The increased performance of TuYV resistant varieties seen in harvest trials and across UK farms in 2019 might not just be down to their ability to withstand increased viral attack, some OSR specialists now believe.
Whilst evidence of growing numbers of Myzus persicae aphids carrying the virus and reports of higher infection levels in plants is clear, it does not explain the full picture, says Mike Farr of DSV UK Ltd.
“Our own tests in conjunction with NIAB and Warwick University this year have shown an average 90% infection level for TuYV across the UK and aphid numbers are definitely rising, but we believe there are other factors at work behind the better performance shown TuYV resistant varieties.
“Many growers who choose to grow the new TuYV resistant variety Temptation this year, for example, report one of it’s biggest advantages was increased autumn vigour and its ability to grow through flea beetle attack so this could be as important as the viral resistance.”
Increased ability to cope with stress could well be an important feature of TuYV resistance, says Warwick University’s Professor John Walsh, who has studied the virus for many years and worked with DSV on testing plants to establish TuYV infection levels.
“The mechanism of resistance is not 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 related to the level of infection, the more you reduce this by, the more yield you will save.”
Such resistance has proved strong and stable across research and effective across the three main genetic groups of the TuYV virus, Professor Walsh points out.
“It may also be ‘genetically linked’ to an ability to cope better with greater levels of abiotic stress.”
Grower Robert Wright’s experiences support this with Temptation’s additional vigour and resilience clearly in evidence and making it the highest yielding oilseed rape at his White House Farm, at Barnby, Suffolk, for two years running.
The variety came out top in both field trials and the commercial crop in 2019 with remarkably consistent yields of 5.3t/ha in both, he explains.
“It was the consistency of the data that first attracted me to Temptation and my experiences of growing it on-farm over the last two years have certainly supported this with it being the highest performer last year and this.
“The farm trials we have carried out comparing a range of varieties showed it yielded around 5% more than its nearest competitor and it’s delivered the same in the field this year.”
“Temptation gets away really quickly and in poorer years it’s one of those varieties that you can visibly see growing more strongly and establishing better – that’s key when you can suffer so much loss early on these days.
The evidence that TuYV resistant varieties carry with them higher vigour and greater ability to cope with stress through the growing season is indisputable, believes DSV’s Mike Farr.
“RL listed Temptation was the outright best performer in trials at Easton Royal in Wiltshire with a yield 5.61t/ha compared to the closest non TuYV resistant variety at 4.33t/ha. Darling and Dazzler, currently on the Candidate List, also did well achieving an average of 4.95t/ha.
“But what is interesting is that there was persistent flea beetle attack during September and October at the trial site and it was apparent the exceptional late autumn vigour of these varieties helped plants grow through this.”
“Most oilseed rape varieties will stop growing when temperatures fall to 5 – 7oC but these TuYV varieties seem to keep growing until much colder conditions prevail. It’s very noticeable when you compare TuYV resistant varieties against others in the field in late Autumn.”
As well as being able to cope with disease and pest stresses better, it is also emerging that TuYV varieties may handle nutrient applications more efficiently, Mike Farr suggests.
“There mounting evidence that they utilise N much better than other varieties. We applied 250kg N/ha in total so growth was strong without any canopy problems resulting.
“The TuYV resistance gene seems to carry with it other significant advantages for growers and more work is required to clarify precisely how and why these result.”