22 November 1996

Forecast says light leaf spot risk highest in north

LIGHT leaf spot sprays on winter oilseed rape are much more likely to be needed in northern England than in the east and south east this season, according to a new forecasting scheme.

Crops in the West Midlands, the south west and Wales are at moderate risk from the fungus.

Based on previous incidence of the disease, location, variety and sowing date, the HGCA-funded scheme estimates that in the east an average of only 7% plants will be infected by early stem extension next March. In the north the figure is 30%.

Much depends on disease carryover, explains Bruce Fitt of IACR-Rothamsted after nine months of the four-year joint ADAS, CSL and SAC project.

"If light leaf spot was a problem in your area last year, it is likely to remain so this season unless fungicides kept stems and pods clean." He advises regular inspection from late November to detect the disease.

Where crops have already been treated against phoma, further anti-light leaf spot sprays are unlikely to be needed until the new year, says Dr Fitt. "The fungicides used against phoma are not absolutely the best against light leaf spot, but they do have some activity. So if you have already sprayed look again after Christmas.

"The take-home message is that in the east growers crop protection strategies should be determined by phoma, and in the north by light leaf spot."

Richard Elsdon of United Oilseeds welcomes the new prediction service. But whichever disease is the target, an inspection at the same time for cabbage stem flea beetle larvae is a must, he warns.

"Look for them on the leaf petioles and if they reach the threshold of three to five a plant, add a synthetic pyrethroid. Otherwise you are storing up a problem for the spring." &#42

Light leaf spot – main driver of fungicide programmes in the north.