Pathogen Responsible for Deadly Irish Potato Famine Discovered

The Irish Potato Famine spread across Ireland during the 1800s, prompting a period of mass starvation, disease and emigration. Now, researchers have discovered the strain of potato blight that caused the famine, revealing a little bit more about Europe's grisly past.

The Irish Potato Famine, which was known as the Great Famine in Ireland, spread from 1845 to 1852. During that time, about one million people died while others fled the country, seeking respite from the constant hunger that was part of their daily lives.

The famine itself was long in coming. The poor of Ireland began to subsist mainly on potatoes--they were cheap, easy to grow and easily harvested. This basic source of starch also possessed all of the vitamins that were necessary for survival. As people continued to harvest the crop, they began to choose only one particular variety--the Irish Lumper. With a lack of genetic diversity and with so many dependent on one crop, the conditions were ripe for the famine.

A potato blight, known as Phytophthora infestans, struck the Irish Lumper. The disease destroyed the potatoes, turning them into mush before they could be harvested. After it was introduced, it spread rapidly. In fact, by late summer and early autumn of 1845, it had spread throughout the greater part of northern and central Europe, Belgium, Holland, northern France and southern England. In 1846, three quarters of the potato harvest was lost to the blight.

Now, researchers have learned a little bit more about the blight that devastated millions. In order to examine the plant pathogen that changed the course of history, they looked at dried plants that were 170 to 120 years old. Despite their age, though, the researchers were able to find many intact pieces of DNA. In fact, they were able to decode the genome of the plant pathogen and the plant host.

"Herbaria represent a rich and untapped source from which we can learn a tremendous amount about the historical distribution of plants and their pests--and also about the history of the people who grew these plants," said Kentaro Yoshida from the Sainsbury Laboratory in Norwich.

Researchers in the past believed that a strain caused US-1 was the cause of the fatal outbreak. Yet the new research seems to point to a different strain--one new to science. Although the new strain was more closely related to US-1 strain than to modern strains, it's still unique.

In fact, it appears that the new strain, known as HERB-1, likely emerged in the early 1800s and spread throughout the globe during the 19^th century. Only in the 20^th century, after new potato varieties were introduced, was HERB-1 eventually replaced by the other infectious strain, US-1.

Researchers didn't just find this new strain, though, they also found out about how it spread and evolved. They discovered that the first contact between Europeans and Americans in Mexico in the sixteenth century coincided with a remarkable increase in the genetic diversity of potato blight. The social upheaval during the time may have led to a spread of the pathogen from its center of origin in Toluca Valley, Mexico. This, in turn, would have accelerated its evolution.

The findings show a little bit more about the history of the potato blight. The study is also applicable to learning a bit more about the dynamics of emerging pathogens.

The findings are published in the journal eLife.