Targeted management of clubroot in oilseed rape

Mapping the distribution of clubroot across a field can aid with the development of targeted strategies. These clubroot field-mapping case studies demonstrate the cost-effectiveness of management approaches under two disease-pressure scenarios.

A winter oilseed rape field in Shropshire had ideal conditions for clubroot development. In particular, the autumn was warm and wet. Significantly, soil temperatures remained above 15^°C during crop establishment. The field was mapped on a grid pattern, using a visual assessment of plant roots. This showed that the level of infection was severe across most of the field area. The severity of the symptoms could also be seen in the above-ground crop.

• Where the whole field is severely affected, the most sustainable management option is to avoid growing clubroot-susceptible crops for at least eight years
• Economic modelling of lime treatment showed it would have been financially beneficial to apply lime to the whole field to reduce clubroot expression in this season (£142/ha benefit from lime treatment, after costs). The economic modelling did not account for other benefits (e.g. nutrients) associated with the lime application
• The other option would have been to grow a clubroot-resistant oilseed rape variety. Bioassay results showed that 93% of the field’s clubroot population was sensitive to the resistance mechanism. However, all resistant varieties use the same genetic source of resistance. Clubroot can overcome this resistance mechanism, especially if used frequently, affecting long-term control

Left image: The severity of clubroot across a Shropshire field was mapped using a disease index of 0–100 (100 = most severe).

Right image: Economics of lime for clubroot treatment. The coloured circles indicate whether there was an economic benefit (green tints) or an economic cost (red tints) per hectare.

A winter oilseed rape field in Staffordshire has patchy clubroot symptoms. Hotspots are quite common for most fields affected by this disease. Patches often start near the entrance to a field, which can be indicative of where the infection began, usually introduced via contaminated machinery. In this field, clubroot spread in the direction of the cultivation pass. Such infection patterns highlight the importance of machinery hygiene to limit spread of infection to ‘clean’ fields. A visual inspection of plant roots was conducted, with plants sampled in a grid pattern and symptom severity mapped.

• The most sustainable management option would be to avoid growing clubroot-susceptible crops. The longer the break, the greater the reduction in the number of viable spores
• Economic modelling showed it would not have been financially beneficial in this season to apply lime to the whole field to reduce clubroot expression. This was calculated to cost £38/ha more than the yield benefit achieved from reductions in clubroot severity
• However, if only the clubroot-infected patches were treated, the model showed that the benefit would be £23/ha, after costs

Left image: The severity of clubroot across a Staffordshire field was mapped using a disease index of 0–100 (100 = most severe).

Right image: Economics of lime for clubroot treatment. The coloured circles indicate whether there was an economic benefit (green tints) or an economic cost (red tints) per hectare.

This table shows the margin-over-treatment costs for lime, applied to the whole field or clubroot-infested parts. The whole-field approach tended to give lower economic responses.

Clubroot assessments: February–April

*Treatment breakeven point: 32% clubroot severity

Lime costs: £80/ha (£20/t spread)

Treatment efficacy: 25%

Rapeseed price: £335/t