7 June 2002

Mosaic virus poses threat to wheat output

By Andrew Blake

SOIL-BORNE wheat mosaic virus (SBWMV) looks set to slash the yield of many popular varieties in a DEFRA/ADAS trial in Wilts. Three-quarters of them have been hit to a noticeable degree.

Symptoms of the incurable soil-borne disease are so bad that output could be halved in over half of the 15 varieties tested, estimates ADAS pathologist David Lockley. "At the moment there are only a limited number of resistant or tolerant varieties available."

Identified in the US as long ago as 1923, the virus has since turned up in China, Japan and more recently in Europe, first showing in the UK about four years ago.

So far, only four sites – the others are in Kent and the Isle of Wight – have been confirmed as affected. But the government is so concerned that it has organised aerial surveys since 2000 to assess its spread, says Mr Lockley.

As with Barley Yellow Mosaic Virus (BaYMV) the symptoms of SBWMV, which is carried by the same Polymyxa graminis soil fungus, are worse after a cold winter.

Crops on heavy soils lying wet are most likely to be affected. "The fungal zoospores are more mobile then," he explains. "In small plot trials in 1999 we saw a 51% yield loss in Equinox."

First signs in plots on the Wilts heavy clay farm this year, where the surrounding crop is the relatively unaffected variety Aardvark, occurred in February. "We began to see a slight yellowing of the leaves, although they were not as strong as the symptoms of BaYMV in a nearby field."

By March it was clear that some varieties were not tillering properly. "The affected plots look-ed thinner and some of the leaves in the worst cases began to roll."

By stem extension (GS31) in April there were obvious differences in height. "Some were only half the height of others," says Mr Lockley.

By last week most of the mosaic patches in the adjacent barley had disappeared. But the wheat virus was still making considerable marks across the trial, with weeds encroaching on the thinnest stands.

Of the varieties tested only Aardvark, Charger, Claire and Hereward can be considered mosaic-resistant, says Mr Lockley. "Shamrock and Recommended List newcomer Xi19 are showing some symptoms and Deben is a bit marginal.

"All the rest are very susceptible." Savannah seems particularly hard hit with distinct leaf rolling and stunted plants. All the plots will be taken to yield.

As with rhizomania in sugar beet, avoiding unnecessary soil movement is the main current defence against the disease, he suggests. "In theory minimum tillage could help because it moves less soil around."

But in practice only very small amounts of infected soil are needed for new outbreaks to occur, he believes.

With no chemical control options likely, that leaves resistant varieties as the best response should the disease spread.

Soil-borne wheat mosaic virus has had a huge impact on most wheat varieties in DEFRA trials on an infected site in Wilts, as ADAS pathologist David Lockley demonstrates. Symptoms include yellow tramlining and mottling of flag leaves.

Breeders give their view

ALL UK wheat breeders are becoming more aware of the mosaic threat, says Nickersons Bill Angus, who admits Claires resistance is purely fortuitous.

"Until recently its not been an issue in the UK. But its now creeping up peoples agendas."

Much more screening for resistance is being carried out, especially in France, he adds. "One benefit is that unlike other disease resistances there doesnt seem to be a yield penalty associated with incorporating it."

Both the other RL newcomers, Access and Solstice, are mosaic-susceptible, notes NIAB pathologist Rosemary Bayles. One-fifth of material in National List trials is resistant. &#42

&#8226 Notifiable disease first confirmed in UK three years ago.

&#8226 Four sites to date.

&#8226 DEFRA aerial surveys since 2000.

&#8226 75% of RL varieties susceptible.

&#8226 Rising breeder interest.

Savannah 9.7

Tanker 9.3

Equinox 8.3

Option 8.0

Napier 8.0

Malacca 8.0

Consort 8.0

Biscay 7.0

Deben 6.7

Shamrock 5.0

Xi19 4.3

Aardvark 2.3

Claire 1.3

Hereward 1.0

Charger 0.0

Source: ADAS/DEFRA. (0-10, 10 worst symptoms)