22 November 1996

Dutch aim to stop the rot

Brown rot is now less of a threat to UK potato crops. Dutch efforts to contain it are bearing fruit and providing pointers for minimising risk. Charles Abel reports from the Netherlands

BROWN rot cases in Dutch seed potatoes have tumbled from 94 last year to just six so far this season. While that raises hopes that eradication is possible, it is more cases than had been hoped.

Since August 200 tubers have been taken from every 25t batch of seed, as well as targeted tests of crops around last years infected fields. The resulting 52,500 samples have been tested by the governments Plant Protection Service.

"With 95% of samples now checked and just six cases it gives us greater confidence in the system," says Jaap Janse, head of bacteriology at the PPS in Wageningen. As last year some of the cases stem from seed transmission, while others are due to dirty water. One crop succumbed when flooded by contaminated water after a canal bank broke.


But Klaas Dijkstra, chairman of the Dutch seed potato promotion board and chairman of leading potato seed company ZPC, is disappointed. "I was hoping for one or two cases, or maybe none."

Even so he is adamant that seed sales have not suffered. Exports last year still amounted to 740,000t. Only Syria closed its borders to all seed imports and that was considered an economic move, rather than a response to brown rot.

"For this year we expect sales of 600,000t, mainly due to poorer crop returns for growers. We cant say brown rot has had any effect on seed sales."

To rebuild confidence and work towards eradicating the disease from the Netherlands, last years full testing programme has been repeated, checking 200 tubers from each 25t lot of seed. "The industry consensus is that we will test again in 1997, and hope to get no cases and then move to a heavy survey in risk areas," he adds.

Computerisation now sees barcode tagging of every sample to speed administration. Last years checks took until March, delaying seed desptach. This year it will be completed this month, in good time for seed movement.

"These results make us more confident in the system we are using and more optimistic that we can eradicate the disease from the Netherlands," says Mr Janse. Sweden provides hope, eliminating the disease within five years.

Further confirmation of the testings accuracy comes from the fact that just two infected stocks were identified by importing nations last year. No cases have been seen in the field this year. Seed from both infected stocks was spotted by official testing in the UK, one confirmed by tests on another part of the same lot in Belgium.

UK tests supported

Mr Janse agrees that testing in the UK is worthwhile. "Scotland really is dependent upon potatoes, so the concern is justified, even if there is some politics, too. The extra testing does give extra confidence."

He blames sampling error for "misses". "The system can never be 100%, but two escapes from 51,000 tests must be seen as very good." Other countries reported positive cases, but further testing showed they were not due to brown rot.

With contaminated seed stocks now all but eliminated, attention is turning to contaminated water. "If you dont touch surface water you shouldnt have a problem," says Mr Janse. Dutch authorities have accordingly banned the use of surface water for irrigation in areas where it is known to be infected and advise not to use it elsewhere. Farmers are complying and accepting the yield loss, says Mr Janse. Any who do not, risk fines and downgrading of seed crops to ware, even if brown rot is not found. Some growers like Jaap de Zeeuw on the north-east polder at Emmeloord are spending £2200 a time to have new boreholes sunk to clean groundwater.

Researchers are now looking at the scope to disinfect surface water for irrigation as part of a £740,000 project over two to three years.

Checks on potato processing plants are also continuing. While final data is not available, Mr Janse confirms that brown rot has been found in nearby water courses.

Similarly, visual checks on all ware potato imports from third countries are ongoing. Lab testing is still being discussed. That is to the consternation of growers, including Mr de Zeeuw, who see fresh imports as the main source of the disease. "There should be a lot more testing of imports, particularly from Mediterranean countries like Egypt," he says. &#42

Over 52,000 tests have been made in the Netherlands to check for brown rot (inset).

Dutch farmer Jaap de Zeeuw – still wants more tests on ware imports.

Dutch researcher Jaap Janse – reckons he has brown rot on the run.


&#8226 52,500 tests.

&#8226 200 tubers/25t seed lot.

&#8226 £46/test cost to grower.

&#8226 Six cases in 1996 so far.

&#8226 94 cases last year.

&#8226 Eradication:

– Continue testing seed.

– Ban some irrigation.

– Monitor factory waste.

– Increased import testing.

Oxford outbreak caused by washings

Washings and peelings from imports are the likely cause of the 1992 Oxfordshire outbreak of potato brown rot, according to David Stead, consultant bacteriologist at the Central Science Laboratory, York. He told the Virus Tested Stem Cutting Growers Association conference he was confident that diseased tissue from Egyptian potatoes was washed down household sinks, entered a sewage plant and passed, unharmed and infectious, into the Thames. The disease then became established in bittersweet roots, which released infectious material into the water. From there it was spread by irrigation to the potato crop. The key was that there was no infection where irrigation was taken further upstream. The odds against brown rot becoming established were formidable, said Dr Stead. The bittersweet roots had to be in water with 10,000 bacteria cells a litre at a minimum temperature of 15C (59F). "It is difficult to see how it survives considering we import most potatoes in winter."

Allan Wright


&#8226 Infected crops destroyed, no roots for 3 years, potatoes for five.

&#8226 Compensation for destroyed crop only – nothing for changed rotation.

&#8226 Some businesses expected to fail as a result.

&#8226 Growers planned legal challenge, but dropped when lost crop compensated.