Growers should select rape varieties carefully after a survey confirmed that one third of susceptible UK crops are infected with Turnips Yellows Virus (TuYV).
Transmitted by aphids, TuYV can result in a 30% yield penalty where high levels of infection occur early in the crop’s life. It is believed that 70% of peach potato aphids (Myzus persicae) carry the virus.
Plant breeder Limagrain has been monitoring TuYV levels in non-resistant or susceptible crops since 2015. Leaf samples are taken both in spring and autumn – and tested using the standard Elisa test.
“We’ve mapped the incidence of TuYV from the UK to Ukraine and seen it build over the years,” says Limagrain marketing manager William Charlton. “We’re getting to a situation where TuYV is now endemic across Europe, and no longer confined to hotspots.”
Samples were taken from 26 locations across the British Isles. Results from this spring’s sampling confirm that one third of all the UK’s non-resistant oilseed rape crops sampled were infected with TuYV.
As expected, high rates of infection of 66-80% were reported in south-east England. But the highest rates of infection – as much as 81-100% – were reported on farms across the Midlands.
High levels of infection were detected as far north as the borders, and as far west as Wales. One third of non-resistant rape crops were infected across Yorkshire, Lincolnshire, Shropshire and Wales.
“Five years ago we would not have seen these cases in the north and west, which just goes to show how TuYV is becoming increasingly widespread, and is now endemic in the UK rape crop, irrespective of region.”
The link between high numbers of aphids in the autumn and corresponding levels of infection is clear, explains Mr Charlton. Infection usually occurs in September to October when aphids are still flying, he adds.
“The earlier a crop is infected, the more severe the symptoms tend to be with a harsher yield penalty as the plants have less seeds or pods. Early drilled crops that are more open, corresponding with mild autumnal conditions, are at the highest risk.”
Aphid numbers have increased because controlling them has become harder with the loss of neonicotinoid seed treatments. Getting the timings right for sprays is difficult, so TuYV inoculum is building up in the environment.
“Once an aphid is infected, it is infected for life, “ says Mr Charlton. “If an uninfected aphid feeds on an infected plant, the aphid becomes infected and so the cycle continues – producing fewer side branches, pods, and seeds per pod.”
Hard to detect
Even severe symptoms can be hard to spot, so TuYV often goes undetected. Combined with the loss of neonicotinoids, this can make it particularly hard to difficult to control early infection.
The value of genetic resistance to TuYV has been proven since Limagrain launched TuYV resistant variety Amalie in 2014, says Mr Charlton. Initially, resistance came with a yield lag, but innovative breeding has since overcome this.
Today, hybrid and conventional TuYV-resistant varieties top the AHDB Recommended List – confirming the value of genetic resistance as a way to protect crops against heavy yield losses from the virus.
Varieties carrying TuYV resistance rely less on insecticides, adds Mr Charlton. In high-risk regions, as much as 80% of the total rape area is drilled with resistant varieties. “Genetic resistance is by far the best way forward.”