Researchers in the University of California Berkeley and the University of Copenhagen are exploring how the pathogen that caused Ireland’s 19th-century potato blight traveled from South America, causing crop destruction on a global scale.

Using some ancient DNA detective work, a new study led by University of California Berkeley postdoctoral researcher Mike D. Martin and University of Copenhagen professor Tom Gilbert has linked the culprit behind the 19th-century Irish potato famine, which was transported to Europe in the 1840s, to a fungus-like organism that originated in South America.

Ireland’s Great Hunger or Great Famine was a period of mass starvation, disease and emigration, between 1845 and 1852. When the potato blight struck Ireland two-fifths of the country were reliant on the crop. During the Famine one million people died and another million emigrated. The population of the island dropped by 20 to 20 percent.

Just how the pathogen, called Phytophthora infestans, made the transatlantic leap to destroy potato crops on a global scale is a "Guns, Germs, and Steel"-like tale of New World exploration and devastation published in the advanced online edition of Molecular Biology and Evolution.

The authors used genome sequences from 71 modern and historical samples of the microbial pathogen, a unique collection culled from worldwide private archives, to construct the ancestral tree of the pathogen. The origin of the species dates back to 1558 AD, the age when the first Europeans explored South America.

The research team further observed that the species was first introduced to 19th-century Europe shortly after it evolved and diversified. They found a close connection between a present-day sister species, P. andina (found only in the highlands of Ecuador and Peru) and the ancient P. infestans that triggered the first global outbreak in 1845 and the catastrophic Irish potato famine.

The authors speculate that after being found in South America, the pathogen either spread from South America directly to the United States, or was simultaneously introduced from Mexico into South America and the US prior to its infestation of Europe.

"We think early European activities in the New World led to the origin of this devastating pathogen. Countless improbable events led to the introduction of this species to Europe in 1845, but our work narrows down the evolutionary possibilities to exactly two," said Martin.

Future work will try to trace the exact route into Europe. "Ultimately finding the precise location where this species evolved could lead potato breeders to discover new genetic tools for improving resistance against potato blight disease," said Martin.

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 University reports that potato blight remains a major global food security threat. It causes an estimated $6 billion in damage per year.

The blight in question infects the leaves and stems, and rots the vegetable (tuber). It can completely destroy a field of potatoes within two weeks. 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.