23 June 1995

Varieties dont change

CHANGES in the susceptibility ratings of oilseed rape varieties to the two main diseases – light leaf spot and stem canker – are not due to a breakdown of resistance.

Claims that a variety "isnt as good as it used to be" cannot be true, stresses PBI Cambridges Mike Bearman. That is because resistance, unlike that to cereal yellow rust, is "multi-gene", he explains. "It is highly unlikely that it will change. You wont suddenly find a variety going down the pan as you might with yellow rust in wheat."

Changes in the NIAB ratings merely reflect the advent of new varieties with better resistance which "reposition" older types in the league table. The onset of higher disease pressure from one year to the next can have the same effect, he adds.