Turnip yellows virus (TuYV) infection reached its highest level ever last year, with an average of 84% of non-TuYV resistant crops affected in early spring 2019 in the UK, according to surveillance by Limagrain, Agrii, Openfield and the Association of Independent Crop Consultants.

Almost all sites analysed had an infection rate between 81-100%. And while the East and South East is where the disease is historically concentrated, sites in Scotland and the South West also showed high levels of infection.

It has not been possible to calculate TuYV infection rates for 2020 due to lockdown affecting the number of samples collected.

However, aphid pressure has been high and virus transmission has been a concern in other crops.

Rothamsted Researchs Brooms Barn site in Suffolk reported levels of peach potato aphid (myzus persicae), the main aphid vector of TuYV, up almost 10-fold in June this year compared to the long-term average, and some sugar beet plants being monitored had 50-100 aphids per plant.

This seasons high aphid numbers could have an impact on new season oilseed rape crops, says Dr Steve Foster, research entomologist at Rothamsted Research.

Tested

The more aphids that are around, the greater chance of them coming into your crop. In 2017 and 2018 we tested aphids from a few sites in Scotland and England to see if they carried the virus , and at that time 20-80% carried it, depending on site.

Nearly every time an aphid goes into a crop that has virus, it will pick it up and potentially spread it in the crop, through secondary infection, to other crop locations and ultimately to following crop seasons. If youve got high levels of virus in the aphids and the crop, inevitably itll just make things worse. Its a numbers game, just like Covid-19.

The changing climate is also having an impact, with aphids flying much later into autumn and earlier in spring, and far fewer cold snaps which can kill aphids. The later autumn flights mean aphids continue to arrive in the crop when it is most vulnerable, Dr Foster says.

The problem with TuYV is its difficult to see symptoms of virus in the crop and it can sometimes be confused with other things, such as a lack of nutrients or drought damage.

Preventing virus transmission has become harder following the loss of neonicotinoid seed treatments, he says.

For turnip yellows, the loss of neonics was really bad news there was no known resistance here in the UK for aphids. In the remaining pesticides available to growers, primarily pyrethroids, there is very high resistance. These compounds also kill your insect friends, so you can end up with a worse situation than you were in before through a lack of beneficials. You dont kill the pest but kill the things that attack the pest.

This only really leaves IPM as a strategy for TuYV control, Dr Foster says. However, while natural enemies can help control aphid numbers through predation, they may not prevent virus transmission.

Diverse field margins and minimised pesticide inputs can help increase natural enemies in field populations, according to AHDB, and weed management is

also important to reduce the risk of green bridge infection.

Dr Foster adds: Varietal resistance is becoming more and more important. It does seem varieties bred to be resistant to TuYV are doing better than those

that arent, suggesting there is a lot of virus in the crop.

OSR crops that were drilled early this summer to get ahead of cabbage stem flea beetle will be most at risk of TuYV, says Will Charlton, Limagrain.

We get the most questions about TuYV the further south you go, and this is possibly to do with the kind of measures people are taking to establish the crop with

flea beetle. There has been quite a bit of drilling in July this year, which exposes the crop to a wider window of risk.

Theory dictates earlier drilled crops are higher risk because they provide a longer period for the aphids to fly into the crop, Mr Charlton says.

Weve seen a lot of problems in sugar beet with virus yellows this summer so, anecdotally, you would expect TuYV pressure to be high this autumn in OSR.

Conditions

Drilling new season OSR crops into good conditions will have been the priority for growers, and Mr Charlton says he would not recommend adjusting drilling dates specifically to manage TuYV while flea beetle is an issue. However, Agrii agronomist Will Foss questions the impact of TuYV on oilseed rape, and says it is an area that needs to be explored further.

There has not been much work done for a while. Our trial in Kent a few years ago always had more virus because its in a brassica growing area and it is down in the South, so you tend to get very high levels of virus, but the question was whether it really significantly impacted on yield.

If it was such a big problem all of the TuYV resistant varieties would rise to the top in the variety trials and that doesnt seem to have happened really.

There are studies that show some varieties without TuYV resistance genes can carry the virus without detriment to yield, he adds.

We theorised that maybe with Recommend List and National List trials there was actually a certain amount of selection going on without us realising it, not

specifically for a gene but for a level of tolerance in the existing varieties, which were rising to the top even with the presence of virus.

If youre assuming it does rob yield, weve now got a lot of varieties with the trait bred in alongside other useful traits and good yields. In the absence of that

there is Biscaya, but using genetics is going to be the main way of dealing with it.

The gene that gives OSR crops their resistance to TuYV was first used in the variety Amalie in 2014 and is now present in all commercial OSR varieties with TuYV resistance But with such varieties now being grown more widely, Prof John Walsh, leader of the Plant--Virus Interactions Group at the University of Warwick, warns of the potential problems of creating strong selection pressure in the virus population and breaking resistance.

He says: Selection pressure has, in the last couple of seasons, started to increase significantly so it is potentially ticking time bomb.

Most resistance breaks down fairly rapidly but there are no hard and fast rules, and some have hardly ever broken down, so you cant predict it.

The best way to mitigate this is to introduce new or alternative sources of resistance into commercial varieties, says Prof Walsh.

The resistance being used now came from Chinese cabbage. We have looked at other OSR varieties and there are no particularly strong sources of resistance to TuYV. However, weve identified a number of sources of resistance from brassica rapa and brassica oleracea. Weve produced two different, resynthesized oilseed rape lines from these resistance sources, both of which have different resistances to each other.

Commercial crops

A Biotechnology and Biological Sciences Research Council Follow-on Funding project at Warwick is enabling researchers to work with four commercial seed companies to incorporate this resynthesized resistance into commercial crops.

This will take at least 10 years because there has to be a lot of backcrossing with the new material and oilseed rape to get a plant that is agronomically acceptable and will yield well.

In the project we are identifying molecular markers so plant breeders can track the resistances. This markerassisted selection will rapidly accelerate the integration of resistance into commercial varieties, says Prof Walsh.