27 November 1998

Pollen beetle control is vital for spring OSR

DESPITE spring oilseed rapes marginal profitability, its treatment against very low infestations of pollen beetle in the north will pay off. But farmers can do nothing to combat the "new" disease, white blister, apart from growing the crop less often.

That is the advice of entomologist Collette Coll and plant pathologist Kerene Sutherland, both of SAC Aberdeen.

"Its essential to keep inputs for the crop as low as possible, but you cant afford to skimp on pollen beetle control," insists Dr Coll. "This pest is more damaging than we thought, so its spray threshold has been reduced from three to one per plant, compared with 15 for winter oilseed rape."

Timing of the pyrethroid spray is also very important, she stresses. Monitoring of the crop must start as soon as the first green flower buds are detectable, usually when the crop has only three or four leaves. Treatment when there is an average of just one beetle/plant is cost-effective, saving around £30/ha (£12/acre) after costs.

"Beetle numbers can go from one to lots very quickly, hence the need for close monitoring."

White blister, a common disease of ware brassicas, first appeared in the crop only two or three years ago. Although spreading, it is still fairly localised, and severely depressed yields of some crops in 1997, reports Dr Sutherland. She believes infection jumped to the crop from brassica weeds like shepherds purse.

"The disease was not much of a problem this year. That was fortunate because theres no approved fungicide for its control," she explains. "We trialled metalaxyl, which is used on ware brassicas, but it was ineffective. The only answer is to grow the crop less frequently."