Clubroot pressure builds

Clubroot is a major issue in rape in Scotand’s arable rotations with over 50% of fields used for oilseed rape carrying the disease – and in areas like Aberdeenshire the incidence is higher with the majority of fields affected. Yield losses on average are around a third of a tonne for every 10% disease severity, but in severe early infections in warm autumns emergence can be so poor that crops are ploughed back in so, at the extreme end, yield losses can be 100%.

Knowing which fields are infected and making basic decisions about good hygiene when moving machinery around the farm is important. Cleaning down equipment is the ideal before moving between fields but if this is impractical then planning operations so that they finish rather than start in an infect field is sensible.  The risk of spreading infection between fields and farms is very high and some machinery can potentially carry tonnes of soil if it isn’t cleaned off thoroughly. Contractors should be particularly careful. Current AHDB funded research at SRUC which is mapping fields for clubroot shows really clearly how often there is a tell tale patch of infection at the gate in an infected field, followed by tails of infection where this has been spread out through cultivations.

Testing fields adds to the information you have to plan with. If a field is infected then drilling a variety listed as clubroot resistant such as Mentor is still the primary means of reducing the risk of yield loss. However, Mentor carries the same resistance mechanism as previous resistant varieties such as Cracker and Mendel and evidence shows that, after two or three uses in the same field, strains of clubroot which can overcome the varietal resistance emerge. Current research shows these strains are common throughout the UK and in fields where resistance has been deployed several times before it ceases to be an effective strategy. Consequently, planning rotations is really important so that you keep fields profitable for the long term and don’t solve the problem temporarily for this year and next, but end up in a worse position than ever within the decade. Clubroot persists for up to 20 years in the soil which is clearly way beyond what can be handled by sensible rotations, but even stretching oilseed rape rotations out in infected fields from three out to five years is enough to make a difference to infection levels and reduce pressure on the varietal resistance mechanism. Clearly the one year in two type approach to growing rape in some fields will really accelerate the risk.

Current advice on clubroot is therefore:

• Test soils for clubroot and pH and use results to plan strategy for farm.
• Pay attention to hygiene and soil movement.
• Rotations of greater than 1 year in 5 are likely to be beneficial.
• Avoid early sowing on infected sites
• Avoid over-reliance on resistant varieties in short rotations
• Only deploy resistant varieties where justified by disease level to avoid over use and selection of virulent clubroot strains
• Maintain higher pHs on infected sites and use long rotations.
• Spot treat infected patches in fields with lime
• Always investigate patches of poor emergence or growth to see if clubroot is the cause

Fiona Burnett for the Farm Advisory Service