Scots face up to a more normal disease attack
IT is all too easy to forget that Britains location rarely provides uniform growing conditions.
So says Stuart Wale of the SAC, Aberdeen. So though much of the south east had a dry, relatively disease-free 1995, rainfall and pressure from fungi was much nearer "normal" in Scotland. "But with plenty of hot, sunny days yields were generally excellent."
Uncertainty over which diseases are likely to cause trouble reinforces the importance of variety choice and crop monitoring, says Dr Wale. "Last year in winter barley, a cool wet spring set rhynchosporium going early. As temperatures rose, mildew became more important. On the whole, though, control was not too difficult with well timed fungicides."
Growers of mildew-prone spring barleys Prisma and Blenheim faced their "usual battle" against summer mildew.
But the expected rhynchosporium on susceptible varieties like Chariot and Derkado failed to develop. "Fungicides, where used, were wasted," comments Dr Wale.
With the wheat area still dominated by Riband, Septoria tritici threatened most crops. "The rainfall pattern meant that often each of the top three leaves was challenged as it emerged and timely fungicide treatment was vital.
"In one of our trials where the disease affected 13.5% of the top three leaves of the untreated control by July 5, a programme applying quarter doses of Impact Excel (chlorothalonil + flutriafol) as each of those leaves emerged boosted yield by 2.55t/ha."
Dr Wale believes that Scottish growers have made few variety changes for this season. "Pastoral and Manitou still dominate the winter barleys. Riband and Hunter are mainstays of the wheat acreage, and Prisma and Chariot will be the top two spring barleys. With each variety the risk of disease-induced yield loss is high." Weather largely dictates the outcome, he says. "The diseases we find in winter crops in early spring may not be those that are important later. But you cannot afford to ignore current disease."
In winter barley mildew is fairly well established, but the mild autumn also encouraged rhynchosporium, notes Dr Wale. "Time and again early spring (mid to late tillering) control of disease, especially rhynchosporium, has proved worthwhile.
"Lowish doses applied at this early timing are generally enough to check disease progress and make treatments at stem extension (GS31) and at flag leaf more cost effective."
"Unusually" mildew has been evident on many Scottish wheats since Christmas, he notes. "It is a difficult disease to control and some growers have been keen to rush in with fungicide. But, as the 1994/95 season in East Anglia showed, things can change. Old leaves can die off and reduce the disease risk.
March mildew action
"If by the end of March mildew is still obvious, then some action to suppress it may be worthwhile with the first chormequat split.
"As far as septoria is concerned, watch the weather from the first node stage onwards," he advises. In wet springs, spraying each of Ribands top three leaves as they come out can be cost effective, as the trials show. "But for many this requires a level of management hard to achieve. Relying on a standard three-spray programme can still be cost effective, but careful monitoring of crop and weather is needed for the best effect."
For spring barley, HGCA work found spraying should be driven by the level of disease present, says Dr Wale. "It showed the optimum time to treat mildew is when 75% of plants are infected. With rhynchosporium the decision is harder because of its patchy nature. Initially a 25% threshold was recommended. In practice 5-10% plants infected is more realistic." *
• Few variety changes – dont ignore current disease.
• Early rhynchosporium control well worthwhile.
• Treat wheat mildew if still obvious at end of March.
• If wet consider protecting each of top three wheat leaves against Septoria tritici.
• Treat mildew in spring barley when 75% of plants infected.
Left: Northern growers faced much more "normal" disease pressure last year than those in the south. But tighter crop monitoring could have avoided unnecessary sprays. Above: Early control of rynchosporium has paid off "time and again", says Stuart Wale.