Archive Article: 1999/04/10

10 April 1999

VOLUNTEER potatoes in sugar beet could increase with a vengeance this year. If potatoes were grown in 1997 and beet is going in this year, what steps should be taken to counteract the weeds?

ELIMINATING fewer than one volunteer potato a sq m will pay for treatment. At that frequency lost beet yield is worth £100/ha; average yield loss is 20%.

In 1998 there were fewer left behind because harvester webs werent opened up so much, but there are still enough to cause severe problems. There werent frosts to wipe them out and not many will have rotted, even in this winters soggy ground.

If youve got potatoes in your sugar beet then treating is worth it, to reduce disease carry-over and potato cyst nematode build-up as well as yield loss.

Dow Shield (clopyralid) has proved reliable for potato control over the years. However, it will help if you know what variety the volunteers are likely to be. Maris Piper, Record and Squire are particularly susceptible to Debut (triflusulfuron-methyl). Use mixes of Debut and Dow Shield for difficult varieties such as Cara and Wilja. They will pay off even though they are more expensive.

With most of the low dose mixes spray when the potatoes are 10cm high. This will be around the time of the second post-emergence beet spray, near the end of April.

For instance, with Dow Shield Id recommend three lots of 0.3 litre/ha of Shield in mixture but, if all the potatoes come through evenly, then wait until they are 15cm high and spray two lots of 0.5 litre.

Mixing Dow Shield with ethofumesate gives more effective control. So does a mix of Debut plus Dow Shield plus ethofumesate. Debut works well with Betanal Progress (desmedipham + ethofumesate + phenmedipham) or Betanal Tandem (ethofumesate + phenmedipham).

Dont be so enthusiastic with the mixtures that you scorch the leaves; you want the treatment to reach the daughter tubers. If youre going to tractor hoe, its best to leave it as long as possible after the last spray – at least two weeks – to give the herbicides time to reach the daughter tubers being produced.

Mike May,

IACR Brooms Barn, Bury St Edmunds, Suffolk.

Rhizo proposals plainly divisive

I AM amazed at the NFU acceptance of the so-called Rhizomania Stewardship Scheme. The proposals to deal with the situation include: retention of existing controls; cessation of compensation payments to any grower who has had a previous outbreak; and allowing quota to be retained but leased out on an annual basis, subject to British Sugar approval.

I accept the current system may be untenable, certainly in the medium to long term but these proposals are divisive and will do very little to limit spread of the disease. If reform is needed then we should take radical decisions now.

Rhizomania is spread by movement of infected soil. The basis of the containment policy is to limit multiplication of the disease and to minimise cross contamination by soil movement. In the early stages of infection this policy works and is accepted by growers fearful of the large yield reductions supposedly caused by the disorder.

Todays situation is completely different. Many crops condemned and destroyed this year would have yielded 25-30t/acre if allowed to mature. Dr Mike Asher of IACR Brooms Barn believes inoculum is present in the soil for 10 years or more before symptoms are detected. He also suggests that wind-blown soil is a major vector in disease spread. These factors mean infected soil is already being taken into factories on roots which are not yet exhibiting the disease. Wind blows will not stop merely because a farmer ceases growing sugar beet. In any event there is no restriction on movement of other root crops such as potatoes or carrots.

The solution, which will have to be faced at some time, is to accept that rhizomania is endemic in both Norfolk and Suffolk and to remove all restrictions in those counties. For the rest of the UK, the containment policy should be maintained.

Within this cordon growers could make their own commercial decisions, using any and all varieties on the NIAB List. Newer resistant seed stock is becoming available which gives high yields while inhibiting multiplication of the virus.

British Sugar could then process all the crop, including rhizomania-infected beet. This would remove the need for any compensation fund in those counties.

Is the idea of processing the whole crop so unthinkable? It already happens throughout continental Europe, and there is no suggestion that sugar produced in their factories is any less pure because of it. There can surely be no objection on any quality aspect because the current British Sugar Review confirms the characteristics of rhizomania include low potash and sodium content, the very qualities BSC are currently advocating to all growers.

The other characteristic – low sugar percentage – is used primarily for each grower to consider how it affects his own gross margin.

After looking at well over 1,000 acres of rhizomania-infected beet this year, I am convinced that in terms of yield and profit loss, rhizomania comes a poor third after weed beet and beet cyst nematode.

The NFU proposals will at best only delay the inevitable, and are at the very least unfair and inequitable in penalising East Anglian growers for a situation for which they were not responsible and over which they have no control.

Chris Sutton,

Risby, Bury St Edmunds, Suffolk.

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