CLUB ROOT

Increasing threat Infection found in SE England in field of resistant variety Mendel Could be shift in fungus race Check for problems this spring Test soils if suspect Check for low pH / poor drainage

Club root infection has been identified in a crop of Mendel – the only commercially available oilseed rape variety with club root resistance – in south-east England.

It has also been found in three German crops, the variety’s UK agent CPB Twyford has confirmed.

But it does not mean Mendel’s club root resistance has broken down, the firm’s Simon Francis stresses. “Mendel’s resistance is not complete and never has been.”

Instead, the most likely explanation for the UK field infection, which is being described as patchy but moderately severe, is that the population of the club root fungus Plasmodiophora brassicae has shifted to contain strains that Mendel doesn’t control. “We’re carrying out tests to identify which strains are present in the farm’s soils.”

Experience with other club root-resistant brassica crops, such as stubble turnips, which all use the same turnip rape resistance source, has shown close-cropping of resistant varieties can select for other strains of the club root fungus, he explains.

‘Investigating’

“We’re investigating back-cropping with the grower to try to understand why this particular farm has club root, while other local growers are growing susceptible varieties with no problems.”

While the exact farm history has not yet been confirmed, there is a suspicion that tight rotations and previous farming practices have created a high-risk situation, he says. “The grower has used the resistance characteristics of Mendel to re-introduce rape onto his farm and has had two crops of Mendel in some fields. This close-cropping could have resulted in a shift.”

Race shifts could also be behind the three German cases. Investigations by Mendel breeder NPZ Lembke has found stubble turnips have been grown in recent years, meaning the shift could have been accelerated.

Concern over the increased threat from club root to UK rape crops, as well as the discovery of the infections in Mendel, has led to CPB Twyford bringing together experts from SAC, ADAS and NIAB to identify a strategy to minimise infections, and using resistant varieties should be the last resort, the working group suggests. “Growers should look at cultural solutions first rather than thinking Mendel is the holy grail.”

Field hygiene

Instead, planning to avoid growing stubble turnips in the rotation to feed stock, lengthening breaks between rape crops, liming to suppress soil acidity – the fungus prefers an acidic environment – and practicing good field hygiene are all good steps to avoid club root build up, Mr Francis advises.

Mendel should only be grown widely where club root is known to be an issue, he says. “Where club root has not been a problem in the past, but where it is suspected, fields should be tested, and where it is creating isolated problems in fields it should be used every other rape crop.”

CLUB ROOT FACTS

Club root occurs in all brassica species, including turnips, kale, cabbage, sprouts & oilseed rape Fungus infects plant roots, causing irregular swellings – galls or clubs – on the main root Prevents plants taking up water and nutrients causing yield loss Greatest yield loss likely in dry summers – can be up 30-40% in OSR Club root (Plasmodiophora brassicae) can persist in soils for 20 years Bad husbandry practices, including continuous cropping, can increase infections from 1% to 100% in three years

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