LATE-BLIGHT, a fungus that destroys potatoes, which had been neutralised decades back, may play havoc with crops in Europe once again. Researchers in southwest Scotland are about to infect a crop of potato hybrids with late-blight to see whether they can resist the disease. (New Scientist, Vol 154, No 2079)

New versions of the deadly fungus recently escaped from the Mexican mountains to other parts of the continent. One particularly aggressive strain has spread across the us in the past five years and is growing resistant to fungicides. It has done hundreds of millions of dollars worth of damage. British researchers believe it could reach Europe this summer.

Other developments with a 'natural pesticide' extracted from potato roots could head off the potato cyst nematode, which almost destroyed the British potato harvest in the 1950s. But the real test is in beating late-blight, which could be on the verge of another pandemic, fuelled by an input of genetic diversity from its Mexican homeland.

Before 1842, potato late-blight, Phytophthora infestans, was known only in Mexico, where it began as a local disease among the potato and tomato fields. But that year it turned up in New England. Three years later it appeared in Belgium, from where it spread throughout Europe and beyond.

"A very small number of clones reached the us in the 1840s, and we think it was a single clone that made it to Europe," says Jim Duncan of the Scottish Crop Research Institute, which is conducting the experiment. Nevertheless, the fungus hit European potatoes hard 150 years ago. It found the most favourable conditions in the Irish monoculture of the lumper potato, a variety that dominated huge tracts of the wet boggy country and provided the bulk of the nourishment for millions of its inhabitants. The lumper had no resistance to late-blight. In the ensuing famine a million people died and two million had to migrate.

In the years since the Irish potato famine, plants breeders have waged a continuing war against late-blight. Until recently, they had one great advantage - the very limited ability of the fungus to evolve. This was because, outside Mexico, the fungus could not breed sexually. The escaped clone, or clones, of the fungus all belonged to the same "mating type", called Al. Without an A2 partner, the Al fungus could only reproduce asexually, producing endless clones of itself. This limited its capacity to evolve by exchanging and rearranging genetic material, making it harder to adapt to change in the environment.

The ability of the new strain to reproduce sexually and lie dormant in soils has alarmed plant breeders fighting late-blight, and moved the disease to the top of the research agenda at the International Potato Centre (ipc) in Lima, Peru, and in potato laboratories around the world.

Worldwide, various forms of late-blight today destroy 14 million tonnes of potatoes a year, with a value of $3 billion, says Peter Gregory, deputy director-general in charge of research at the ipc. "It is poised to strike hardest at millions of poor people in developing countries who rely on potatoes but can least afford to buy expensive chemicals to keep the fungus in check," he says.

Farmers who can afford it are fighting back in the only way they know - by spraying ever more fungicide on their crops. In Central America, 25 sprayings a season have been reported as desperate farmers have been trying to tame the fungus. In potato fields in the highlands of Java and Sumatra in Indonesia, says Duncan, they sometimes spray fungicide on crops more than twice a week.

A century's search for a lasting resistance to blight has not stopped the fungus menacing crops around the world. Every time supposedly resistant cultivars were developed, new races of the fungus quickly got the upper hand. Researchers believe that tl* battle against late-blight is far from being won.