Project reveals true incidence of turnip yellows virus in rape

Up to three-quarters of oilseed rape crops being monitored on Velcourt farm sites as part of a HGCA project are estimated to be infected with turnip yellows virus this season.

Trapping results that also found about 30% of Myzus persicae aphids were carrying the virus.

It has prompted the organisations involved in the HGCA-funded project to urge growers to take the threat of turnip yellows virus (TuYV) seriously. The aim of the project is to investigate the impact, incidence and distribution of the virus, the symptoms of which are often misdiagnosed as nutritional deficiencies.

Mark Stevens of Brooms Barn explains that TuYV is transmitted by aphids feeding in the autumn, but adds that symptom expression is usually delayed until stem extension in the following spring.

"You get a characteristic purpling and reddening of the leaf margin, together with interveinal discolouration. Weve recorded a yield suppression of anywhere between 12% and 30% in our work to date, so the impact of the virus shouldnt be underestimated."

The involvement of Velcourt in the project has allowed a survey of leaf samples from the companys managed farms, which together with insect trapping, has shown that the virus is a countrywide problem in the main vector Myzus persicae and the crop itself.

"Of the farms sampled, there were several sites in Lincolnshire and Kent which were hotspots, with 70% of crops infected (see map). Our intention now is to look at the impact of that level of infection on yields," says Dr Stevens.

This years cold winter will have taken out many aphids, he accepts. "There will be few around in the early part of 2010. But a dry and calm autumn, such as we had in 2009, works in their favour."

In such conditions, aphid migration into oilseed rape crops carries on through September, October and November, he reports. "Theres a wide host range for both the virus and the aphids responsible for its spread, which allows levels to build up."

A trial conducted in 2008 involving 10 varieties gave an indication of varietal differences in symptom expression - something which Dr Stevens will be looking at in more detail this year.

"The other focus of our work this year is to try to assess the impact of the virus according to the time that the aphids arrive. So we will be monitoring crops which are inoculated with aphids in mid-September, mid-October, mid-November and late January."

Apart from a loss in yield, infected plants are shorter, have fewer pods and fewer seeds, he adds. "Theres also some evidence that oil quality is adversely affected."

Control of the virus-transmitting aphids can be achieved with seed treatments and foliar sprays. "Its the second-generation neonicotinoid seed treatments, such as Modesto (clothianidin + beta-cyfluthrin) and Cruiser (thiamethoxam + fludioxinil + mefenoxam), which have the persistence required. They give around eight weeks protection."

But large numbers of aphids flying into crops over a prolonged period may require a follow-up spray, he notes.

Keith Norman, technical director of Velcourt says that seed treatments were introduced across all the companys farms as standard practice this year, after seeing the results of monitoring work.

"One of the farms with the highest level of virus has only just started growing oilseed rape," he reports. "Forecasting aphid-borne virus is difficult, so weve started to look on TuYV in the same way as we view barley yellow dwarf virus."

Adrian Cottey of Bayer CropScience adds that sowing date and seed rate are important factors when it comes to using seed treatments to reduce virus levels.

"Crops sown at lower seed rates are more at risk from TuYV," he says. "In these situations, you need a better seed treatment, as the lower seed rate increases the pressure."

Work done with Modesto has shown an average yield benefit of 8.4%, or 0.3t/ha. "Thats twice the level achieved with its predecessor Chinook. We know that this newer chemistry is active on aphids."

He estimates that around 30% of the UK winter oilseed rape crop was treated with Modesto in its first season of commercial use. "Its the only seed treatment with virus reduction on the label."