Clubroot is a perennial problem in many parts of Scotland but it is now affecting oilseed rape crops outwith the traditional hotspot areas, according to experts.

The disease is typically most severe in wetter regions with a history of mixed cropping, such as here and in northern or western England, but isolated cases occur across the UK. Losses can exceed 50% of yield potential in the worst-affected crops.

“Clubroot is still a relatively niche problem, compared to diseases like light leaf spot, but we’re seeing more cases around the country and for growers in hotspot areas there’s a high proportion who have at least one field affected,” pointed out Limagrain’s Vasilis Gegas.

“It mirrors what we’re seeing elsewhere in Europe where clubroot incidence is increasing, possibly as a consequence of climate change resulting in more frequent, milder and wetter winters. That is why the disease is our most heavily-invested oilseed rape breeding trait, aside from turnip yellows virus.”

Resistant hybrid varieties such as Archimedes, offer a valuable option for growing OSR on infected land while newcomers like Alasco, are showing enough promise to virtually eliminate the 'yield drag' associated with resistant varieties in the past, claimed Dr Gegas.

“Alasco is the next generation of clubroot resistant varieties, with a yield close to mainstream varieties and oil content 3% higher than Archimedes.”

It has been developed specifically for northern areas, with robust light leaf spot resistance, short stiff straw, pod shatter resistance and early maturity.

Agrii northern seed sales manager, Rodger Shirreff, who will be trying out Alasco for the first time this autumn said Archimedes had served growers well by allowing decent yields from clubroot-infected land.

Its vigorous establishment is particularly beneficial as it allows the taproot to establish quickly, providing a solid foundation to build on. It is also one of the quicker varieties to resume growth in the spring, he noted.

“Clubroot is a widespread issue in Scotland, that’s been perpetuated by tight – one year in four – rotations. But it’s not limited to Scotland. We’re also seeing increasing problems further south, so more growers are moving to resistant varieties.”

But while genetic resistance is effective, Dr Gegas warned that the clubroot pathogen (plasmodiophora brassicae) is capable of evolving and overcoming resistance in certain circumstances, so genetics must be supported with good husbandry and stewardship.

He advised against growing resistant varieties unless clubroot is a significant problem and added that extended rotations of at least one year in seven or eight, and careful soil management, must also be used to control disease.

“There’s a lot to gain from making soil conditions unfavourable for clubroot by liming to increase the pH to 6/7, prior to drilling or improving drainage of waterlogged areas. Wider rotations alone aren’t the answer because the pathogen can survive for such a long time in soil.”

Commercial soil tests can be useful for confirming presence of the clubroot pathogen, but should only be a guide: “We often find many soils contain the pathogen, but that doesn’t mean disease will develop or there will be an impact on yield, as it still requires the right environmental conditions.

“Last autumn was mild and wet so we saw a lot of symptoms and if climate change leads to frequent, milder and wetter winters, we could well see a wider spread of the disease.”

He also added that volunteers from previous oilseed rape crops can harbour the disease and need to be controlled.