28 May 1999

KEEPINGVIRUSATBAY

DONT rely on later sowing dates or higher seed rates to reduce the impact of barley mosaic virus, advises Richard Overthrow of Arable Research Centres.

Results from a three-year HGCA-funded trial conducted by ARC, which will be revealed at Cereals 99, confirm that growing a resistant variety is the best way to combat the effects of both barley yellow mosaic virus (BaYMV) and barley mild mosaic virus (BaMMV).

"Although delayed drilling can reduce infection and alleviate the symptoms, the weather conditions at the time also make a difference," says Mr Overthrow. "A good, mild autumn favours the virus as well as the barley, while a bad autumn means that barley yields are depressed.

"So sowing date only has limited scope and shouldnt be relied on at infected sites."

Boost seed rate

Using a higher seed rate to offset any crop damage failed to show any response in the trials, continues Mr Overthrow. "If infection is widespread, the only option is to grow a resistant variety."

He adds that every county in England is known to be affected. "It is also found in Scotland and Wales, so no barley grower can afford to be complacent. Yields can be halved by the virus."

Although it starts as a small patch within a field, barley mosaic virus usually spreads quickly. "It is spread by soil movement, so cultivation equipment, tractor tyres, birds and dogs are just some of the ways that it gets transported around the farm.

Wont go away

"And once there, it doesnt go away. Work has shown that a 20 year break makes no difference."

Mr Overthrow admits that barley mosaic virus is easy to confuse with other things. "Most commonly, it is diagnosed as a manganese deficiency. But once there has been no response to a foliar spray, growers tend to recognise it."

Symptoms appear after a cold snap, he advises. "That might be as early as December but it is usually in late January or early February. A pale green mottling, which is expressed most strongly on the youngest leaves, is the first sign."

For confirmation, samples need to be sent away to a laboratory. "Once you know what you are dealing with, the right variety can be selected. Both mild mosaic virus and yellow mosaic virus can co-exist in a field, but that is not common."

Around half of the current barley varieties are resistant, points out Mr Overthrow. "But their performance does vary, especially under high infection pressure. And even susceptible varieties can be influenced differently by the type of virus present.

"Fanfare, for example, was badly affected by mild mosaic virus but performed well on land infected with the yellow mosaic virus. Epic, a resistant variety, did well at the mild location but not in the yellow mosaic virus trials."

New race seen

He concludes by warning growers that another race has recently been detected. Race 2, a breakdown race, is believed to have come from barley yellow mosaic virus. "Theres currently no resistance to Race 2 in varieties, so think twice about growing barley on soil found to be infected with it." &#42

Cereal mosaic viruses have

been thrust back into the

limelight with the discovery

of the first ever wheat

mosaic virus in the UK in

Wilts earlier this month.

Here we examine current

understanding on this

troublesome, yet often over-

looked, group of diseases