Scottish growers on heightened BYDV alert20 September 2019
High aphid numbers over the summer, coupled with short turnaround times between crops after the late Scottish harvest, and the loss of neonicotinoid insecticide seed treatment chemistry, are all set to put this autumn’s cereal crops at heightened risk of barley yellow dwarf virus (BYDV) infection.
So says Alistair Gordon, regional technical manager in Scotland for agronomy firm Robertson Crop Services, part of the ProCam group.
He says even when a neonicotinoid seed treatment could be used in cereals prior to this autumn, crops emerging in early October in northern areas of Scotland, or later in milder southern areas, were particularly susceptible to BYDV infection.
Now, without that initial protection from a neonicotinoid seed treatment against BYDV-carrying aphids, he believes even crops emerging later will be at heightened risk.
“Historically, a neonicotinoid seed treatment gave an important period of protection,” says Mr Gordon, “but we’re now moving into unfamiliar territory. We will have to rely much more on timely applications of aphicide sprays.
“Additionally, because of the late harvest in Scotland this year, the turnaround to get winter crops planted is shorter. This leaves less time to destroy green bridges between crops, which allows more aphids to survive.
“At some point, frosts will become severe enough to check aphid activity. But until that point, it is important that crops are monitored closely and any threatening aphid populations dealt with promptly. Even crops emerging later into October could be at risk.”
When spraying against BYDV-carrying aphids, Mr Gordon urges growers to choose a pyrethroid insecticide with good persistence to achieve maximum duration of control. However, a further issue for Scottish growers is aphid resistance, he adds.
“BYDV is carried by three different aphids – the grain aphid, the bird cherry-oat aphid and the rose-grain aphid. Growers in Scotland are looking out primarily for the grain aphid. However, resistance has become more of an issue recently, so correct spray timing and limiting the number of applications has become even more important.
“A T-sum is a useful tool to help with the timing of insecticides. This uses accumulated daily temperatures to predict aphid risk and is calculated by subtracting 3 degrees centigrade from the average temperature each day and adding the result to the running total. When the T-sum reaches 170, it’s an indicator the second wingless aphid generation could be starting to emerge, which spreads the virus. Where there’s a risk, growers should be walking crops daily, but only spray if aphids are present.”
For convenience, Mr Gordon says an aphicide can often be applied at the same time as a foliar manganese spray, which is frequently needed to prevent manganese deficiency occurring in light soils.
“Correcting manganese deficiency in autumn can also make cereal crops less susceptible to mildew problems in spring,” he adds.