More mosaic than feared?
By Andrew Blake
SOIL-BORNE mosaic viruses which can slash winter barley yields by 25% or more could be more widespread than many growers believe, according to a Berks-based crop consultant.
Since most other control options are impractical, a good choice of tolerant and resistant varieties is desirable, warns Association of Independent Crop Consultants member Laurence Sim. Only a quarter of currently recommended winter barleys are tolerant or resistant, and two of those are becoming outclassed, he notes.
An HGCA aerial survey in 1988 suggested Barley Yellow Mosaic Virus and Barley Mild Mosaic Virus infected about 13% of Cotswolds barley fields. However, a recent review of his own clients fields within Oxon, Berks and N Hants found 19% infected with either BaYMV or BaMMV, says Mr Sim.
"But some fields had not grown barley," he says. "So that figure would have been higher if it had been based solely on barley fields."
The 1988 finding could also have been an underestimate because fields in resistant varieties may have gone undetected, he adds. Yield losses from the disease in Germany have been up to 50%.
"We first recognised mosaic virus in our area in a field next to one of our group members in 1980. The second confirmed case was found within the group in 1981 – about 20 miles away."
Since then nearly five extra fields a year have become infected within the Phoenix Agronomy Groups 12 farms and 387 fields.
"Once on a farm, the disease seems to spread rapidly, especially on heavy land. On some farms all or nearly all the barley fields are infected.
Mosaic disease from either of the viruses, which are indistinguishable in the field, shows up as yellow crop patches in Jan, Feb and March, but only after cold spells. "Youngest leaves display unique elongated light flecking. Sometimes the leaves become rolled and appear spiky," says Mr Sim. "The patches tend to fade in the spring, but the stunting may persist."
The viruses are carried in the spores of a common soil fungus which grows and multiplies on barley roots, he explains. Because the resting spores can persist for at least 15 years without a barley host so too can the virus. "That limits the value of rotational control."
Delayed drilling can reduce the severity of BaMMV and boost the output of susceptible varieties. "But it is not an attractive option because earlier sown resistant types still outyield them."
Specific tests to identify which virus is present would be needed to take advantage of differential tolerance between varieties to BaYMV and BaMMV. "That approach would depend on only one virus being present. In many situations there are mixtures," he adds.
In theory good hygiene, as with rhizomania in sugar beet, should help prevent the viruses getting onto farms in the first place. "In theory too growers should be able to stop them spreading within farms. In practice precautions are not taken. The symptoms are fleeting, not seen in other crops, and so it is often a case of out of sight, out of mind".
The problem can clearly be avoided by dropping the crop altogether, says Mr Sim. "But on many light sandy soils where winter barley is the mainstay, the choice is very limited."
That leaves tolerant or resistant varieties as the best alternatives. Deciding when to introduce them requires only a simple calculation, he suggests.
"Assuming a yield differential between resistant and susceptible varieties of 4% and a yield loss in infected patches of 30%, the `break even area is 13%. In other words once you have more than 13% of a field infected you should switch types."
BARLEY MOSAIC DISEASE
* More common than realised.
* Yields trimmed severely.
* Few practical controls.
* Infected area rising 1.2%/yr?
* More resistant varieties needed.
Barley mosaic viruses could be more widespread than growers realise and finding varieties that resist yield loss is becoming more tricky, says independent adviser, Laurence Sim, seen here with local grower, Tony Carter of Millets Farm, Garford, Oxon, pondering current options.
• More common than realised.
• Yields trimmed severely.
• Few practical controls.
• Infected area rising 1.2%/yr?
• More resistant varieties needed.