Turnip Yellow Virus

Oilseed rape growers must take the threat of Turnip Yellows Virus seriously and recognise the yield penalty that can result from infection.

That's the message from all the organisations involved in an HGCA-funded project investigating the impact, incidence and distribution of the virus, the symptoms of which are often misdiagnosed as nutritional deficiencies.

Dr Mark Stevens of Broom's Barn explains that Turnip Yellows Virus (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. We've recorded a yield suppression of anywhere between 12-30% in our work to date, so the impact of the virus shouldn't be underestimated."

The involvement of Velcourt in the project has allowed a survey of leaf samples from the company's managed farms, together with insect trapping showing that the virus is a countrywide problem in the 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. Our intention now is to look at the impact of that level of infection on yields," says Dr Stevens.

Some 30% of the Myzus persicae aphids found at the Velcourt farm sites were carrying TuYV, he confirms. "This then translated into between 36-78% of the crop having the virus. We know from previous monitoring that there can be as many as seven out of ten aphids carrying the virus at any one time."

This year's very cold winter will have taken out aphids, he accepts. "There will be very 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. "There's 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 ten 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 and asses 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 less seeds, he adds. "There's also some evidence that oil quality is adversely affected."

Control of the virus transmitting aphids can be achieved with seed treatments and foliar sprays. "It’s the second generation neonicotinoid seed treatments, such as Modesto and Cruiser, 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 company's farms as standard practice this year, after seeing the results of the 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 very difficult, so we've started to look on TuYV in the same way as we view BYDV."

Adrian Cottey of Bayer CropScience adds that sowing date and seed rate are very 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. "That's twice the level achieved with its predecessor Chinook. We know that this newer chemistry is very 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. "It's the only seed treatment with virus reduction on the label."

Panel - Velcourt at Cereals 2010

Visitors to the Velcourt stand at Cereals 2010 will be able to see for themselves the symptoms of TuYV.

Oilseed rape plots will be inoculated with virus-carrying aphids, some of which will have been treated with current seed treatment choices, Modesto and Cruiser.

"This virus is an unknown area of yield loss in oilseed rape," says Keith Norman. "Our aim is to show the impact it can have and how seed treatments can be used to prevent infection. We will have infected demonstration plots at Cereals with and without seed treatments"