ALERT FOR RAMULARIA
Will spring barley growers be better forewarned about ramularia this season? David Millar reports from the Dundee Crop Protection Conference.
A FEW might be prepared to put their yield losses last year down to sunburn, too much ultraviolet light, or unidentified stresses, but many growers seem certain to include ramularia as a target in their spray programmes.
The jury may be out for some time on the wilder theories about the brown spotting that showed up in 1998s wet season but consultants north of the Border admit that growers with crops at risk are likely to take the safest option of a strobilurin protectant against ramularia.
"It all depends what the weather is going to do," says Dr Keith Dawson of CSC CropCare. "We are telling people to apply their T1 spray as normal and then consider what diseases should be targeted with a later spray. If it is dry then I dont think ramularia will be a problem."
However, he admits many growers are adamant they wont take the risk of waiting and will be using either azoxystrobin or kresoxim-methyl to catch this late developing foliar disease first positively identified in Scotland and later Ireland in 1998.
Stuart Wale, of the Scottish Agricultural College, is unconvinced that ramularia is the total answer to unexplained crop losses but SAC has geared up to give early warning of the disease. Samples from fields in its Adopt-a-Crop scheme are being monitored with laboratory tests for ramularia infection.
If it is present again in 1999, Dr Wale hopes it can be picked up by early stem extension in susceptible varieties such as Chariot or Optic which account for a substantial part of the Scottish malting barley acreage. Trials show that strobilurins have an effect on ramularia so a spray application at the flag leaf emergence to early boot stages should be effective.
The Dundee Conference also heard from Dr Peter Mercer of the Department of Agriculture in Northern Ireland that growers can continue to use reduced doses of sprays against powdery mildew on varieties with in-built natural resistance to the fungus.
However, he warned against trying to eliminate the mildew spray completely. The resistant variety Riviera showed more mildew than expected last year, and responded to fungicide slightly more than the similarly resistant Chariot. Both varieties owe their resistance to the mlo gene.
"Is the mlo gene on its way out?" asked Dr Mercer. "I hope not; I think it is just a blip but I am not too sure."
In two years of trials, firstly with Chad and Chariot, and then with Felicie and Riviera, he demonstrated that variety had a significant effect on fungicide requirement. The effect was so pronounced that untreated Chariot outyielded Chad receiving a full dose of fungicide in 1997. However, there was little difference anyway between a full rate and half-rate fungicide applied to the Chad.
In both years a mixture of tebuconazole and triadimenol proved to be the most effective fungicide, followed by azoxystrobin. Yields in 1998 were significantly higher with these two fungicides than with the tridemorph also used.
Dr Stuart Wale, of SAC, warned that in winter barley at least growers and advisers should be aware of the significance of NIAB ratings when choosing varieties on the strength of their disease resistance. The rating system was such that the difference in rhynchosporium susceptibility between a variety rated at three by NIAB and one with a rating of four was much greater than the difference between varieties rated at six and seven, respectively.
The difference between a variety rated at five and another at four could be worth a quarter of a dose of fungicide.