19 August 1997

Timing vital for OSRsprays

FIRST spray timing is critical to the success of an oilseed rape disease control programme. Get it wrong and you could lose up to 1t/ha (8cwt/acre).

"Spraying at the right time gives good control and a typical yield response of 1t/ha," ADASs Dr Peter Gladders told a recent DuPont seminar. "Delaying the first application loses most of that response."

Light leaf spot and phoma stem canker are the two main concerns. "Each is responsible for yield losses worth about £40m/year. Poor spray timings often make the problem worse.

The risk of both diseases this season is similar to last year, he said. "All the signs suggest it will be a fairly low risk year in the south and east for light leaf spot, but crops in the north and Scotland will be at a high risk.

"Canker, which tends to be overlooked, could be a threat because late rains increased inoculum. Growers must get out into the field and look for the signs. If its there, spraying must take place within five days."

A classic warning sign for canker is more than 50mm (2in) of rain in September and October, which increases spore release.

Dr David Ellerton of distributor ProCam supported advice to check crops in the field for disease pressure. Light leaf spot symptoms were 6-8 weeks later than usual last year, leading to poor spray timings which explain some of this years poor rape yields, he said.

Timings can be optimised by intensifying field checks once the threat of light leaf spot has been established from ADAS or other advisers warnings, he added.

"Walk the field and pick up some leaves, incubate them overnight in a polythene bag and if spores are obvious, start spraying. If theres nothing, go back to the field in a couple of weeks and check again.

Indeed, more than one fungicide spray may be needed to hit both diseases hard when their peak pressure does not coincide. Three and four-way splits worked well in HGCA-funded trials last year.

"It all depends how epidemics evolve," says Dr Gladders. "The important thing is to take out the early phase of both these diseases. If spraying starts in October, a top-up before Christmas will be required."

Louise Impey

OSR DISEASE CONTROL

&#8226 Precise timing vital.

&#8226 Bad timing can lose 1t/ha.

&#8226 Check for phoma and light leaf spot separately.

&#8226 Match sprays to pressure.

&#8226 Multiple applications?