27 October 1995

Bad timing squanders cash

when spraying for septoria

By Robert Harris

NEW evidence suggests that spraying fungicides to control septoria is often a waste of time and money.

Many applications are poorly timed or unnecessary. To counter that, more accurate prediction of disease development is needed, to allow better spray timing.

Fungicide use is high, IACR-Long Ashtons David Royle told delegates at a recent Arable Research Institute Association meeting. About 95% of all winter wheat crops have been treated for septoria since the mid-1980s. Despite that, there have been severe attacks in some years. "That suggests we are not using fungicides in the way we should."

Long term forecasting would be useful, thought Dr Royle. Research suggests the number of days when the temperature falls below freezing between mid-November and early December "correlates strongly" to the amount of disease occurring on leaf 2 later in the season.

That could provide an early warning which would be fine-tuned using more accurate short-term predictions.

Those would assess the ease with which inoculum can spread up from the base of the crop during the growing season.

Several factors affect the speed and severity of that spread, he explained. Rain splash encourages disease, but raindrop penetration depends on crop cover. Sowing date, variety and nitrogen usage all affect this, said Dr Royle.

Plant structure is important too. In varieties with short internodes the flag leaf may rub against the tips of older leaves as it emerges. If these older leaves are infected, they will transfer inoculum directly to the new leaf.

The time leaves emerge can also determine severity of attack. The disease needs a three week latent period before it develops, so there are only two or three infection cycles in the life of a leaf.

If this happens soon after emergence, a further attack three weeks later will allow disease to take off. Later developing leaves may miss this first strike, and the second attack will come too late to cause severe loss.

"We can identify risk factors easily, but they interact in complex ways. Each crop needs to be carefully assessed. Fungicide choice, rate and timing can then be tailored to precise need," said Dr Royle.