beware blotch

3 March 2000

Hit rhyncho early but beware blotch

beware blotch

Rhynchosporium and net

blotch can devastate barley

yields, especially in the

south-west. John Burns

reports ADAS consultant

Bill Butlers approach in Devon and Cornwall

BARLEY growers should be ready to react to rhynchosporium before GS31, but net blotch can generally wait for traditional timings, says a leading agronomist.

"Until last year rhynchosporium was consistently our most damaging disease on barley," says Devon and Cornwall-based ADAS arable consultant Bill Butler. "Then last year it didnt appear and we still dont know why. But it would be risky to assume it will never appear again."

In areas prone to warm wet winters, rhynchosporium often starts building up before GS31, he warns. Failure to nip it in the bud before the T1 timing can result in significant yield loss, he warns.

"It appears in circular patches that go yellow. The leaves die and the crop thins out. From a distance these circles could be mistaken for BYDV infection."

Opus (epoxiconazole) at 0.25 litres/ha should be used to keep it in check, he suggests.

At GS31 on most barley in his area Mr Butler recommends 0.5 litres/ha of Opus alone, unless rhynchosporium is severe or there are signs of mildew or brown rust. Then 0.5 litres/ha of fenpropimorph (as in Corbel or Mistral) will be added.

If rhynchosporium is threatening at awn emergence a further 0.5 litres/ha of Opus will be used.

Although early rhynchosporium infection is unusual in drier or colder parts of the country, Mr Butler recommends walking crops regularly to check for infection.

Net blotch tends to follow cold wet winters and merits a different approach, notes Mr Butler. Early infection is ignored. "I have never seen it bad enough to kill leaves and I would wait until the standard GS31 timing with 0.5 litres/ha Opus plus 0.5 litres/ha Corbel or Mistral."

But net blotch can be devastating later in the season, burning off flag leaves and even killing the awns. "It happens very quickly. It can take the top off a crop over a few days," explains Mr Butler.

From awn emergence and later, so long as the crop is still green, Mr Butler advises hitting net blotch hard at the first sign of the disease with a minimum of 0.75 litres/ha of Opus, or even 1.0 litre/ha if the net blotch is moving very fast. Unix (cyprodinil) is also very good, he adds.

"So a mix of Opus and Unix might well be the bees knees."

Strobilurin Amistar (azoxystrobin) is known to be good against both rhynchosporium and net blotch, but it is not a knockdown treatment for established disease, he stresses. "You have to catch the infection very early."

Adding Opus would make it ideal for late net blotch, but at the 0.5-0.6 litre/ha Amistar rate and 0.7 litre/ha of Opus that he considers would be needed, total cost would hit £30/ha.

"At forecast grain prices Im not sure it would be worth doing. The alternative would be an extra 0.25 litre/ha of Opus."

Spring barley is rarely badly hit by net blotch but rhynchosporium can be serious and Mr Butler warns against trying to make savings on fungicides.

"Almost certainly you will need fenpropimorph with the Opus at GS31 because spring barleys are more susceptible to brown rust and mildew. And rhynchosporium spreads so fast in spring barley Id be tempted to get the fungicide on as soon as you are anywhere near GS31, regardless of what disease is there."

Once disease is established yield is lost, and a flag leaf or early awn spray should be considered. "I cant see where any savings could be made over the winter barley programme." &#42


&#8226 Hit rhynchosporium early.

&#8226 Nail net blotch at flag leaf.

&#8226 Opus best triazole.

&#8226 Amistar cost questioned.

Competitive buying

Product price is a key driver in determining any spray decision and part of the agronomists role, Mr Butler maintains. By combining clients orders he reckons to gain more competitive quotes which in turn increases the margin over fungicides, even if some programmes are a compromise to fit in with the bulk order. This season he hopes to pick up Amistar (azoxystrobin) at £30/litre, Opus at £25-30/litre and Corbel at £18-£20/litre, but it will all depend on the order size, he adds.

Dont wait until GS31 with rhynchosporium, says ADASs Bill Butler (left), seen here advising former FW barometer grower in the south-west, Stewart Haylor. Recommendations tend to be based on disease seen in the crop, rather than varietal resistance because of the high disease pressure in the region.

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