15 March 2002

P-strain erodes variety tolerance

TWO cases of the more aggressive P-strain of rhizomania have been identified in UK grown sugar beet raising questions about whether current tolerant varieties will be able to cope with the presence of the disease.

DEFRA screening of roots taken from new rhizomania outbreaks revealed two out of 68 sites identified last autumn to have the P-strain, previously unknown in the UK.

"It was picked up at two sites near Norwich, about 10 miles apart," says IACR-Brooms Barn plant pathologist Mike Asher.

"This strain is more aggressive and pathogenic. It decreases yield and increases virus content a little more than the normal strains of the disease."

Outside of the UK the P-strain has only been identified in Japan, China, Kazakhstan and around Pithiviers in the Loire region of France, after which the strain is named.

Yields of varieties tolerant to the more common A and B strains of rhizomania, such as those listed in the UK could be slashed by the P-strain.

"Tolerance seems to have been significantly eroded," acknowledges Dr Asher. "The key question is how badly."

Another resistance line being developed by breeders seems able to cope with the P-strain of virus but only one such variety is available so far in Europe and it is not suitable for UK conditions, he says.

The existence of the new strain in the UK was announced to an industry meeting of British Sugar, NFU and key growers in October by DEFRA plant health officials.

DEFRA maintains its decision not to defend the UKs rhizomania free zone status had already been made and this new strain of disease had little bearing on that. &#42

&#8226 More aggressive than common A and B strains.

&#8226 Two outbreaks near Norwich.

&#8226 Only identified in UK and France within EU.

&#8226 Overcomes current variety tolerances.