Mild autumn increases BYDV risk
Winter barley and winter wheat growers are being warned of a high risk year for barley yellow dwarf virus (BYDV).
A warm summer and early mild autumn, as well as many early-drilled crops have increased the level of risk.
David Stormonth, technical manager for Interfarm UK, said crops that did not receive a seed treatment would need a pyrethroid “sooner rather than later”.
“Those with a seed treatment should be monitored just as carefully,” he added.
Early-drilled crops that were given a seed treatment may shortly need a follow-up treatment with a persistent pyrethroid insecticide.
“A seed treatment such as clothianidin will give six to eight weeks’ protection against aphids from the time of drilling,” he explained.
“Growers of early-sown winter barley and winter wheat will usually need a post-emergence top-up with a pyrethroid spray to give a further six weeks’ protection.”
Mr Stormonth said ADAS recommended using the T-sum spray threshold to calculate when the follow-up spray was needed.
“This is calculated by taking the daily mean temperature, subtracting three from it and then adding the result to the accumulating total until it reaches 170,” he said.
“For crops treated with a seed treatment, this calculation should start six weeks after sowing, but crops without a seed treatment, it starts on the day of crop emergence, when aphids are able to colonise the plants.”
For example, crops emerging in the last week of September reached a T-sum of 170 by mid-October.
Dan Dines, an AICC agronomist with Wessex Agronomy Services, advising growers in Dorset and Wiltshire, said it was difficult to assess whether aphid numbers would be higher this season.
But the risk will increase if the mild weather continues into November, he added.
For crops without a seed dressing, he has been recommending a treatment with an appropriate insecticide.
However, for early-drilled crops which were given a seed dressing, such as Deter (clothianidin), the picture is less clear.
“The six-week protection period for early drilled crops finishes next week and most will probably get a follow-up insecticide as the risk is there,” he said.
But Mr Dines urged growers to consider whether they really needed to spray an insecticide.
“You should consider whether the crop has had a seed dressing and when it was drilled,” he said.
“But you have also got to work on a preventative basis. Discovering BYDV in your crop in the spring isn’t much fun.”
He recommended an informal risk assessment with an agronomist before deciding to spray an insecticide.