Light leaf spot could be costing oilseed rape growers, even southern ones, more than they think. But a new type of dual resistance could soon help them manage the disease better.

In Recommended List candidate varieties, conventional Imola and hybrid Tuscan, CPB Twyford believes it has a new level of defence.

Despite widespread use of fungicides, UK producers lose 15m-20m a year to light leaf spot, according to Rothamsted’s Neal Evans.

Its cyclic nature makes it hard to predict, and how plants resist it is poorly understood.

“The dry winter has reduced this season’s risk quite dramatically in some areas, but it’s still a problem in individual crops particularly in the north and Scotland.”

An HGCA-funded study shows the fungus is a key reason for farm yields lagging behind new varieties’ potential, adds the SAC’s Elaine Booth.

“Much of the variation in yield of the commercial crop is linked to the presence of light leaf spot.”

The study covered the whole of the UK, not just the north, where the disease tends to be more important than phoma stem canker.

Until now Elan has been the only Recommended List variety meriting an 8 rating for resistance, notes CPBT’s Peter Werner.

But seedling and field tests on the new varieties, incorporating two novel genes from a wild cabbage, suggest Imola and Tuscan deserve ratings of 8.2 and 8.3, respectively, he says.

“We think one gene blocks the infection and the other arrests its development.”

Because two mechanisms are at work the resistance should be hard to overcome, adds Dr Werner.

Seed of both varieties, which have outyielded Winner in northern National List trials, will be available this year primarily through Frontier.

“In my 23 years in the industry I haven’t seen this level of resistance coming through,” says the firm’s David Waite.

andrew.blake@rbi.co.uk