An international team of scientists has finally solved one of history’s greatest mysteries: What caused the devastating Irish potato famine of 1845? The research team, which published its findings in the journal eLife this week, used DNA sequencing of plant specimens dating from the mid-19th century to identify the pathogen that led to the death of nearly 1 million people and the mass emigration of another 2 million from Ireland by 1855. The discovery marks the first time scientists have successfully sequenced a plant’s genome from preserved samples and opens the door for further research into the evolution of pathogens and the spread of plant disease around the world.

Scientists have long known that it was a strain of Phytophthora infestans (or P. infestans) that caused the widespread devastation of potato crops in Ireland and northern Europe beginning in 1845. P. infestans infects the plant through its leaves, leaving behind shriveled, inedible tubers. The most likely culprit, they believed, was a strain known as US-1, which even today is responsible for billions of dollars of crop damage each year. To solve the mystery, molecular biologists from the United Kingdom, Germany and the United States examined DNA extracted from nearly a dozen botanical specimens dating back as far as 1845 and held in museum collections in the UK and Germany, which were then sent to the Sainsbury Laboratory in Norwich, England. After sequencing the genome of the 19th century samples and comparing them with modern blights, including US-1, they were able to trace the genetic evolution of P. infestans around the world and across centuries.

The researchers concluded that it wasn’t in fact US-1 that caused the blight, but a previously unknown strain, HERB-1, which had originated in the Americas (most likely in Mexico’s Toluca Valley) sometime in the early 19th century before spreading to Europe in the 1840s. HERB-1, they believe, was responsible for the Great Famine and hundreds of other potato crop failures around the world. It wasn’t until the early 20th century that improvements in crop breeding yielded potato varieties that proved resistant to HERB-1 that the deadly infection was stopped in its tracks. Scientists believe that the HERB-1 strain is now extinct.

First domesticated in southern Peru and Bolivia more than 7,000 years ago, the potato began its long trek out of South America in the late 16th century following the Spanish conquest of the Inca. Though some Europeans were skeptical of the newly arrived tuber, they were quickly won over by the plant’s benefits. Potatoes were slow to spoil, had three times the caloric value of grain and were cheap and easy to grow on both large farms and small, backyard lots. When a series of non-potato crop failures struck northern Europe in the late 18th century, millions of farmers switched to the more durable spud as their staple crop.

Nowhere was dependency on the potato more widespread than in Ireland, where it eventually became the sole subsistence food for one-third of the country. Impoverished tenant farmers, struggling to grow enough food to feed their families on plots of land as small as one acre, turned to the potato en masse, thanks to its ability to grow in even the worst soil. Requiring calorie-heavy diets to carry out their punishing workloads, they were soon consuming between 40 and 60 potatoes every day. And the potato wasn’t just used for human consumption: Ireland’s primary export to its British overlords was cattle, and more than a third of all potatoes harvested were used to feed livestock.

By the early 19th century, however, the potato had begun to show a tendency toward crop failure, with Ireland and much of northern Europe experience smaller blights in the decades leading up to the Great Famine. While the effects of these failures were largely ameliorated in many countries thanks to their cultivation of a wide variety of different potatoes, Ireland was left vulnerable to these blights due to its dependence on just one type, the Irish Lumper. When HERB-1, which had already wreaked havoc on crops in Mexico and the United States, made its way across the Atlantic sometime in 1844, its effect was immediate—and devastating. Within a year, potato crops across France, Belgium and Holland had been affected and by late 1845 between one-third and one-half of Ireland’s fields had been wiped out. The destruction continued the following year, when three-quarters of that year’s harvest was destroyed and the first starvation deaths were reported.

As the crisis grew, British relief efforts only made things worse: The emergency importation of grain failed to prevent further deaths due to Ireland’s lack of working mills to process the food; absentee British landlords evicted thousands of starving peasants when they were unable to pay rent; and a series of workhouses and charity homes established to care for the most vulnerable were poorly managed, becoming squalid centers of disease and death. By 1851 1 million Irish—nearly one-eight of the population—were dead from starvation or disease. Emigration from the country, which had steadily increased in the years leading up to the famine, ballooned, and by 1855 2 million people had fled, swelling the immigrant Irish populations of Canada, the United States, Australia and elsewhere. Even today, more than 150 years later, Ireland’s population has still not recovered its pre-famine level. Those that stayed behind, haunted by their country’s suffering, would form the basis of an Irish independence movement that continued into the 20th century.

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