The cradle of potato late blight and the Great Irish Famine of the 1840s has been tracked to a pretty, alpine valley in central Mexico. The Toluca valley, ringed in by picturesque mountains a short distance from Mexico City, is now known to be the ancestral home of one of the most costly and deadly plant diseases in human history.

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A team from Oregon State University and the US Department of Aagriculture's ARS research service have tracked Phytophthora infestans back to its 'ancestral home' in the valley, where it co-evolved with potatoes over hundreds or maybe thousands of years, before spreading to much of the rest of the world.

Knowing the origin of the pathogen does more than just fill in some knowledge gaps of agricultural history, though. It provides new avenues to discover resistance genes, and helps explain the mechanisms of repeated emergence of the disease, which remains to this day the most costly potato pathogen in the world.

Potato late blight continues to be a major threat to global food security and at least £3.6 billion a year is spent in combating the disease. However, the researchers believe that, in tracking its origin, they can find the key to conquering blight. Blight is one of the first agricultural diseases to have its origin successfully traced.  

"This is immensely important," said Oregon State University professor Niklaus Grunwald, who led the study, "This is just a textbook example of a centre of origin for a pathogen, and it's a real treat. I can't think of another system so well understood. This should allow us to make significant headway in finding additional genes that provide resistance to P. infestans."

Finding ways to genetically resist the potato late blight, scientists say, could help reduce the use of fungicides, and the expense and environmental concerns associated with them.

There had been competing theories about where blight may have come from, with the leading candidates being the Mexican valley, or areas in South America where the potato itself actually evolved thousands of years ago. However, gene sequencing research pointed towards Toluca, where the disease is thought to have co-evolved with plants that were distant cousins of modern potatoes, which produced tubers but were more often thought of as a weed than a vegetable crop.

The Irish potato famine that began in 1845 led to the death of 1 million people, and led another 1 million to emigrate to the U.S. The famine was exacerbated by lack of potato biodiversity, as some of the varieties most vulnerable to blight were also the varieties most widely cultivated. Modern mores in agricultural production are increasing homogeneity in the world's major agricultural crops, which in turn is increasing the risk disease poses to food production.