital micronutrient. Formulating biocontrol agents as seed dressings would be ideal and is being sought in Canada and China.
Strains of the common soil bacterium Bacillus subtilis are well known as strong antagonists of P. brassicae. German scientists used them in the 1960s as a way of protecting glasshouse-grown kohlrabi. This microbe is available as Serenade from Bayer. It is approved in Britain for controlling foliar diseases of soft fruit.
More than 5,000 soil-borne microbes have so far been tested in Canada, including the biofungicide Prestop (Clonstachys rosea). Combined with resistant cultivars, biofungicides reduced disease and raised yield. On their own there was little effect. The Canadian research funds horizon-searching work alongside immediately applicable studies. A flow of new knowledge describing the pathogen's life cycle, means of multiplication and details of host-pathogen interactions is laying a firm basis for future clubroot control in vegetables.
Professor Geoff Dixon is managing director of GreneGene international.
