31 March 2000

Dutch in battle to control spud blight epidemic

By Charles Abel

POTATO blight has become so aggressive in Holland that growers are being fined for leaving dumps uncleared, government-back-ed scientists are revising control programmes and breeders are co-operating to find more resistant varieties.

"Between 1975 and 1998 potato blight populations became three times more aggressive and the trend is accelerating," Dutch blight expert Wilbert Flier told a technical briefing organised by Cyanamid last week. "The same is likely to be happening in the UK." Last years uncharacteristic failures of Maris Piper support his view.

Dr Flier attributes blights growing aggression to the arrival of A2 blight from Mexico in 1976. That has been crossing with existing A1 blight ever since, creating a vast range of new pathotypes. Over-wintering resting spores are also produced, which may explain very early blight outbreaks.

In Holland, blight now takes four days to complete a disease cycle, compared with seven before. New strains are also active below 5C and above 25C. Tuber breakdowns are more severe, more rapid and more frequent.

Varietal resistance is also being eroded, only Santes seeming unaffected.

The Dutch potato industrys response is to clamp down hard on blight sources. Last year growers were advised to destroy dumps in March, inspectors fining growers up to £1000 where a herbicide or black plastic sheeting had not been used.

Genetic fingerprinting is also being used to trace the source of original blight infections. Last year infections across an area of 100km by 200km were all traced back to a single crop grown under polythene, explained Dr Flier.

Growers with high risk crops are now advised to use 3-4 applications of a semi-systemic product in the 2-3 weeks before the canopy closes to protect stems.

UK growers would be well advised to do the same, Dr Flier suggested. &#42

BLIGHT ALERT

&#8226 Three times more aggressive.

&#8226 Dumps and sheeted earlies a key source.

&#8226 Blight police fine growers £1000 for uncontrolled dumps.

&#8226 Focus on early control.

More aggressive blight strains put tubers at extra risk, says Wilbert Flier of Dutch Plant Research International at Wageningen.