BALLERINA OFF BALANCE
With new rhizomania outbreaks now confirmed, we report on a disappointing debut for a varietal counter to the disease.
EARLY promise of relief from rhizomania by growing the first recommended variety with in-bred resistance has faded with confirmation of outbreaks in the newcomer.
By early September, two of this years 14 confirmed new outbreaks of the devastating disease involved Ballerina. This is the first time MAFF has allowed the partially resistant variety on farms which previously had outbreaks, although the beet cannot be grown on fields known to be infected.
In this first year of commercial use, the first NIAB-recommended, rhizomania-resistant sugar beet variety doesnt seem to be coping as well on this side of the English Channel as on the Continent. However, further varieties with beefed-up ability to resist virus build-up are being tested for launch in 2000.
"After trials on infected Dutch fields we were expecting great things from Ballerina so it is disappointing to discover it could be risky growing it here," says Dr Mike Asher, of IACR Brooms Barn at Higham in Suffolk. "We felt it would be safe to grow it on fields with low infection levels. But, after hearing that two of the new outbreaks are in crops of Ballerina, growers may think twice before they grow it.
"Although varieties with similar resistance are performing well on the Continent, conditions here are very different and the UK now looks to be a tough testing ground for new resistant types."
All 87 outbreaks prior to this season, and all the new ones confirmed by early September – up to 30 in total are expected – have been on light land farms. Lack of adequate soil moisture increases the damage caused by the virus which needs warmth and moisture. In Holland, the disease mainly affects beet on marine silts in the polders and reclaimed delta areas.
The UK crop is concentrated on lighter soils. Dr Asher is expecting many more cases this year, compared with just 10 new outbreaks in 1997. He measures temperature at 5cm depth, the level at which rootlets develop, and is able to relate accumulated day degrees over 10íC in April, May and June to disease confirmed in early autumn.
Trials in the UK in 1997 had already indicated Ballerinas yield in lightly infected fields would not match those achieved on the Continent. Yield of the susceptible variety Roberta was slashed by 80% but Ballerina also returned 24% less yield than a healthy crop.
"This came as a big surprise to us as Continental trials suggested the reduction would be about 5%," says Dr Asher. "We are now convinced the reason for the extra loss here is because our beet-growing soils where rhizomania occurs are different. The virus nips off the tap root and, to compensate, the plant develops the characteristic bearding.
"By introducing the virus resistance, breeders have helped an infected crop retain root shape, but they are still much smaller than healthy ones, so shallower rooting. If sandy soils suffer any lack of moisture the crop is hit with a double whammy and is not able to provide an equivalent yield."