Turnip Yellows virus (TuYV) may be a hidden threat to UK crops and there are signs that it’s on the increase. CPM reports.
My feeling is we’re seeing just the tip of the iceberg and things could be a lot worse than we realise.
By Rob Jones
Crop viruses can be devastating and difficult to control because by the time symptoms occur, it’s too late to do anything. Oilseed rape growers are being warned that Turnip Yellows virus (TuYV) seems to be on the rise.
The reasons for this are multifold. Monitoring indicates high proportions of the aphid population have been found to be carrying the virus and TuYV resistant OSR varieties have been performing extremely well this harvest, perhaps indicating a TuYV yield penalty in some crops without resistance to the virus. These findings are fuelling concerns that the virus could have more of a stranglehold on UK production than was previously realised.
Sarah Hawthorne says the only realistic option for controlling the virus lies in developing TuYV tolerant and resistant varieties.
According to Alex Greenslade of the Rothamsted Insect Survey, while numbers of peach potato aphid (Myzus persicae) ay have varied in recent years, some specific trends are becoming evident.
“The numbers of M. persicae caught in our trap fluctuates greatly from year to year and it’s difficult to tell what’s happening in terms of a population trend when the data is this variable. But we’re generally seeing the aphid flight season is getting longer, they’re flying earlier in the spring and staying around later in the autumn.”
Last year aphid populations were generally high, he adds. “M. persicae saw high population peaks last year, but they were generally in-line with some of the high numbers of the aphid we’ve seen over the past decade, particularly in years like 2011 and 2015 which showed even larger peaks.
“Numbers were at the high end of previous autumn catches over the past decade but similar to the numbers in 2007 and 2009. Again, these populations fluctuate substantially each year.”
What’s more concerning is the proportion of these populations that actually carry the virus, says Rothamsted research entomologist Dr Steve Foster. “It’s very worrying. We tested at a number of UK sites last year and the proportion of M. persicae that were carrying the TuYV virus ranged from 50% to over 80%.
“These results can only lead you to believe that high levels of TuYV may be the result in crops, which could be having a profound effect on production.”
To compound the problem, aphids resistant to common insecticides are carrying the same amount of virus as non-resistant insects, he says. “Over 85% of aphids tested were resistant to common insecticides like pirimicarb and pyrethroids, which means there’s a high population of aphids carrying high levels of TuYV virus that are proving very difficult to control.”
As far back as 2008, a then HGCA-funded research project carried out by Dr Mark Stevens, Graham McGrann and Bill Clark at Broom’s Barn Research Centre in Suffolk suggested that it’s likely TuYV is one of the principal reasons why commercial OSR crops don’t reach their genetic yield potential.
Recent regulatory changes can only have made the situation worse, believes Steve. “The ban on neonicotinoid seed dressings and the ongoing loss of chemistry containing effective insecticides is probably contributing to the problem becoming much more prevalent.”
The situation is compounded by the fact that the virus is visually very difficult to detect in the growing crop, he points out. “Often you only realise there’s a problem when the damage to yield has occurred and it’s too late to do anything about it.
“The bottom line is we need to be testing virus levels in the crop as well as in aphids, to build up a true picture of what’s happening. There’s an urgent need for funding and commitment to a major project in this area. My feeling is we’re seeing just the tip of the iceberg at the moment and things could be a lot worse than we realise.”
In continental Europe, TuYV has been blamed for yield losses of up to one third but it’s too simple to pin the reason on reduced insecticide treatments, says Nadine Wellman of Germany-based seed breeders DSV.
“Previous studies have suggested that there’s nothing new about TuYV infection in OSR but the incidence appears to have increased significantly in recent years.
“Contributory factors have certainly included the modern political framework that has resulted in a lack of insecticidal seed treatments, but climate change has generally resulted in higher temperatures in the autumn combined with milder winters and this is an important factor too. Increased cultivation of potential host plants has not helped either”, she adds.
“Up until 2014, insecticidal seed treatments were a relatively effective means of preventing a viral infection and subsequent yield losses. Moving forward, using an insecticidal treatment to control aphids makes less sense because it doesn’t target the vectors specifically and it’s well known that M. persicae is already resistant to several insecticides.”
According to Sarah Hawthorne of DSV in the UK, the only realistic option for controlling the virus lies in developing TuYV tolerant and resistant varieties. “We’re taking this seriously and we’re confident virtually all new varieties introduced for the UK market in the future will be TuYV resistant.
“Trials across Europe have shown encouraging yield results for the first of these, called Temptation. The variety has been successfully registered in France and is currently going through AHDB Recommended List trials in the UK.”
What’s interesting is how well the variety is doing in current UK trials, she highlights, and it’s often out-yielding well established top performers.
“With harvest results in from eleven RL sites, the 2018 data puts Temptation as the joint highest performer this year, with an average gross output of 107% of control, representing a yield of 5.48t/ha.”
“This is exactly what started to happen on the continent and was really the beginning of the wider industry becoming much more aware of the situation with TuYV.”
The TuYV resistance mechanism works by simply slowing down the speed of multiplication of the virus within the plant, she says.
“It’s been proven to be effective through testing with various scientific institutions in glasshouse tests throughout Europe, as well as under natural infection conditions in the field. By far the most important proof for the efficacy of the resistance is the results achieved by the resistant varieties in both trials and on-farm.”
Lincs grower John Lucas of Welbourn Farms, witnessed this in the field this year, with Temptation achieving spot yields up to 5.8t/ha compared to Dariot which just nudged ahead at 6.0t/ha.
“The best sample we saw this year was Temptation. Whilst Temptation and Dariot did well, so too did Incentive with spot rates on the combine ranging between 5.0 – 6.4t/ha. Dualis also impressed with 4.5 – 6.8t/ha,” he reports.
How does the TuYV infection take hold?
The peach-potato aphid Myzus persicae is one of the main vectors of TuYV, although it can also be transmitted by many other aphid species including the cabbage aphid.
M. persicae colonise the crop in autumn shortly after emergence of the OSR. They migrate from cover crops and/or summer crops and are already infected when they transfer to the winter OSR crop.
In warm autumn weather, wingless aphid populations can explode before winter sets in, leading to large-scale infection. As temperatures start to fall, the aphids migrate to suitable trees to lay overwintering eggs.
In early spring as temperatures start to rise, the aphid progeny leave their winter quarters, acquire the virus from the infected OSR crop and spread it further. At the same time, the aphids transmit the virus to weeds and other crops, which then provide a further source of infection.
As UK autumns become increasingly warmer and winters milder, aphids manage to persist in the OSR crops in autumn and even over-winter and continue to reproduce. The incidence and spread of TuYV infections are largely dependent on the weather. Seasonal and climatic conditions determine aphid activity and the spread of the virus in OSR.
Temptation tops the tables
Although not all harvest information is complete, 2018 data from the first eleven sites to be harvested in AHDB trials have shown the TuYV resistant variety Temptation to be the joint top performer this year.
It is also the highest performing variety in French trials this year and third in Poland and Hungary.
Temptation is a restored hybrid with multi-gene resistance to phoma and light leaf spot and has a high oil content. The variety has good early vigour, typical of other DSV varieties like Incentive 45 and Sparrow 45 – so called because of their outstanding growth in the first 45 days after drilling. This makes it suitable for medium to late drilling.
