Science: WHAT A LOAD OF ROT

In the last year, millions of tons of potatoes have been destroyed by new super-strains of blight. Roger Dobson reports on the fight to prevent a second famine

IT WAS A busy morning in the bustling Dutch port during that long hot summer of 1976. Freighters and tankers from all over the world were departing and arriving, or were docked in the blistering heat, waiting to be unloaded by the stripped-to-the-waist shore gangs.

Most were regular callers carrying their usual shipments of sugar from the West Indies, iron ore from South Africa, coal from the US, cars from Japan, and exotic fruit from Australia. But waiting among them was one ship with something that had not been seen before. In the cavernous black holds of that cargo ship, newly arrived from Mexico, were several thousand tons of potatoes, imported by an unknown entrepreneur to try to make up for that year's terrible harvest in northern Europe.

It had been the hottest, driest summer on record. Britain was suffering drought orders, and Denis Howell had been appointed to oversee the crisis. As well as causing water rationing, the drought had also devastated root crops such as potatoes which rely heavily on having enough water to grow. It was against this background that the ship full of potatoes arrived.

But the holds of the ship from Mexico carried not only potatoes, they also carried new types of a virulent fungus which is now believed to have mated and created strains of super-blight that have now spread around the world. In the last year alone, potato blight is estimated to have devastated 14 billion tons of potatoes at a cost of pounds 1bn.

Given the rapid progress of the killer blight, scientists are now working around the clock to create potatoes which are immune to it. Ironically, the re-emergence of blight as a major problem comes exactly 150 years after the Irish potato famine which led to the deaths of one million people through famine and to a further 1.5 million emigrating to America. That original Irish blight is now also thought to have originated in Mexico and to have been accidentally imported in the mid-19th century, on a specimen plant brought to Europe by a botanist.

The potato blight fungus - Phytophthora infestans - comes in two forms, A1 and A2 and in the last century, it was only A1 that came to Europe from Mexico. It caused such devastation in Ireland principally because, in the absence of 20th-century fungicides, its spread was rapid and uncontrollable.

The fungus that causes blight is extremely virulent and its spores can travel thousands of miles on the wind. These spores thrive in wet weather, first on the leaves of the potato plant and then by penetrating the foliage and being washed down into the tubers. Not only does it kill the foliage, but it also rots the tubers. As Irish farmers found in 1845 and 1848, even lifting the potatoes of affected plants does not help, because they go on to rot in store.

Over the years, the development of fungicides and pest management schemes has meant that blight caused by the A1 fungus became much more controllable, but the 1976 arrival of the cargo ship from Mexico changed that. That cargo brought the A2 variety to Europe, as well as more A1. Crucially, the arrival of both types meant that the fungus could reproduce sexually for the first time.

"Before 1976 and the blunder that allowed that ship of potatoes to come to Europe we had only the single strain and it reproduced clonally. But since that ship arrived we've found many different strains," says Dr David Shaw, senior lecturer in biological science at the University of Wales, Bangor and a world expert on the blight.

"Once they reproduce sexually, they produce varied offspring and it is then able to evolve and adapt, which it couldn't do clonally and it could go on to evolve to tackle any resistance the breeders may come up with."

Worse still, in the US researchers believe the new genetic variation means the fungus is evolving to create strains that will be immune to the most powerful fungicides, as well as destroying any natural immunity in the plants. The theory is that over the last 150 years the Irish famine fungus has become less aggressive, and that its more virulent offsprings will take over. "When you have compatibility and sexual reproduction, it throws up new mixes of genes. It seems that sexual reproduction between compatible strains is now occurring more readily and we are therefore likely to have more variations which will give the fungus a greater pool of diversity from which new types will arrive and may be able to survive better," says Professor Paul Davies, vice principal of the Royal Agricultural College. An added worry is that the new strains can spread so rapidly. In the US, a strain arrived in New York state in 1992 which has now spread to 23 other states and to parts of Canada.

Dr Shaw and his team have spent the last three years combing the UK looking for signs of new strains and have so far found more than 100. Each fungus is grown in the laboratory and typed according to its DNA. "One of the worries we have is the possibility that some of these new types are worse pathogens and more destructive than anything we have seen before. Research in America suggests the rotting process is happening more rapidly and resulting in a faster epidemic," says Dr Shaw.

A doomsday scenario is that the mating process produces contaminated spores which can survive the winter and live in the soil for up to 100 years, affecting future plants for generations.

Because of growing concern, several teams of plant geneticists, co-ordinated by the International Potato Center in Peru (which has 200 indigenous species) are working on genetically modifying the potato to make it more resistant to blight. It is relatively easy to manipulate, and lines have already been bred which are more resistant to Colorado Beetle or which carry less starch. And a team of researchers in Mexico has bred a potato which does carry some resistance to blight, but it is not thought suitable for large- scale commercial production. In Europe, types of potatoes are being created which have greater tolerance to blight, thus the effects of disease can be kept at bay long enough for the tubers to be harvested.

The problem now is that as the fungus can breed sexually, it is better equipped to adapt for its own survival, however much potatoes are genetically altered.

While another famine on the scale of the Irish tragedy is highly improbable, the long-term effects could still bring economic disaster to millions, says Professor Gail Schumann, Plant Pathologist at the University of Massachusetts.