Managing the BYDV risk

During February, the UK experienced the highest and the lowest temperatures of the winter season. According to the Met Office, cold conditions from the
east brought temperatures down to -23degC in Braemar, Aberdeenshire, on February 11.

This was followed by a southerly flow that brought warm weather from the Canaries and Africa and led to the season’s highest temperature of 18.4degC at Santon Downham, Suffolk, on February 24.

But according to Dr Mark McCarthy, head of the Met Office National Climate Information Centre, the kind of big temperature dips experienced in Braemar this year will be more of a rarity in future.

“Minimum temperatures of below -20degC were more frequent historically, but have become scarcer, while winter temperatures above 18degC have become a little more regular, with four of the last five winters recording such events.

“Our winters are changing and, as we have seen, we can still receive cold snaps, it’s just those extremes won’t be quite as cold or as frequent as they once would have been,” he says.

As colder-than-average temperatures are known to curb aphid populations, growers based in areas that have experienced big temperature dips are likely to see aphid numbers impacted and the appearance of these pests possibly delayed.

This has been shown recently with the decision not to authorise the emergency use of neonicotinoid seed dressings in sugar beet because the virus yellows threat is deemed so low.

However, in some parts of the UK, where temperatures did not dip as low as those observed in Scotland, growers should remain vigilant for aphids appearing in early spring.

Aphids

Geoffrey Bastard, commercial technical manager with FMC, warns there are aphids about.

“It’s remained relatively mild in certain places, especially in the South West, which often doesn’t get as hard winters as some other parts of the UK. This means
aphid populations may have survived throughout the winter in these areas,” he says.

Temperatures required to significantly decrease BYDV aphid species are -8degC for grain aphid and -0.5degC for bird cherry oat aphid. Grain aphid are the prime vector in the mid-north UK and bird cherry aphid in the South West.

Many autumn-sown cereals reached the crucial T-SUM 170 (the accumulated mean daily temperatures in degC) in October-November, meaning a second generation of aphids was potentially spreading the BYDV virus. A badly infected crop could suffer yield losses of 10-15%.

“If autumn-sown crops are infected with BYDV we should start to see the effects of it in the next few weeks. As crops start growing, the plants that are infected won’t grow at the same rate. You might start to see patches where the crop might begin to go yellow and just generally appear unwell,” says Mr Bastard.

If a significant area of the field is infected, a decision will need to be made about how to treat those patches.

“Those plants that are infected will not provide a positive return on investment from fertiliser and fungicide applications compared to the healthier parts of the crop,” he says.

Spring crops

When it comes to spring crops, later-drilled crops, such as those planted between the beginning and the middle of April, can also become a target for BYDV-carrying aphids.

This is because they are at a susceptible growth stage when overwintered eggs of the bird cherry oat aphid hatch and move into new crops.

Fortunately, the winter host of this species does not hold the virus, meaning this generation can be infection-free and generally presents a much lower risk to crops. In contrast, the hardier grain aphid can survive all year on grasses and remain a source of BYDV infection, says Mr Bastard.

Aphid flight picks up when temperatures rise above 11degC. Therefore, rapidly rising spring temperatures mean aphid populations can quickly increase and there is a risk of migrating individuals picking up BYDV infection from other crops and spreading it to new plantings, he adds. However, early drilled spring crops are likely to get through the BYDV high-risk period.

“This is because the risk to the plant is reduced when it reaches growth stage 31. However, if crop development is hampered by severe dry weather or other
growth restriction it can be susceptible to aphid damage as we get into the later spring,” he says.

Crop health

When it comes to limiting the potential damage of virusspreading aphids in spring cereals, crop health is of the utmost importance.

“Drilling early is key, as is making sure the crop has nothing holding it back. Adequate nutrition is crucial to helping crops get through the growth stages quick enough for BYDV not to be a risk.”

Mr Bastard advises growers to ensure seedbeds receive optimum nutrition, with any deficiencies of micronutrients, such as manganese and zinc, rectified.

“Manganese is an important element that helps cereals build up their dry matter. Zinc aids the plants’ growth and development and also boosts their resilience
against diseases,” he says.

“Those with fields in high-risk areas should be drilling at optimum seed rates, with good quality seed stock that has a good thousand grain weight and germination percentage. This way, growers can rest assured they’ve done all they can to raise a healthy crop.”

As the weather starts to warm up, growers should be mindful of when aphid populations are likely to increase. If aphid numbers in the field are significant at the 2-3 leaf stage of spring-planted cereals and the risk of BYDV infection is high, then a well-timed application of a plant protection product may be required.

Mr Bastard adds that the broad-spectrum insecticide Karis (lambda-cyhalothrin) can be used for the control of aphids on spring wheat, barley and oats. 

However, populations of beneficial insects, such as lacewing and ladybird, will also be rapidly developing in warming spring weather and their population should be assessed as part of an integrated pest management-based decision, he says.

“Cereal growers need to keep a close eye on their crops for signs of BYDV infection and do everything they can to ensure newly-planted crops get the best possible start.

“If BYDV damage is evident in winter crops, they should consult with their agronomist, who will help them make an informed decision on how best to treat the crop going forward.”