With incidences of clubroot occurring this season in areas which have hitherto been visibly free of the problem, varietal choice, combined with liming of acidic soils, remains the key countermeasure against the threat it poses to oilseed rape performance, according to agronomists and researchers.
"Clubroot reports appear to be coming from a wider area, and there has been an increase in the number of incidences reported since Christmas,' says ADAS Boxworth's Peter Gladders. "In some areas there are cases on farms that have never before seen clubroot.
"In England, while the dry, alkaline soils of the east mean the area is relatively free of the problem, there is a strong trend in the West Midlands, where higher rainfall, the greater predominance of mixed farms, and generally acid soils combine to create an ideal environment.
"But it's not restricted to the area by any means – the south-west, south Wales, North Yorkshire, Northumberland and the Midlands are all seeing higher incidences. The cold winter held the disease back a little, so we didn't see wilting of foliage in the autumn, but the effects soon became apparent once conditions warmed up.
"When rape was first grown in the UK in the 1970s, a one in five rotation was usual, but one year in three or even one in two is now common, and this isn't helping. The clubroot fungus spores can stay undetected in soils for many years and, once the issue has shown itself on-farm, a resistant variety such as Cracker is the only way of addressing the problem.'
An ongoing HGCA report is working on identifying the issues surrounding clubroot suppression, including liming and, to a lesser extent, the application of trace elements such as boron, reports Dr Gladders. But results have shown inherent varietal clubroot resistance can give double the yield response of any soil treatments, he says.
"Often the issue is with soil acidity, and soils of pH 6.3-6.4 are simply too acid for rape when clubroot is present. Lime treatments increase soil pH, but also work by increasing available calcium. Clubroot can occur on soils with high pH – pH 8 and above – on Lincs silt soils, where Perlka or Limex applications can still improve clubroot control. The latter's smaller particles address the problem faster than coarser conventional lime.
"It's important to weigh up the cost of treatment against the cost of crop loss: big galls on tap roots and purple plants can lead to total failures in the worst cases. Even where there are simply acid hotspots within a field, spot lime treatment is recommended, to prevent an issue arising in future.'
On farms where clubroot has appeared recently, fields should be tested ahead of sowing, and varieties selected upon the result, says Dr Gladders.
"In addition to club root resistance, Cracker offers good yield and oil content, as well as light leaf spot resistance,' he notes. "If yield losses of 5-10 per cent can be attributed to club root, then it is worth switching varieties.'
Andrew Gilchrist, of Scottish Agronomy, notes that a recent SAC survey showed half of soil samples testing positive for clubroot. "We are seeing more and more incidences, and tighter rotations haven't helped. Neither have the wetter autumns – one year in three or four – of recent seasons. We have a large number of fields that are heavily infested.
"Cracker looks to be the next step on from Mendel and, in that respect, it gives growers in clubroot-affected areas a means of keeping rape in their rotation. But it also performs well in other respects – it's at the top end of the scale for yield, it produces high oil content samples, and has good light leaf spot resistance.
"While Aberdeenshire has the highest incidence of clubroot, the problem is becoming more widespread throughout Scotland. Most soils are nowhere near the pH 7.0 + that discourages clubroot development, and liming appears to have limited effect. Genetics plays a bigger part than any other factor in terms of control. "With limited alternative winter break crops, and clubroot effects that can vary from negligible to devastating, causing yield penalties of perhaps 0.5t/ha in a mild infestation, a new variety that's resistant to the disease will give arable farmers greater scope to continue benefiting from the inclusion of rape in their rotations.'