Crops    |   Disease    |   Oilseed Rape   

European farmers have seen yield losses of up to 30% in recent years resulting from Turnip Yellow virus in oilseed rape so it’s ability to cause serious damage to production and profits is in little doubt. But with increasingly limited means of controlling the virus’s Myzus persicae aphid vectors, the question of how much of a threat it is to UK growers is now being taken very seriously by scientists, agronomists and breeders alike. Of growing concern are the high numbers of the aphid Myzus persicae seen at key times in recent years combined with worrying new data about the number of these carrying the virus.

According to Alex Greenslade of the Rothamsted Insect Survey, whilst numbers of Myzus persicae have varied in recent years, some specific trends are becoming evident. “The numbers of Myzus persicae caught in our trap fluctuate greatly from year to year and it is difficult to tell what is happening in terms of a population trend when the data is this variable. “What we do generally see is that the aphid flight season is getting longer. They are flying earlier in the spring and staying around later in the autumn.” What is equally 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 recently and the proportion of Myzus persicae we caught that were carrying the TuYV virus ranged from 50% to over 80%. “These high levels can only lead you to believe that high levels are resulting in crops which could be having a profound effect on production.” In addition, aphids resistant to common insecticides are carrying the same amount of virus as non-resistant insects, he says. “Over 85% of aphids we tested were resistant to common insecticides like pirimicarb and pyrethroids which means there is a high population of aphids out there 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 Mark Stevens, Graham McGrann and Bill Clark at Broom’s Barn Research Centre in Suffolk surmised  ‘It is likely that TuYV is one of the principal reasons why commercial oilseed rape crops do not reach their genetic yield potential.’ But recent events can only have made the situation worse, Steve Foster believes “You can only conclude that the ban on Neonicotinoid seed dressings and the ongoing loss of chemistry containing effective insecticides is contributing to the problem becoming much more prevalent.” “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 significant yield losses but it’s too simple to pin it simply on reduced insecticide treatments, says Nadine Wellman of Germany-based seed breeders DSV. “Previous studies have suggested that there is nothing new about TuYV infection in oilseed rape 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, an insecticidal treatment to control aphids makes less sense, however, because it does not target the vectors specifically and it is well known that Myzus persicae is already resistant to several insecticides.”

According to Sarah Hawthorne of DSV in the UK, the only realistic option of 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 with the variety just being added to the RL in December last year.”

What is interesting is how consistently well the variety has been doing in recent UK trials, often outyielding well established top performers, she says. “This is exactly what started to happen on the continent and was really the beginning of the wider industry becoming much more aware of what was happening with TuYV.” Temptation has one of the strongest all-round production and agronomic packages in the new 2019/20 AHDB RL, she says. “In the challenging year of 2018, Temptation was a top three contender for yield in AHDB harvest trials with an average yield of 5.45t/ha – just marginally behind the 5.52t/ha of the leading variety. “It also has an exceptional oil content with 46.5% in 2018 AHDB harvest trials and 46.0% in the 2019/20 RL – the joint highest oil content in the list.”

Temptation’s production potential is supported by an outstanding agronomic package featuring strong resistance to phoma stem canker and light leaf spot in addition to the TuYV resistance, she adds. “The problem is that TuYv is very difficult to spot in growing crops and impossible to treat once you’ve seen it – by then the damage is done.”

Temptation delivers 20% higher yield

A trial area of DSV Temptation was the highest yielding oilseed rape variety on August Farms’ Signet Hill Farm at Burford in Oxfordshire in 2018 delivering an additional 20%, over the crop’s average for the year. Farming 330 hectares of combinable crops, Nick August operates a four-year rotation based around 60ha – 80ha blocks of land comprising of mainly Cotswold Brash soils, with wheat followed by OSR, wheat and then a break crop, which in 2018 was spring barley. “We have not got a particular problem with charlock and resisted growing Clearfield varieties for 2018 harvest,” he explains. “With the field headlands around the tramline trials drilled with a conventional variety, DSV suggested I grow Temptation as one of four varieties in replicated trials.”

The 2018 oilseed crop was established on 29 August, about the normal time for the farm, Nick recalls. The variety emerged well, grew away vigorously in the autumn and looked very well going into winter, he says. “The crop received the farm’s standard fertiliser programme for OSR, totalling 200kgN/ha, and progressed well in the spring, developing a distinctive appearance with a nice canopy and strong stems. “It had a long flowering period and stayed greener for longer than the neighbouring plots of Sparrow which yielded nearly as well.”

Temptation grew reasonably tall and received a standard application of pod sealant prior to harvest, being direct cut on 17 July, which was two weeks earlier than normal. Ultimately the best of several trial varieties grown on the farm, Temptation went on to yield 3.8t/ha – compared to the farm’s average of 3.2t/ha for the year. “This was very respectable given the drought conditions which lasted from the middle of April until harvest and had a severe negative impact on cereal yields.”

Based on the results from the 2018 harvest, Nick is now growing 40ha of Temptation in place of the previous conventional variety.

Source: DSV

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