Use strobs to combat mystery leaf spot,
By Shelley Wright
WHY leaf spotting occurs on spring barley in Scotland and the north is still unknown and exact yield losses are hard to quantify.
But the damage has been so severe on some crops that scientists are advising a strobilurin plus triazole spray before symptoms are seen this season.
"Optimum spraying time to control leaf spot is at growth stages 39-45," says SAC senior cereal pathologist Simon Oxley.
But the spots and loss of green leaf area appear only after heading. "In some varieties you get severe necrosis as well as the spots. It is usually worst on leaf two."
Triazoles alone seem to have little impact but all three strobilurins give an effective reduction in spots and protect green leaf area. But cutting back early applications in anticipation of that later expense would be a false economy, he warns.
Diseases such as mildew and rhynchosporium still need tackling, but, unfortunately, protection from T1 does not last to leaf spot season, he says.
"We do not know for sure if we will see the same degree of leaf spot this year but I would say that anyone who had a problem last year should not take any risks and should spray again. The protectant element is everything with this disease."
Once spots have appeared, there seems little that can be done to get rid of them, but a strobilurin fungicide at that stage still helps preserve remaining green leaf area, he notes.
Dr Oxleys work is part of a three-year research programme on leaf spot in conjunction with scientists in Austria, Germany, Ireland and Norway where the disease has also emerged in recent years.
Theories about the cause vary from it being ramularia alone, to a combination of ramularia and environmental and physiological factors.
"We know that sunlight has an effect, with trials last year showing that crops exposed to the sun suffered more leaf necrosis than those in the shade," he says.
Chariot has been hardest hit, with lots of spots and dramatic loss of green leaf area leading to yield loss and higher screenings. Other varieties, such as Optic, showed many spots, but the plants stayed much greener. Initial results of variety trials suggest none is resistant to leaf spot.
The fact that malting varieties seem worst hit implies lower nitrogen applications could be playing a role. Also the disease seems confined to the wetter areas of Europe.
Such weather influences need resolving. "It may be a reaction to certain weather conditions, although I do not think we are looking solely at sun-scorch, more a prolonged run of wet weather followed by sun," says Dr Oxley.
It is not clear if the weather effects are due to changes on the ramularia fungus or on the plant itself, he adds. *
LEAF SPOT TIPS
• Use strob + triazole at GS39-45.
• Base spray decision on previous incidence.
• Chariot worst hit.
• Weather and nitrogen factors?
SAC strob + triazole results
Variety: Prisma Optic Chariot
% spots yield % spots yield % spots yield
Untreated 55 5.73 38 4.82 63 5.43
Non-Strob 36 7.32 36 7.08 56 6.87
Strobilurin 13 7.88 33 7.66 55 7.26
Source: SAC trials. % Spots = ramularia and other speckles on leaf 2, Jul 27 1999.
Strobilurin plus triazole should be applied before tell-tale leaf spots start to appear on spring barley (inset), says SACs Simon Oxley.