Oilseed rape disease turnip yellows on the rise after mild autumn

One major yield-sapping disease of oilseed rape is more widespread this season due to a mild autumn that favoured the aphids that carry the disease, and growers are being urged to look at resistant varieties.

A nationwide survey of turnip yellows virus (TuYV) showed the disease, which can cut yields by up to 30%, was detected from Perthshire down to Dorset and at higher levels than the previous season.

The survey across 14 sites was undertaken by plant breeder Limagrain and agronomy group Agrii, and clearly showed the link between disease incidence and the mild autumn weather favourable for aphids.

See also: Latest turnip yellows resistant OSR shows increased yield

Vasilis Gegas, Limagrain’s oilseed rape breeder, says the pattern over the past four years has been that when there are plenty of aphids in crops than growers should expect high infection levels.

“One of the reasons the disease is becoming widespread is that the timing of insecticides can be difficult as the window of aphid activity is from July to November,” he told Farmers Weekly.

Resistant varieties

He argues that genetics offers the best protection against the disease, and there are now a number of oilseed rape varieties that are resistant to turnip yellows.

Early signs of the disease are purpling of the leaves and later symptoms show interveinal yellowing and reddening of leaf margins.

“More than half the sites showed samples to be highly to severely infected, and none of the sites showed no disease,” said David Leaper, Agrii’s seed technical manager.

Vasilis Gegas (left) and David Leaper (right)

Disease incidence ranged from 100% in Norfolk, with high levels also seen in Scotland, Yorkshire, Suffolk and Kent, to lower levels in Somerset and Lincolnshire.

The incidence can be related to the use of autumn insecticides.

All 14 trial sites showed signs of the virus and compared mainstream rapeseed variety Exalte with a new TuYV-resistant variety Architect.

The first resistant variety, Amalie, from Limagrain was tested in the 2015 harvest year, but its gross output yield at 98% lagged behind top variety Elgar at 109% in the AHDB Recommended List.

Hybrid and conventional 

Now two new varieties from Limagrain – hybrid Architect and conventional Annalise – are candidates for the new list to be published at the end of this year, and both have higher yields than Amalie.

“The trait is a very efficient way of controlling the disease rather than using short-acting contact insecticides,” Mr Leaper added.

He likes the look of Architect as the variety shows good early autumn growth so may be useful in the fight against cabbage stem flea beetle.

Turnip yellows disease is spread by peach potato aphids (Myzus persicae) and had been largely controlled by neonicotinoid insecticide seed treatments, which also control cabbage stem flea beetle.

The European Commission imposed a ban in December 2013 on the three neonicotinoids that were used as seed dressing on bee-attractive crops, such as oilseed rape, due to their perceived harmful effect on bees.

Growers in four flea beetle hotspot counties – Suffolk, Cambridgeshire, Bedfordshire and Hertfordshire – were allowed to use neonicotinoids under emergency legislation in the harvest 2016 season, but no exemptions are allowed this season.

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