RHIZO SLIPS THE
Rhizomania is breaking through the UKs strict control measures. Gilly Johnson puts the case for a re-think on how we tackle this soil-borne beet disease.
BEET growers beware – rhizomania has taken a turn for the worse.Its not the number of new farms with outbreaks discovered this season thats the worry – at eight, the score is not unexpected. More shocking is that rhizomania has now been discovered in fields with no obvious visible symptoms, on farms where strict hygiene precautions have been maintained.
And it has appeared on organic soils for the first time. Until now, rhizomania had been confined to sandy soils.
The problems experienced by Elveden Farms, near Thetford, Suffolk, this season should ring warning bells for all beet growers. They suggest that the disease is likely to be far more widespread than is officially recognised.
Last year MAFFs survey showed a suspect field at Elveden Farms, picked out owing to the distinctive pale patches in the crop. This triggered a more detailed inspection and rhizomania was confirmed on six fields, 100ha (247 acres) in all.
Because the estate was now an outbreak area, this season the Ministrys plant health officials visited all Elveden Farms beet fields, to give the crop a thorough inspection – no mean feat on a 4,000ha (9,884-acre) farm with 1,000ha (2,470 acres) of beet.
The inspectors dug up roots, and checked every corner of every beet crop. Some fields which were clear of visible symptoms and would have passed the aerial inspection, were later found to be infected with rhizomania.
Elvedens farms controller Lindsay Hargreaves is alarmed at the implications. "It begs the question as to how widespread rhizomania really is in the UK. The symptoms in some of our fields were atypical – I would not have known there was anything amiss during the late summer."
By mid-October, and armed with fore-knowledge, the infected fields did become more obvious to Mr Hargreaves, with plants displaying the erect leaf symptom, characteristic of rhizomania, that he would otherwise have mistaken for stress. But root bearding was minimal – and this is what most growers would use as a yardstick.
On taking a knife to root tips, infected beet did show a slight darkening of the central vascular system. However, these small signs of infection would have arrived far too late for many early harvested commercial crops.
Understandably, Mr Hargreaves is having second thoughts about beet policy on the estate. "Last year we had felt reasonably confident about continuing with sugar beet under the containment strategy. As a consequence of events, we drilled 90ha (222 acres) of the rhizomania-tolerant variety Ballerina to lessen the risk of the spread of inoculum. But rhizomania has been confirmed even in the Ballerina this season."
In all, 35 fields totalling 680ha (1,680 acres) have been identified as carrying the disease at Elveden. About 400ha (988 acres) is being destroyed.
Plant pathologist Dr Mike Asher of IACR Brooms Barn cant offer definitive explanations for the sudden explosion in rhizomania foci at Elveden Farms. But armed with expert knowledge of the disease, hes piecing together some theories. Hes ruling out infection spreading from a single point source.
"Judging by the infected fields over the entire farming area of the estate, its conceivable that what were seeing is the legacy of inoculum which arrived perhaps as far back as the 1980s, spread subsequently by wind-blown soils."
The time lag could be due partly to the precautionary tactics taken by Elveden Farms. The estate has operated a strict hygiene policy since rhizomania was first discovered in the UK in the 1980s. Concrete disinfectant wheel dips were installed in 1992, and no contractors have been employed on the beet crop to minimise risk of soil contamination.
"From the beginning, we have actively supported the containment policy," says Mr Hargreaves. "We considered it was in the best interests of the whole industry. But our experience this year shows that growers can no longer rely on containment."
Irrigation is thought to exacerbate rhizomania symptoms; Elveden Farms stopped irrigating beet in the mid-1980s, despite the drought-prone nature of most of the light soils. This season was the first exception, when only the rhizomania-tolerant variety Ballerina was given irrigation. The aim was to boost yields on a low potential site, without increasing the risk. This plan backfired, because Ballerina succumbed.
Under the NFUs rhizomania levy system, Elveden Farms will receive some compensation for crops destroyed. This season growers have paid 1p/t into the fund which is distributed to those with rhizomania outbreaks. British Sugar does not contribute to this scheme. The levy is raised the year following crop destruction; next years bill for this seasons problems could be double, or even treble the current amount.
The UK currently enjoys official status as rhizomania-free, at least until the end of 1999 when this status comes up for review. Other EU member states have criticised the Governments rhizomania containment policy as creating a barrier to trade; it allows the UK to check and police all imports bearing soil at the point of entry. This does offer advantages, because other plant health problems can be monitored at the same time. Being classed as "rhizomania-free" also widens the UKs export potential to other countries.
However, it is widely anticipated that the UKs status will be changed after 1999. Rhizomania has now been officially confirmed on more than 3,800ha (9,160 acres).
"Up to now, the containment policy has worked," says Dr Asher. "The growth in rhizomania has been slow. But we are now reaching the point where an alternative strategy may help growers more."
Comparisons can be made with Holland, where rhizomania has become a common problem. The use of resistant or tolerant varieties is widespread: last season 28% of the crop area was down to resistant beet; this year the figure is expected to be 50%. Dutch growers accept a yield penalty of perhaps 5-7% compared with what they might expect from conventional varieties grown without rhizomania present.
In the UK, Ballerina is the only rhizomania-tolerant variety approved. However, two others – Rosana from Delitzsch and Rebecca from the English Sugar Beet Seed Company come up for NIAB judgement shortly.
All the sugar beet breeding companies have said they are now developing only varieties with rhizomania tolerance for the future. Also under development are varieties with enhanced resistance, which withstand the soil fungus which carries rhizomania, as well as the virus itself.
In the light of this seasons experiences, the NFU and British Sugar are currently exploring possible options for a future rhizomania policy. "We have two main aims," says the NFUs sugar beet negotiator, Matt Twidale. "First, we want to minimise the hurt felt by members whose crops are affected, and second we wish to reduce disease spread. The solution must lie in a joint industry initiative."
• MAFF has issued the following statement to Crops: "The Ministry is reviewing its policy on the statutory controls of rhizomania disease, and is in discussion with industry representatives on a number of possible options for the future of the UK protected zone."
Theres only one more beet crop until 1999, when the UKs rhizomania-free status is reviewed. Will that signal the end of this disease as the worst nightmare for beet growers – and the beginning of learning how best to live with it?