Clubroot management in crops

Pathogen

Plasmodiophora brassicae

Hosts

Clubroot affects all cultivated and wild cruciferous plants. In addition to oilseed rape, all vegetable brassica species are affected. Other susceptible broad-acre arable crops include turnip, swede, Brussels sprouts, cauliflower, calabrese and mustard. Numerous weed species, such as charlock and shepherd’s-purse, are also common hosts.

Symptoms

The first symptoms usually occur within six weeks of planting, provided soil temperatures are greater than 15oC. In oilseed rape, symptoms commonly start in late autumn. Roots become swollen and distorted, and develop small, irregular, white-coloured, solid galls. These are present on taproots and/or lateral roots. As the season progresses, galls may enlarge and discolour, before starting to rot.

Above-ground symptoms do not usually develop until later in the season. Typical symptoms include stunting and yellowing. Under dry conditions, plants may wilt, especially when galling is severe. Distinct patches of poor growth are often visible. Plant loss occurs in the most severely affected areas and, occasionally, the whole field may fail.

Life cycle

Clubroot is a soilborne pathogen that produces resting spores. These spores have thick walls and help the pathogen survive for about 15 years in the soil. Chemicals, released by the roots of host plants, cause nearby resting spores to germinate and release motile spores (zoospores). These move through soil water and infect the host’s root hairs, where a secondary spore stage occurs. These spores invade the outer layer of the root (the cortex) and form structures called secondary plasmodia. These structures cause the root cortical cells to enlarge and increase the rate of cell division. Ultimately, this results in the formation of the characteristic clubroot galls. These galls decay during the season and release large numbers of resting spores back into the soil.

Importance

Clubroot is a global problem and has increased in recent years. For example, many new UK cases were reported in 2016, often on farms with no history of the disease. The trend for shorter rotations, along with milder and wetter winters, have probably contributed to the rise.

Although yield losses may not be significant at a national level, losses of 10% are common in infected fields. However, losses can exceed 50% and crops can even fail completely, in extreme circumstances. Yield losses of 0.03 t/ha per 1% of oilseed rape plants infected have been reported. In species grown for their storage roots, quality losses, due to visual appearance, also occur.

Risk factors

The following factors are associated with an increased clubroot risk:

Field/soil factors

  • Fields with a history of clubroot infection
  • Warm (above 15oC) and wet soils – particularly, compacted, poorly drained soils and flood-prone fields
  • High numbers of susceptible cruciferous weeds and volunteers
  • Acidic soils (pH less than 7)
  • Boron-deficient soils

Contaminating factors

  • Poor machinery hygiene, resulting in transfer of infected soil
  • Use of manure from animals fed on infected produce
  • Use of infected green mulches, composts and straw
  • Movement of infected vegetables, such as feed swedes and turnips, onto clean land

Cropping factors

  • Short rotations (less than 1 in 4)
  • Use of susceptible crops and varieties
  • Repeat use of clubroot-resistant varieties, especially in short rotations
  • Early drilled oilseed rape

Management

Use an integrated management strategy to ensure the long-term profitability and sustainability of clubroot-susceptible crops in the rotation. In practical terms, this means adopting sustainable rotations, judicious use of varietal resistance, diligent monitoring, surveillance and testing, combined with good biosecurity measures, and the use of wider agronomic practices, such as drainage and liming.

Identify high-risk areas

Early detection of clubroot infection is extremely important. As patches of poor growth or establishment can have multiple causes, it is important to investigate them. Where clubroot is suspected or confirmed, establish hygiene measures around the infected area immediately and adapt rotational and agronomic plans, accordingly.

Soil tests

Conduct soil tests prior to planting any susceptible crop. Prioritise high-risk areas for tests, such as wet hollows, gateways/field entrances, recently flooded fields (especially those near infected fields) and where civil infrastructure projects are planned.

Tests are based on traditional assay techniques, such as growing susceptible bait plants in suspected infected soil, or molecular diagnostics. SRUC, FERA, PGRO and Eurofins all offer testing services.  Molecular test results can show the quantity of spores per gram of soil. Although suggested spore risk thresholds have been set for vegetable brassicas, no validated thresholds exist for oilseed rape.

Visual tests

If symptoms are severe enough to result in visible above-ground symptoms, NDVI (Normalized Difference Vegetation Index) techniques can be used to help detect them (see Figure 3). However, such symptoms are not always present, especially in oilseed rape, so root systems should be monitored for galls. This is best conducted over the winter period, when fields are easier to walk and monitor.

Use the field’s tramlines to establish a sampling grid. Ideally, sampling points should be no more than 50 m apart. At each sampling point, pull up and inspect 10 plants.

Clubroot maps

Use test results to create field maps of clubroot patches. Since clubroot areas persists for at least 15 years, knowledge of patches remains useful for several seasons.

As patches remain relatively stable within a season, targeted treatment of affected areas can be cost-effective. Targeted treatments include the use of liming, resistant varieties or fallow/grass.

Minimise contamination

Movement of infected soil or water is the main cause of clubroot introduction and spread.

On average, farm equipment transfers 250 kg of soil, most of which is deposited close to gateways and field entrances. Ensure farm staff and contractors follow good hygiene protocols and strict biosecurity procedures. Plan machinery movements to avoid travel from infested to clean fields. Restrict access to severely infested fields. Where land is rented, ensure tenants understand and manage risks in the same manner as farmer-owned land.

Where non-agricultural personnel need to access land, ensure they have sufficient biosecurity awareness, as contamination risks can be relatively high. For example, civil infrastructure projects can involve a high level of machinery movement. In such situations, additional hygiene measures may be necessary, such as hardstanding and wash-down facilities for vehicles and machinery.

Infected animal manures, composts, green mulches and straw can also introduce spores. It is important to understand the infection risk associated with these materials. Avoid the movement of feed swedes and turnips onto clean land. Risk associated with digestate use is low, particularly if it is compliant with PAS100 or PAS110 standards, but a risk remains, especially where high volumes are spread. When working with vegetable transplants, use compost with good provenance that is guaranteed clubroot-free.

As clubroot zoospores move through soil water, ensure soils are not compacted or waterlogged.

Maintain soil pH above 7

Crops grown in acidic soils are at greater risk of developing severe symptoms. Check soil pH routinely, particularly before establishing a susceptible crop. Aim to maintain the pH to above 7: even small increases (0.5–1 pH units) in pH can decrease clubroot severity. Calcium also has a direct effect on the pathogen.

Agricultural lime products, which are associated with a spike in pH and available calcium at drilling, can significantly reduce clubroot infection. High doses of lime (applied at 8 t/ha) can reduce clubroot severity by 25%. AHDB research shows that targeted treatment of the worst clubroot-affected patches can improve economic returns, compared with whole-field approaches.

As boron also has some activity against clubroot, any deficiencies in the soil should be corrected.

Extend the rotation

In clubroot-affected land, extend the rotation to a break of at least four years between susceptible brassica crops. Clubroot resting spores can remain viable in the soil for at least 15 years but have a half-life of approximately 3.5 years. Consequently, the longer the break, the greater the reduction in the number of viable spores. Thus, extending the rotation is often the most sustainable long-term management strategy.

Cover crop mixes that contain susceptible brassica hosts should be avoided on infested land. Agricultural radish is resistant to clubroot but the mechanism is the same as in oilseed rape. There is limited evidence for reduced soil inoculum after growing radish and the advice is not to grow it in severely infested fields. Spring oilseed rape is also susceptible.

Use resistant varieties judiciously

Where a susceptible crop is grown in an infected field, a resistant swede, vegetable brassica or oilseed rape variety should be selected. However, resistant varieties are not immune to attack and small galls are likely to develop. Based on a single dominant gene, the same mechanism (Mendel) is present in all resistant crop varieties. However, recent research shows there is a high pathotype diversity in the UK and resistance is not effective against all pathotypes of clubroot. Where resistance is deployed frequently in rotations or in very heavily infested soil, resistance-breaking strains are more likely to evolve and establish. This has already started to occur in some parts of the UK. As a rule of thumb, a resistant variety should have less than 30% infection compared with a susceptible variety (volunteers and off-types should not be included in the observation). Resistance breakdown is most likely to appear in patches. These patches can act as an early warning of changes in the pathogen population. Extend the rotation, where resistance breakdown is suspected.

Across Europe, resistant varieties account for about 5% of oilseed rape seed sales. The purchase of certified seed ensures that susceptible plant numbers are minimised in a resistant variety seed batch. Resistant varieties should not be home-saved for seed.

Avoid early sowing on infested sites

Infection is most likely in warm, wet soils (optimum temperature 16–25oC) when conditions enable zoospores to disperse through soil water. Disease activity reduces as soil temperatures drop below the optimum. Typically, winter-grown crops are most susceptible to infection from August to mid-September. Crops are most vulnerable at the seedling growth stage (one to two leaves unfolded). Delayed drilling minimises the potential window of infection and helps the crop to avoid clubroot.

Control weeds and volunteers early

Cruciferous weeds, such as charlock and shepherd’s-purse, are common hosts to clubroot, along with volunteer oilseed rape. Consequently, they should be managed within and between susceptible crops. Early weed control (7–14 days post emergence), by either herbicide application or shallow disking, reduces the number of resting spores in the soil.

Fungicides

Fungicide and biocontrol options are not available for clubroot control. Calcium cyanamide, which is registered for use as a fertiliser, is reported to have some incidental effect on clubroot.

Action points

  • Keep accurate crop records of clubroot occurrence, location and intensity, and where varietal resistance has been deployed in field
  • Monitor crops carefully and assess the levels of clubroot present, especially for higher-risk fields
  • Use field maps to identify hot spots to management strategies
  • At sites with higher frequencies of susceptible crops in a rotation, increase the frequency and detail of tests
  • If levels of infection start to increase, change strategy – especially, where ‘resistant’ varieties have been deployed
  • For resistant varieties, use certified seed to minimise susceptible plant numbers. Do not home-save seed of resistant varieties
  • Manage volunteers and susceptible weeds, within and between susceptible crops, as early as possible
  • Be mindful of other susceptible crop choices when planning rotations – spring rape is vulnerable and cover crop mixes often contain susceptible species
  • Make decisions based on long-term profitability and sustainability of a field, not on a single season’s predicted margin

Targeted management

Whole-field treatment approaches can give lower economic returns, compared with the targeted treatment of clubroot-infected patches. Mapping the distribution of clubroot across a field can aid with the development of targeted strategies.Take a look at these clubroot field-mapping case studies, which demonstrate the cost effectiveness of a targeted management approach under two different scenarios.

Further information

Clubroot tests

SRUC Fera PGRO Eurofins

Figure 1. Clubroot symptoms in oilseed rape

Figure 2. Inspection of an oilseed rape plant for clubroot symptoms

Figure 3. NDVI image of a field of oilseed rape, taken in April (background). Coloured circles show the disease severity (0 to 100 scale), determined by a visual crop assessment.

Figure 4. In this field, clubroot has spread in the direction of cultivation

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Oilseed rape disease management guidance

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