Current rhizomania-tolerant sugar beet varieties will almost inevitably be overcome by the virus eventually, a leading researcher told delegates at a joint Rothamsted Research Association/Applied Research Forum workshop.

But future varieties could be bred with resistance to the soil-borne fungus, Polymyxa betae, that transmits the viral infection, if promising research can be built on.

Tolerance in current varieties was based on one single dominant gene, Broom's Barn's Mike Asher said.

"There are concerns about its durability."

Over 2m hectares of rhizo-resistant sugar beet in Europe are grown based on this gene.

"In the UK we are going to see a rapid increase in its use.

Seed orders for next year indicate a 20% market share, double that of this year."

But in Holland and the USA there is some evidence of the resistance being challenged, particularly on land where high levels of inoculum had built up.

"It is almost inevitable it will be overcome at some point in the future, we just don't know when, so we need to think about finding another approach to resistance."

Other "resistant" genes have been identified, he said.

"One is slightly better than the existing gene, but it is on the same chromosome, so you cannot have both together and durability will still be an issue."

But a gene has been isolated that could be bred into sugar beet varieties to make them resistant to Polymyxa betae, he said.

"Preliminary evidence suggests there is some resistance to the virus conferred from plants with Polymxya resistance gene."

A DEFRA Sustainable Arable LINK project with BBRO and BBSRC was investigating its potential, he said.

"Within a year we should know whether we have managed to confer virus resistance, and to what extent, and whether it is simple genetically."

The latter would be easier for breeders to use the resistance in varieties, he noted.

"If we could add it to existing resistant varieties it would help protect and potentially increase their effectiveness."

mike.abram@rbi.co.uk