TuYV – The unnoticed epidemic

 

Turnip yellows virus may be more of a problem than previously thought following a nationwide analysis of oilseed rape crops. CPM looks at the implications for growers.

Resistant varieties actually reduce levels of virus in the plant population.

By Rob Jones

Up and down the country, oilseed rape crops have been investigated for the presence of turnip yellows virus (TuYV). According to the results of the tests carried out at the University of Warwick and NIAB, over 90% of the UK crop now carries the virus.

The results follow previous reports that the numbers of the main virus vector, Myzus persicae, have been at high levels in recent years. A large proportion of these are now carrying TuYV, raising concerns that this will inevitably end up in growing crops.

Experience in Europe has shown the virus can result in yield losses as high as 30% and in the UK, AHDB studies estimate losses due to TuYV at 15%, amounting to around £80 million every year. It’s an issue that it is now concerning researchers, agronomists and growers alike.

“The situation is very worrying,” says Rothamsted Research entomologist Dr Steve Foster. “We’ve tested crops at a number of UK sites in recent years and the proportion of M. persicae carrying the TuYV virus has ranged from 50% to over 80%.

“These high levels can only lead you to believe that the virus is being carried into crops, which could be having a profound effect on production.”

To complicate matters, aphids resistant to pyrethroid insecticides are carrying the same amount of virus as non-resistant insects, he says.

“Over 85% of aphids we tested were resistant to pyrethroids, which means there’s a large population of aphids out there carrying high levels of TuYV virus that are proving very difficult to control. We need to be testing virus levels in the crop, as well as testing aphids for virus, to build up a true picture of what is happening and there’s an urgent need for funding and commitment to a major project in this area,” he says.

In the latest round of tests initiated by seed breeder DSV, actual levels of virus present in the growing crop have now been established and they present a very disturbing picture, says Sarah Hawthorne of DSV UK.

“We looked at crops due to be harvested this summer across a range of locations from N Yorks, down to Wilts and from Shrops in the West to Lincs in the East. Leaf samples were taken in the field and these were analysed at the University of Warwick Life Sciences department and by NIAB,” explains Sarah.

“All samples were taken randomly from crops that looked to be healthy and growing strongly. Out of the 60 samples taken, 55 of these were found to be positive for TuYV – which represents a 91.7% level of infection.

We believe this is representative of the whole of the UK, which suggests the level of virus now carried in the UK crop is much higher than previously thought,” she comments.

As well as taking part in the analysis, Professor John Walsh of the University of Warwick has carried out many years of research into the problem and he believes the UK is now facing a ‘perfect storm’ for rapid development of TuYV in crops.

“We have a worrying alignment of factors, including large numbers of aphids carrying high levels of virus, the loss of systemic control from neonic seed treatments and growing resistance to existing insecticides. In addition, the generally milder conditions we’re now experiencing is making the situation worse, with infection easily spread from other crops and weeds.

He believes it’s very clear the industry has to focus on solutions to address the increased risk from TuYV and sees resistant varieties as the main tool.

“The mechanism of TuYV resistance isn’t fully understood, but it appears to work by the plant being able to reduce levels of virus in its phloem rather than just being able to function as normal with high levels of virus in its system.

“Resistant varieties actually reduce levels of virus in the plant population and, as yield loss is linearly related to the level of infection, the more you reduce this by, the more yield you will save,” he adds.

The TuYV resistance has proved strong and stable in research, as well as being effective across the three main genetic groups of the virus, John points out.

“It may be ‘genetically linked’ to an ability to cope better with greater levels of abiotic stress. Although current resistance is thought to be due to one dominant gene, Warwick’s new sources of resistance have been found to be on more than one chromosome. This means they could be carried by more than one gene, which could be contributing to resistance and suggests they could remain robust.”

With the increase in aphids, higher proportions of these carrying the virus and now higher levels of plant infection in evidence, it’s no surprise the 2019/20 AHDB Recommended List saw TuYV varieties coming to the fore in the UK for the first time, says OSR specialist Simon Kröger from DSV in Germany.

“It’s an issue we’ve been wrestling with across Europe for many years and now, unfortunately, it looks like it’s becoming a growing problem in the UK. It’s one of those things that can be working in the background and depressing yields without growers fully understanding why,” he says.

“The virus makes the plants use the assimilates of photosynthesis for the replication of new viruses rather than making them available for producing plant biomass, which in turn leads to reduced growth and smaller seeds.

“It’s only when the level of infection reaches a certain threshold that you see obvious signs and by then it’s too late and yield losses of 20-30% aren’t uncommon. Often there are no indications at all that infection is present,” he highlights.

DSV’s newly listed variety Temptation is the first of a line of high gross output TuYV resistant OSR varieties entering the European testing system, he says.

“As well as Temptation being added to the latest RL as suitable for the whole of the UK, DSV triple-layer varieties Darling and Dazzler, which also have TuYV resistance, are now on the UK candidate list. In the future it’s likely that all new DSV varieties will carry TuYV resistance.

“We first started looking at the issue in the early 2000s, but only at a relatively low intensity. When it became clear that options for insecticide use were becoming increasingly limited, but insect pests and the diseases transmitted by them were becoming a major problem, the work was intensified,” explains Simon.

“As well as being able to handle high infections of TuYV without suffering yield loss, we’re finding such varieties have better tolerance to abiotic stress in the field. This means they’re generally more resilient across a range of varying growing conditions, which makes them ideal for modern OSR production.”

John believes greater awareness of the threat posed by TuYV in the UK is important. “I often think about TuYV as being an unseen virus, as it often shows no symptoms, so growers are unaware of its presence in the crop. Two plants could have 100% infection but only one could show any of the classic signs that we recognise as being indicative of the disease.

“But although you can’t see it, it doesn’t mean TuYV isn’t there. All the indications are that it will remain a problem for UK growers for some years and yield loss can be significant if the virus isn’t addressed.

“With reduced use of insecticides to control the vectors likely in the future, milder growing conditions generally and indications that high levels of infection are now being seen in aphids and plants alike, growers have got to take the use of TuYV resistant varieties very seriously moving forward,” he concludes.