Tighter rotations, greater production of field-scale vegetables and increased use of cover crops combined with the recent wet weather are making clubroot more of a problem than ever for Scottish OSR growers.
Clubroot remains a major challenge for oilseed rape growers in Scotland and is a contributing factor in production of the crop now being much lower than at its peak, believes Scottish Agronomy managing director Adam Christie.
“In Scotland we are so short of potential break crops that we cannot afford to lose oilseed rape and farming businesses must have it as an option.
“The 2023 oilseed rape area was the largest for at least 20 years, around 41,000 hectares, but the average yield of 4.1t/ha was 5% less than in 2022, giving a total production of 166,000 tonnes.
"Cabbage stem flea beetle and winter stem weevil can be issues for oilseed rape growers in Scotland, but the two biggest concerns are Light Leaf Spot and Clubroot.
"Clubroot is a long-term issue which goes back decades to the time when turnips were widely grown to feed sheep, stored in field clamps after lifting and transported to other areas.
"The problem can be compounded as field-scale vegetables are widely grown in many regions. The significant increase in cover crops during recent years, many of which contain a brassica component that can host clubroot, has exacerbated the problem.
"I have even identified clubroot in a field which had only ever produced one crop of oilseed rape many years previously, so it is vital to check every field, cruciferous weeds and volunteers for gall.
"The disease loves wet winters and protracted rainfall this season has provided ideal conditions for it to develop, making life more difficult for growers.
"On average sixty per-cent of our soils can be infected with clubroot and in some regions, such as Aberdeenshire, the incidence is even higher."
Wider rotations and cleaning machinery between fields or farms can help reduce the spread of clubroot from infected areas, he points out.
"If growers are unsure of the status of their soils in respect of infection they should have them tested. The cost is relatively low and if doing so allows a variety that yields 0.5t/ha more to be grown it will have been worthwhile.
"At Scottish Agronomy we have years of research data which show that when it comes to growing oilseed rape in clubroot infected soils nothing is more effective than selecting resistant varieties, which now account for 30% to 40% of the total area."
Protecting genetics
Sarah Hawthorne of plant breeders DSV UK agrees, saying modern clubroot tolerant varieties can be a great ally in helping control the disease, but they need using with care.
“No official advice is available on how to deploy clubroot-tolerant varieties, but they should not be over-used.
"Growers should do their bit to protect the genetics of clubroot tolerant varieties by only planting them where the disease is a known issue and after addressing other causal issues such as drainage, calcium and boron levels, controlling weeds effectively and delaying sowing.
“Where clubroot tolerant varieties are used frequently or in heavily infested soil, strains which break this resistance are more likely to develop.
"Where it is suspected that this has happened the rotation should be extended and repeated use of tolerant varieties avoided.
“We do not advise early drilling as this can also put pressure on the protection offered by clubroot tolerant varieties.
"Where possible such varieties should not be drilled before the third week of August and grown at a minimum 1:5 rotation in soil free from brassicas."
Clubroot has definitely become more apparent on farms during the last five years, but the latest clubroot tolerant varieties have much to offer, she points out.
"One of the latest varieties helping bridge the gap between clubroot protected and normal varieties, for example, is DSV Cromputer which is currently on the 2024/25 AHDB Candidate List.
"A high vigour variety which is fast to establish and produces tall plants with a compact canopy, it offers a significant yield increase over existing clubroot-resistant varieties, combined with TuYV (Turnip Yellow Virus) protection and improved stem canker resistance."
Later flowering than its competitors, DSV Cromputer offers growers the ability to split the work load with spring sprays and lower risk the risk from frost damage. Strong yielding (Gross Output – 99.9% and Seed Yield E/W – 99.8%) it also scores well for oil content (45.4%), Light Leaf Spot (6) and Stem Canker (6), she adds.
“Due to the rapid development of the lower branches of Cromputer CR we recommend sowing at 40 – 50 seeds/m2.
"The variety’s vigorous autumn growth allows it to be drilled over a broad range of dates without any issues and in spring a PGR is only required on fertile sites to ensure efficient harvestability."
Wet season has increased clubroot risk
Clubroot resistant varieties of oilseed rape have been available for more than two decades and are now widely grown throughout Scotland, says Professor Neil Havis, a plant pathologist with Scotland’s Rural College (SRUC).
"Mendel, the single resistance gene, is holding up well and with newer clubroot-tolerant varieties being higher yielding there’s much less of a penalty, if any, for growing them.
"More risk-averse farming businesses generally favour that approach, but to protect the Mendel gene it is important that resistant varieties are only grown where clubroot is a known problem and a minimum rotation of one year in four is maintained.”
But choosing which variety to grow is just one element of an integrated management approach required to mitigate against the disease, he stresses.
"Although clubroot is not as devastating in Scotland as in some parts of England, controlling it requires careful management through a combination of rotation, liming and varietal choice.
"It is important to combine these actions with chemical, biological and mechanical methods to control cruciferous weeds which can potentially host the pathogen and increase soil borne spore levels.
"Clubroot has a half-life of seven years and the trend to shorter crop rotations over the last two decades has contributed to a significant build-up of the disease.
"Ten or fifteen years ago m