14 September 2001

New blight threat anything but late

POTATO blight should not be called late blight. It is anything but late these days, says Dutch blight expert Wilbert Flier of Plant Research International, Wageningen.

Rapid evolution of the disease through mutation and sexual reproduction has led to strains that can be carried on seed, thrive at lower temperatures and hit crops earlier. The average number of blight sprays used each season in Holland has rocketed from 12 to 20 in five years as a result.

Critical control periods are also more frequent, says Dr Flier. "Smith periods are history." As little as three hours of favourable conditions can lead to infection.

To combat the extra blight pressure, dumps are now illegal in Holland and blight control is compulsory in crops and volunteers.

But it is feared seed may provide the initial infection. A total of 13 blight outbreaks were att-ributed to infected seed this season. "We were astonished to find this. We always thought seed was relatively safe," says Dr Flier.

German work suggests treating seed with blight fungicide just prior to planting could work. But no such approval exists.

Varietal disease resistance ratings are no longer reliable, Dr Flier adds, because the isolates used to develop them are not as aggressive as some strains in farmers fields.

Whether the same exists in the UK is unclear. David Shaw, of the University of Wales, admits that when funding for disease surveys stopped in 1998 A1 dominated, but A2 was present in pockets. Now it could be widespread.

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