Scientists have discovered a gene which protects potatoes from blight, a disease which led to crop failure and famine in 19th Century Ireland.
Potato crops were wiped out by the disease in 1845
The gene was taken from a species of wild Mexican potato which has evolved a natural resistance to late blight.
Inserted into commercially grown potatoes, it effectively protects the plants from the devastating fungus.
More than one million people died of starvation when Irish potato crops failed because of the disease in 1845, sparking a wave of emigration.
When plants become infected with blight, lesions appear on their leaves and stems. Eventually the potatoes rot and become inedible.
All the commonly cultivated varieties of potato are highly susceptible to late blight, a fungal family that normally kills any plant it infects.
The protective gene was found in a species of wild potato called Solanum bulbocastanum.
Scientists believe the plant co-evolved in Mexico alongside late blight, and developed a strain that was immune to the disease.
Plant experts at the University of Wisconsin-Madison in the United States cloned the gene and used it to engineer blight-resistant potatoes.
Insertion of the single gene protected the plants from the full range of late blight infestations.
Professor John Helgeson, who led the research, said: "So far, the plants have been resistant to everything we have thrown at them."
The discovery was reported in Proceedings of the National Academy of Sciences.
Prior to the 1990s, chemical fungicides were available that effectively held late blight at bay.
But new strains have emerged, testing the limits of technology and forcing American farmers to treat potato fields as many as a dozen times a season.
In warmer climates, such as Mexico, fields may be treated as often as 25 times a year with costly and toxic chemicals.
It was the emergence of one of the new strains that, ironically, led to the discovery of the Mexican gene.
The disease swept through the United States in 1994, compelling the farming industry to develop new fungicides.
The scientists said it would be "all but impossible" to produce blight-resistant potatoes using conventional breeding techniques rather than through genetic engineering.
Potato blight is still a major problem in many countries - affecting crops in Russia, Mexico, Ireland, Ecuador and the US.