The pathogen that led to the potato blight in Ireland at the time of the Great Hunger in the 1840s has been traced to central Mexico.

Knowing the exact origin of the pathogen will help plant breeders identify local plants with late blight resistance that could be bred into commercial potatoes. Cornell reports that potato blight remains a major global food security threat. It causes an estimated $6 billion in damage per year.

Researchers from Oregon State University, the USDA Agricultural Research Service and five other institutions carried out genetic analyses. They found that the pathogen in question, Phytophthora infestans, hails from Toluca Valley in central Mexico and not the Andes as previously believed.

The blight in infects the leaves and stems, and rots the vegetable (tuber). It can completely destroy a field of potatoes within two weeks. The study concludes that Phytophthora infestans originated in this valley and co-evolved with potatoes over hundreds or maybe a few thousand years.

A single clonal lineage of the pathogen Phytophthora infestans had invaded the United States by 1843. By the summer of 1845 the potato late blight had established itself in Europe, including Ireland.

Niklaus Grunwald, who is a courtesy professor in the Department of Botany and Plant Pathology in the College of Agricultural Sciences at Oregon State University, a researcher with the USDA Agricultural Research Service, and lead author on the study, told the findings from this study are “immensely important.”

"This is just a textbook example of a center of origin for a pathogen, and it's a real treat," Grunwald said.

"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."

The study, published on June 2 in the “Proceedings of the National Academy of Sciences”, shows how the researchers identified the pathogen’s origins by studying the DNA of more than 100 samples across the world, in order to understand their ancestral relationships.

The results showed that the Phytophthora infestans found in the Andes had migrated south from Mexico. It’s rare to pinpoint the exact origin of a pathogen. Having done so scientists can now work towards developing more resistant crops.

William Fry, professor of plant pathology and a co-author of the paper, is optimistic. He said “Populations of wild plants contain a wide array of resistance genes.”