The potato late blight, properly known as Phytophthora infestans, has a new weapon - sex. Scientists fear that this may allow it to attack crops earlier in the growing season in more sites, and become better at developing resistance to the pesticides used to control it.
The blight arrived in Europe's Low Countries from America in June 1845. By September, spores carried on wind and rain reached Ireland, whose fast-growing rural population was uniquely dependent on potatoes. It wiped out the crop that year and the next, killing at least 1.5 million people - about one-fifth of the population - through starvation and diseases such as typhus.
The fungus has remained a feared foe of potato farmers around the world ever since. Given warm, wet conditions it can turn potatoes into mush and transform a lush green field of plants into a rotting mass in a few days. It is held at bay with large applications of fungicide, and attempts to breed more resistant crops have achieved only partial success. On 10 September - the date the blight was first reported in Ireland - the Irish President, Mary Robinson, will open a conference of potato experts from around the world.
Today the fungus poses a particular threat to potato farming in Third World countries, whose populations are as poor and as rapidly growing as that of mid-19th century Ireland.
The post-1845 European strain of the fungus, known as A1, discovered sex in 1976. Until then it had been reproducing with asexual spores, but that year a different strain, A2, arrived in a potato shipment from Mexico and spread across the Continent to Britain and Ireland.
These two strains can mate with each other as well as reproducing asexually. Dr Richard Shattock, an expert on potato late blight at the University College of Wales, Bangor, explained that after mating the two strains produced oospores - microscopic eggs- which could lie in the soil over winter and infect the next crop to be planted.
A1 and A2 cannot do this on their own; to survive they must remain in a potato, waiting for the warm, wet conditions that promote growth and the production of asexual spores. Piles of discarded potatoes can often be the source of the disease.
Dr Shattock said: "It's been shown that these two strains can mate and produce these oospores, and that they can survive the winter and infect a new crop the next year. What we don't know is whether this is happening yet.''
Searching for the oospores can be like looking for a needle in a haystack - they are three-hundredths of one millimetre across.
As well as giving blight a better chance of surviving winter in the open, sex allows strains to swap genes - and so possibly develop fungicide resistance more swiftly. "It leads to greater genetic diversity - there is concern about that," said Dr Shattock.
Meanwhile, across Europe, the price of potatoes is at its highest in 20 years - though the cause is poor potato-growing weather in 1994 and low confidence among farmers; at 15-33p a pound in UK supermarkets they are more than 50 per cent dearer than at this time last year.Reuse content