2 November 2001

Extra virus threat to yields

By Andrew Blake

YIELD-SLASHING aphid-borne barley yellow dwarf virus (BYDV), is set to explode after the mildest October for over 300 years.

Early unsprayed cereal sowings and those with Secur (imidacloprid) seed treatments running out of steam are most at risk. Much depends on growers ability to control the virus-bearing pests soon, say experts.

The Met Office says this October has been the warmest since records began in 1659.

BYDV, spread through fields as infected aphids multiply in warm weather, can eliminate all yield in affected patches, says the Arable Research Centres Richard Overthrow. The aim must be to stop the patches joining to become tennis-court sized or worse, he advises. "In the past I have seen crops completely wiped out by BYDV.

"Soil temperatures during October at Cirencester were 2.5C above average so there are more crops this year at high risk."

"A mild October is always bad news when it comes to BYDV," says ADAS entomologist Jon Oakley. "Where growers have been unable to spray, aphids have done terribly well and even crops sown in October have been infected quite quickly.

"It is probably too soon to panic. Even at high temperatures it takes six weeks for all plants to be infected. But growers need to take advantage of any dry spells to spray as soon as possible."

The threat in some cases is so high that it may be worth separating aphicide and herbicide treatments, says Bayer technical support manager, David Ames. "The correct timing for herbicides is often a bit arbitrary for BYDV control."

With few winter cereals sown last season beyond September and disease levels in spring very low BYDV may not be at the forefront of growers minds this autumn, he believes.

Mr Overthrow agrees. "People are telling us they cant see any aphids. But it is always difficult to find them, and we are saying do not cut back on your sprays."

In the mild south-west ADASs Bill Butler is especially concerned for crops after grassland ripped up because of foot-and-mouth.

Unless properly controlled with an insecticide or through sward desiccation before being ploughed down, buried aphids bearing BYDV can wreak havoc in newly germinating cereals, he warns.

Unlike those flying into crops, of which at most about 10% carry the virus, the buried pests are nearly always 100% infective. Even where wheats are intended for whole-crop feeding, at least two aphicide sprays will be required, he says. &#42

No aphicide required as contractor Brian Porter treats weeds in Secur-treated wheat at Sentry Farmings Burrows Hill Farm, Chatteris, Cambs. But a follow-up BYDV spray will be needed before long, says foreman Trevor Scott.

Defence warning

Bayer estimates that 15-16% of UK cereals sown in September received Secur treatment offering up to six weeks protection against aphids. But dilution of the chemical in larger plants from early low seed rate sowings means that defence could have gone within three or four weeks, warns Mr Ames.

Because of its upward movement in plants Secur also offers little protection against buried aphids attacking cereal roots, he says.

VIRUS THREAT

&#8226 Warmth encouraging aphids.

&#8226 Wet land restricting spraying.

&#8226 Seed treatment effect wearing off.

&#8226 Untreated crops especially at risk.