Turnip yellows virus (TuYV) is now a major threat to oilseed rape yields and growers should carefully monitor crops this autumn for aphids which spread the disease and treat with an insecticide if present.

Predominantly carried and spread by the peach potato aphid (Myzus persicae), there are a number of symptoms in crops infected with TuYV.

These include reddening of leaf margins, purple colouring of leaf margins and interveinal yellowing and reddening, but is often symptomless.

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From the same family as cereal disease barley yellow dwarf virus, it causes stunted plants, resulting in up to 30% yield loss and is estimated to cost the industry between £67m-£180m each year.

Hidden problem

University of Warwick’s John Walsh says TuYV is a “hidden problem”, with growers often unaware crops are infected as it occurs across fields and can be attributed to problems such as nutrient deficiency.

“It’s often very evenly spread and our research has found that 100% of sampled plants can be infected across a field in some years,” he explains.

Professor Walsh adds the virus is also prevalent throughout the UK, from Cornwall to the north of Scotland, with hotspots in Humber, Lincolnshire, Suffolk and Kent.

Huge reservoir

The virus can be hosted by more than 150 separate plants species in 20 plant families, including common arable weeds such as shepherd’s purse, fumitory, mayweed, groundsel and chickweed, to name but a few.

Combine this with the prevalence of Myzus aphids in the UK, of which an estimated 60-80% of the population carry TuYV for life, the virus reservoir and infection efficiency is huge.

Neonicotinoid insecticide seed treatments provided growers with about 85% reduction in aphids when the crop is at its most vulnerable stage, significantly reducing virus spread.

They have now been banned, however, and leave just foliar insecticides for direct control and with Myzus 100% resistant to pyrethroids, only Plenum (pymetrozine) and Biscaya (thiacloprid) provide effective and approved options.

Myzus has two main migration periods, one in the summer that peaks around June and the other in autumn from mid-September onwards, which can last until November in an open season.

Potential risk

Fera Science entomologist Larissa Collins says yellow water traps run on behalf of AHDB Potatoes through the summer caught very high numbers in some regions, particularly in the Midlands and Northern England.

She explains although there is no model to make a scientific prediction that risk will be increased summer aphid counts were high, logic would say this may be the case and vigilance will be required against the pest this autumn.

“Those aphids have got to go somewhere. We can’t be sure of the risk to oilseed rape, but it is definitely something to keep an eye on as crops begin to emerge,” says Dr Collins.

Spray timing

Syngenta field technical specialist Max Newbert says conditions over the coming weeks will have a major influence on the level of risk from the pest and the virus it carries.

“It could be low if it becomes wet and windy, but if conditions are favourable, there could be a lot around.

“It is very hard to time sprays and it will require a lot of hard work.

“Growers and agronomists will have to keep an eye on crops, Rothamsted Research aphid bulletins and also have the option of yellow water traps in the field,” he explains.

In addition to these monitoring measures, there is also a Warwick Myzus online bulletin that shows the proportion of aphids carrying TuYV this season.

Dr Newbert adds that sprays such as Plenum need to be applied just before peak migration.

“It is all about autumn infection with turnip yellows virus, as the earlier it occurs the greater the yield loss, so timing is critical.”