As there are no adequate chemical solutions, growers will have to rely on good agronomy combined with resistant varieties in order to control Verticillium wilt in oilseed rape.
Dr Faye Ritchie of ADAS says “The only way forward, particularly in areas of high risk, is to establish a strong crop that can withstand disease better through good agronomy and to choose a variety that has proven resistance to the disease. No individual genes resistant to Verticillium wilt have been identified as yet, so resistance is thought to be multigene.”
She reports that high risk areas for Verticillium are the Eastern counties, the South East and Midlands, although it has been identified on farms in other counties as far north as Yorkshire. “It is widespread across many areas of Europe, where yield loss of up to 50% has been reported in severe cases. In the 2013 DEFRA oilseed rape disease survey, 24% of crops surveyed in England and Wales had Verticillium wilt symptoms before harvest, compared with 10% in 2012 and 18% in 2011. A recent HGCA study reported severely affected plants showed decreases in thousand seed weight of up to a quarter, highlighting potential yield losses through premature ripening. This is significant as, in some severely affected crops, over 90% of plants were reported to have symptoms of Verticillium wilt prior to harvest last year.”
“It is difficult to identify Verticillium wilt at the moment. It may well be there, but we can’t see it. However, growers can look for symptoms in crops prior to harvest, to gauge the level of risk for future crops. Early symptoms to look for include vertical yellow or brown stripes up the stem and along branches. The classic signs are microsclerotia on the surface of stems close to desiccation time. Microsclerotia can survive in the soil for over 10 years, making crop rotation less effective,” says Dr. Ritchie.
Although not part of the HGCA Recommended List criteria, resistance to Verticillum wilt has been identified in some varieties and there are significant differences between them, with varieties such as Catana, Es Alienor and Es Cubic being resistant, whereas Castille and Excalibur were susceptible.
Grainseed have a particularly strong breeding portfolio of Verticillium resistant oilseed rape varieties including Es Alienor, Es Alegria and Es Cubic. “In ADAS and the AICC trials, Alienor and Alegria show excellent resistance to this damaging disease. Cubic showed the same high level of resistance as Catana, which is regarded as the control variety,” reports Neil Groom, Technical Director for Grainseeds.
“Growers should grow oilseed rape varieties with a good overall disease resistance package, because where Light Leaf spot and Phoma are both obvious, Verticillium symptoms are more severely expressed. Also Phoma is still the major disease threat to UK crops. Alienor has a 7 rating for Phoma stem canker and a 6 rating for Light Leaf Spot, giving it an excellent combined disease resistance package. Alegria and Cubic also have a 7 rating for Phoma and a 5 for Light Leaf Spot. Genetic resistance to Phoma and Verticillium in these varieties is multi-gene and so unlikely to break down.”
“This year we have no neonicotinoid seed dressings to protect crops as they come through and so many growers will be opting to grow varieties with known autumn vigour anyway. Alienor, Alegria and Cubic all have proven Verticillium resistance, high Phoma resistance plus excellent autumn vigour – the best combination for protecting yield, reducing growing costs and increasing flexibility of your spray programmes,” points out Neil.