Strob dose set to drop

APPROPRIATE DOSES of strobilurin fungicides in winter wheat are likely to fall again next season, or be more targeted to specific situations.

But a leading manufacturer insists yield responses require at least a half dose when used at T1 or T2.

Appropriate dose trials show strobilurin performance against Septoria tritici has declined over the two seasons since resistance was detected, says ADAS plant pathologist Neil Paveley. “The impact is to reduce the appropriate dose. There is no point in putting rates up attempting to control uncontrollable disease.

“Initial analysis of yield data suggests a quarter dose, maybe once or twice, will be the appropriate dose of strobilurins next season,” he says.

That is not the finding from Syngenta trials, stresses the firm’s Rod Burke. “To see the yield response you would expect from strobilurins you need a minimum half dose in a single application or 80% dose [0.8 litres/ha Amistar] over the whole season.” Yield responses from single applications are around 0.4t/ha, he says.

Strobilurin will be more targeted next season, suspects ProCam technical director David Ellerton. “But identifying those situations where there will be a response needs more analysis of last season’s results.”

Using strobilurins against other diseases, such as take-all, eyespot and rusts, or for physiological responses could justify higher doses, he adds, although he expects strobilurin dose to decrease in general. “The economics are crucial. You have to justify every single situation.”

In contrast, doses of triazole fungicides are likely to increase next season, says Dr Paveley, despite a gradual, if disputed, shift in efficacy. “A full label dose in appropriate dose trials in 2003 struggled to reach 80% control.” Between 1995 and 1997 a quarter dose achieved the same control. “The good news in 2004 is there no is evidence of an additional significant shift.”

But unlike strobilurins, the effect of a gradual decrease in triazole effectiveness should be to increase dose, he stresses. “It is still possible to control disease with [higher rates of] triazoles.”