28 May 1999

Mildew breakdown fear premature

SUGGESTIONS that the mlo mildew resistance gene in spring barleys like Chariot and Derkado is failing (Arable, May 21) are premature, says NIAB plant pathologist John Clarkson.

"It is possible there may have been a small shift in the mildew population towards increased virulence to some mlo cultivars, but it is too early to talk of breakdown of what, so far, is a very effective, durable, mildew resistance. Use of fungicides to control mildew in mlo varieties should not normally be necessary. The main advantage of such good field resistance is that farmers can make some savings in sprays. Spraying is not likely to be a particularly effective defence of the resistance in any case," he adds.

Increased incidence of mildew in mlo variety Riviera last season and again this year, prompted the suggestions. But these outbreaks cannot be consistently reproduced on other mlo cultivars and Mr Clarkson reassures growers that the UK Cereal Pathogen Virulence Survey is monitoring mildew populations closely.

One possible explanation of any increase in mildew incidence in mlo varieties could be a temporary breakdown following a crops relief from water stress. That has been noted by SCRI researchers. &#42