More acidic soils, tighter rotations and a changing climate are just some of the reasons why clubroot is on the increase and spreading to new regions, according to new study. Abby Kellett reports.
While club root’s yield-robbing effects were once confined to regions of high rainfall and areas with a mixed farming history, a recent Agrii investigation reveals almost two-thirds of agronomists are seeing an increase in club root incidence, largely in fields which have never before been affected by the disease.
Agrii seed technical manager David Leaper says: “We are encountering problems in areas and under conditions where club root has not been an issue in the past. For instance, both our Somerset and Derbyshire iFarms have seen the classic symptoms for the first time this season.”
Agronomists taking part in the study identified a number of factors believed to be responsible for the surge in club root incidence:
According to ADAS pathologist Julie Smith, a change in autumn weather has been a big driver in the recent upsurge.
She says: “Together with sufficient moisture, we are seeing autumn soil temperatures remaining above the 16degC threshold needed for infection for longer than usual.
“Last year, temperatures did not fall below this level until early October, at Rosemaund at least. Therefore early sown crops were exposed to infection for about six weeks compared to just one week in 2014,” she says.
Going forward, experts predict the effects of climate change, including warmer temperatures and more frequent flood events, are only going to aid the survival and spread of the club root pathogen.
“Unfortunately, modelling based on climate change predictions suggest the club root growing season will be extended in future years,” says Ms Smith.
Since club root can have a significant impact on crop performance, experts say growers should regularly test soils for the club root pathogen and put cultural control measures in place in high risk areas to avoid inoculum build up.
Ms Smith says: “Across sites and seasons, club root typically causes losses of 0.3 tonnes per hectare for every 10% of plants affected. Losses are most severe where dry summers place a particular strain on compromised root systems.”
Lengthening rotations is the most sustainable strategy for managing club root as soil amendments and varietal control can be inconsistent, according to AHDB which suggests growing OSR no more than once every five years.
Besides OSR, cruciferous weeds are also effective carriers of the club root pathogen, as are other brassicas commonly used in cover crop mixes, such as oil radish and mustard. Therefore these species should be avoided in high risk areas.
In AHDB trials, boosting soil pH and available calcium at drilling successfully reduced club root infection. Research found neutral or alkaline soils are most effective in reducing club root in OSR and vegetable crops.
Agrii agronomist Chris Letts says: “There is no substitute for getting out into the field and testing widely for any areas of acidity, especially areas where crop performance has been obviously falling below the field average.”
Growers are advised to consider the wider rotation when raising pH above 6.5, as following cereal and potato crops can suffer from nutrient deficiencies.
Source: Will Vaughan-France (Monsanto), Julie Smith (ADAS) and David Leaper (Agrii)
Where club root risk is high, growers can opt to use varieties which are resistant to common strains of club root. Currently, Mentor from LS Plant Breeding (LSPB) is the only winter OSR variety with club root resistance on the AHDB Recommended List (RL) for the East/West and North regions. Previously-listed varieties include Mendel and Cracker.
The increasing incidence of club root is a cause for concern says LSPB’s Theo Labuda.
“It is a bit worrying. If you grow oilseed rape in a close rotation you are building up problems for the future, but rotations have become longer.
“The problem is club root is soil-borne and in the soil you get a whole range of races. A resistant variety will not be completely resistant, selection will happen and races will build up. There is only one source of club root resistance in oilseed rape.”
Predicting club root ‘will get worse’, Mr Labuda urges growers to follow agronomy guidelines and warns where club root infection is severe, affected fields may have to be taken out of oilseed rape for 15-20 years.
There are new resistant varieties in the pipeline, with Monsanto and Limagrain among breeders developing club root-resistant lines.
New RL candidate variety, DK Pliny is also showing promise, according to Dekalb technical specialist, Will Vaughan-France.
He says: “DK Pliny combines resistance to common club root strains with the sort of resilient agronomics previously only available in our susceptible varieties. Most importantly, vigorous establishment and root development provide the best tolerance of infections from other strains of the disease.”
Since trials have shown varietal resistance to club root is under pressure in some areas of the UK and has broken down where it is commonly used, a combination of cultural methods should be adopted for long-term control, he says.
“Over-reliance on varieties which all share the same original, multi-genic source of resistance must be avoided if we are to safeguard this hugely valuable resource from breakdown,” adds Mr Vaughan-France.
At Lower End Farm, Northampton, grower Tim Care is battling a club root problem which emerged three years ago and now affects 17% of his 400ha arable area.
A single isolated patch of infection was identified in 2014 because of its poor wheat performance the previous season. On inspection, OSR roots displayed clear signs of club root and soil tests showed the area to be acidic, despite the field having no history of pH problems.
The following season, infections were identified in a further four fields, with the worst field yielding just 2.9t/ha compared to averages of up to 4.5t/ha in nearby unaffected ground. Mr Care says: “What made matters worse, the previous crop of rape in the same field two years before gave us our best-ever spot yields at well over 5t/ha.
“Having been alternating wheat and rape in our rotation for more than 20 years, I suppose it was inevitable we’d hit the buffers at some stage. Especially so as, if I’m honest, my brother and I hadn’t been as diligent with the liming as we should have been, relying instead on variable application and testing every five years.
“Our move from ploughing to minimum tillage may not have helped either, concentrating any build-up of club root inoculum in the upper, warmer soil layers,” he says.
To tackle the disease, OSR production has been reduced to once every three or four years, depending on field risk.
According to Mr Letts, the change is already having an effect. “I am happy to say an extra year or two between OSR crops in the Lower End Farm rotation is already paying dividends in terms of crop yields.”
Additionally, soil testing and liming is now a routine practice and drainage efforts have been stepped up. Where club root is known to be a problem, Mr Care delays sowing until late-August and uses resistant varieties on selective fields.
Mr Letts says: “I think Tim has been able to nip club root problems firmly in the bud by spotting them early and taking concerted action to address them. Driven by the black-grass imperative, a good measure of rotational ploughing and spring cropping should definitely be helping on the club root front too.”