Cabbage stem flea beetle is not the only pest troubling oilseed rape crops, with peach potato aphid posing a threat as a vector for turnip yellows virus.
Increasing aphid activity means oilseed rape growers face a tough decision about whether to treat crops over coming weeks to reduce the risks from turnip yellows virus (TuYV).
The symptoms of the virus, spread by the peach potato aphid (myzus persicae), can be very difficult to identify in the field, but Hutchinsons technical support manager Neil Watson believes it is becoming more prevalent and the damage to yield from early autumn infection must not be underestimated.
Analysis by Mr Watson shows seven out of 10 of the top-yielding oilseed rape varieties on the AHDB Recommended List have the TuYV resistance trait, and on average yield 5-15 per cent more than non-resistant varieties. It may be partly down to better genetics in resistant varieties, but he says there is also a strong implication that TuYV is present and going undetected.
He says: “TuYV is often seen as a problem that mainly affects southern England, but can be an issue even as far north as Croft in North Yorkshire.”
The potential damage from infection depends on the variety, whether it contains the resistant trait or not, numbers of myzus persicae in flight throughout the autumn, and crop size at time of infection, adds Mr Watson.
Further analysis of aphid flight over the past four years reveals there are typically two peaks in activity; one in early October and another about three weeks later, he says.
Mild conditions and continued aphid activity could further extend the period of infection, so he advises growers to monitor crops carefully over coming weeks and be prepared to treat crops without the resistance trait once, potentially twice if necessary.
“The earlier the infection, the greater the virus risk, so if you do decide to spray it pays to do so earlier in the season. Products often do not offer much persistence, so if you have treated crops in early October, it may be necessary to follow this up later in the month if aphid risk warrants it.”
Widespread resistance to pyrethroids among myzus persicae populations means the only effective chemical controls are products based on thiacloprid or flonicamid, both of which cost more than traditional pyrethroid treatments.
Understandably, many growers may therefore be reluctant to make such an investment where crops are struggling to establish in the presence of cabbage stem flea beetle or other pests, such as pigeons or slugs.
Ultimately, it comes down to a judgement call between grower and agronomist, says Hutchinsons technical development director Dr David Ellerton.
He says: "It depends on your attitude to risk and whether you think the crop is worth treating. If it is a decent crop, sensitive variety and there are a lot of aphids around, then it may be worth protecting it.
“It is admittedly a harder decision if the crop is still struggling to establish because of flea beetle."
Going forward, growers concerned about TuYV risk must look to grow a resistant variety, he adds.
"The fact they are performing so well in Recommended List trials suggests there is more of the virus around and it could be holding back yields."
York has been a recent hotspot for peach potato aphid activity in AHDB monitoring, and local Hutchinsons agronomist David Stead says all crops are being monitored closely.
Mr Stead says: “For us, we are on the borderline as to whether aphid treatment in oilseed rape can be justified or not.
"This year might be one when some growers can justify treatment because of the numbers of aphids flying, if they are not growing varieties which hold the resistance trait."
Turnip yellows virus key points:
- Virus is spread by the peach potato aphid (myzus persicae)
- It is present throughout the UK, but areas in southern England are at highest risk
- It can potentially reduce yields by 15-30 per cent
- Smaller or early-sown crops are at greater risk
- Reduce the risk by growing a resistant variety or controlling aphids in autumn
- Two sprays may be necessary to cover peaks in aphid flight