Friday, December 6, 2019

Three-point checklist follows heightened BYDV risks

November 6, 2019 by  
Filed under Crops

Rising aphid numbers this autumn prompted agronomy firm ProCam to compile a three-point checklist of barley yellow dwarf virus (BYDV) risk factors for growers managing the problem in the absence of neonicotinoid seed treatments.

ProCam regional technical manager Paul Gruber said all cereal crops – not just winter barley – were at risk from the virus following aphid activity during late September and into October. Growers should keep an eye on crops, he added.

“Previously, when a neonicotinoid seed treatment could be used, it provided about 6-8 weeks protection of emerging cereal plants against aphids. Without this safety margin, it has become even more important to be vigilant of the sources of BYDV risk.”

Unexpected sources

Aphid migration from grassweeds, volunteer cereals or grass leys into newly-planted cereal crops –  either in the same field or nearby fields – is a prime source of BYDV infection. But growers should also be aware of other infection sources.

These include grassy margins and buffer strips around field boundaries, which will provide a haven for aphids right next to unprotected, newly-emerging cereals. Increased amounts of spring crops introduced into rotations to combat blackgrass will increase the risk too.

“Their late harvest shortens the interval that aphids need to survive between crops. Increased amounts of second wheat in rotations where oilseed rape has fallen out of favour will have a similar effect,” said Mr Gruber.

“Remember also that aphids can transfer to the new crop through the soil, so if ploughing to bury green bridge material then 4-6 weeks should elapse before drilling the next crop to allow enough time for them to die underground.

“Ideally, green stubbles should be killed off fully with glyphosate.”

Depending on location, crops planted after mid-October were once considered at lower risk of BYDV infection. This is because November frosts checked aphid populations. But recent warmer autumns means this hasn’t always been the case.

“We’ve seen that even with a relatively low number of aphids, if it stays mild through the winter, the population keeps ticking over and continues to spread the virus within the crop.”

A temperature of -1.5C will reduce bird cherry aphid numbers, while several days at -5C are needed to take out grain aphids. Similarly, although heavy rain can wash aphids off cereal plants, this is only temporary. They can quickly recolonise.

Misjudged spray timings

With chemical control of grass weeds now so heavily focused on pre-emergence herbicide applications, it has become easy to overlook the importance of correct post-emergence spray timings to treat aphids, warns Mr Gruber.

While this may have been less of an issue when crops had neonicotinoid seed treatment protection, BYDV management will now be much more reliant on precise aphicide spray timing, he points out.

“Correct aphicide timing will be crucial to control the second generation of aphids before it has chance to spread the virus through the crop,” says Mr Gruber.

“Use a T-sum tool – which utilises accumulated daily temperatures to predict the emergence of this second generation – to help with timings. However, there is also no substitute for walking crops regularly to monitor aphids.”

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