29 March 1996

Brown rot not beat yet despite new strategies

Huge efforts are being made to combat potato brown rot in the Netherlands. But shortfalls remain, as

Charles Abel quizzed his hosts during a recent visit

A CASCADE of measures is being introduced by the Dutch seed potato industry in a bid to combat the bacterial disease brown rot. But the problem is unlikely to be eradicated this year, experts admit.

Much of the strategy is possible thanks to the countrys comprehensive seed certification scheme. That has helped scientists trace the original cause of infection for 72 of the 86 outbreaks which have hit 23 varieties.

Most outbreaks can be directly attributed to contaminated stocks, with the majority stemming from one lot of Bildtstar grown in 1991. Irrigation and transmission on machinery have also been implicated.

The new strategy involves:

&#8226 Testing all seed lots.

&#8226 Controls on infected farms.

&#8226 Controls in regions at risk.

&#8226 General industry monitoring.

Building upon the emergency testing of over 53,000 seed samples in 1995/96, a co-ordinated programme will test each 25t seed lot this coming autumn. Only if all lots on a farm are disease-free will marketing be permitted, explains Jan Schans, phyto-sanitary specialist at the Plant Protection Service in Wageningen.

If a lot tests positive all seed on the farm is destroyed. Sister lines will also be re-checked on other farms. If two or more test positive it will be assumed the original parent stock was infected and all sister stocks will then be destroyed.

Seed potatoes may not be grown on infected fields for five years, with only grass, cereals or alfalfa allowed for the first three years. Maize is banned because it can harbour the disease. Other fields on the same farm are banned from seed for a year and solanaceous weeds and volunteers must be controlled. Machinery must be disinfected and best hygiene practised.

A farmer compensation fund is currently collecting £100/ha (£40/acre) from seed growers and £6/ha (£2.50/acre) from ware growers to pay £7000/ha (£2800/acre) to affected growers.

Farms considered to be at particular risk, due to irrigation, use of sister stocks or machinery sharing face other measures. Growing crops, including ware and starch, will be tested intensively, machinery must be disinfected and ware and seed kept separate in storage to avoid cross contamination.

Most importantly surface waters will be tested for brown rot. If it is suspected or confirmed farms within the catchment area will be banned from irrigating with surface water. Crops already irrigated will be withdrawn from sale.

Further measures include testing all seed and in-vitro planting stock, random surveys of ware and starch crops at harvest and all processing facilities where domestic and imported produce is washed, sorted or processed, explains Mr Schans.

Applied quickly

The measures are only proposed at present and must go before the EU standing committee on plant health in April. But Marie-Josee Jenniskens, head of phyto-sanitary development at the Plant Protection Service in Wageningen, is confident they will be applied quickly. "The industry wants them," she points out.

"We have a problem, we do not deny it. But we are trying to combat it. Im confident we will eradicate the disease, although not this year perhaps. But this way we can keep a very keen eye on new sources or dispersal of the pathogen," says Jaap Janse, head of microbiology at the Plant Protection Service in Wageningen.

&#8226 86 cases confirmed, 51 on seed and 35 on ware farms.

&#8226 Seed main cause of spread.

&#8226 Irrigation and machinery also to blame.

&#8226 54,000 seed tests in 1995/96 by 300 people over 6 months.

&#8226 Even more tests for 1996/97.

&#8226 Irrigation bans likely.

&#8226 Strict hygiene required.

Dutch seed grower Jaap DeZeeuw reckons he has brown rot licked thanks to strict farm hygiene, irrigating from bore holes only, choosing seed carefully and comprehensive testing. Costs add up to £50-60/ha, plus the loss of 3.5t of seed for testing – every batch is sampled, no matter how small. "But you just cant take any risks," he says.