By Professor Fiona Burnett

Head of crop and soil systems and professor applied plant pathology at SRUC

Clubroot has long been a serious problem to oilseed rape growers in Scotland for a long time, but the pattern of warm autumn and winters that we have had over recent years has favoured its early development.

This manifests itself as patches of poor growth or missing plants in crops, with high numbers of reports coming in to the SRUC’s Crop Clinic last season.

Clubroot will keep developing in roots until soil temperatures drop below 15°C and so in mild autumns it can stay active right through to December. This is exacerbating the long-term issues we have had with clubroot in Scotland.

More than 50% of oilseed rape fields carry some degree of infection and in some areas where oilseed rape rotations have been short, the severity of disease is high. Losses can exceed 50% in affected patches and where crops are ploughed in because the problem is so severe early on, then losses are obviously complete.

Longer rotations help with most soil-borne problems, but because clubroot can persist for 15-20 years, longer rotations are not a get out of jail card in normal arable rotations.

That said, short rotations of one year in three or less will certainly exacerbate the problem and increasingly growers are opting for longer breaks between rape or other susceptible crops, like vegetable brassicas.

Historically, there has been much interest in the use of soil amendments to manage the problem. High levels of calcium or high pH (in excess of 7) can help reduce clubroot in high value cash crops like transplanted veg, but the effects in rape have been more limited as really high pH levels aren’t feasible in normal arable rotations and also because by the time oilseed rape germinates and triggers clubroot to infect, any spike in pH or calcium applied at drilling has somewhat dissipated.

The key control measure is the use of resistant varieties, with a succession such as Mendel, Cracker and Mendleson giving good control where they are initially deployed. But, an emerging issue is that they all carry the same resistance mechanism, so in fields where they have previously been used, strains of clubroot which can overcome the resistance mechanism build up.

Recent testing at SRUC and ADAS in a joint AHDB funded project show that ‘Mendel breaking’ strains of clubroot are common throughout the UK.

Resistant varieties remain the most effective control measure but the results emphasis that rotation lengths need to be kept as long as possible to avoid over reliance on the resistance mechanism.

Another issue for growers is the patchy nature of the disease, so it is hard to judge what the likely yield loss will be over the whole field and hence whether a resistant variety is necessary. Field mapping might help with this in the future and the AHDB project underway is letting us map fields with the idea being that affected areas could be treated or drilled separately.

Basic messages about good hygiene are important too. Always investigate the causes of poor patches of growth and where clubroot is confirmed then try and avoid spreading it within the field or between fields by cleaning down machinery.

If you know clubroot is present then try and lengthen rotations in that field and avoid over-reliance of resistant varieties. Planning a long-term strategy for the field is sensible.

*This column is sponsored by the Farm Advisory Service.