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Researchers have finally identified the specific pathogen responsible for the blight behind the Irish Potato Famine by sequencing its genome. It's the first ancient plant genome to be successfully sequenced, a breakthrough that could help fight modern agricultural diseases.

Between 1845 and 1852, a disease struck Ireland's potato crops, rendering entire harvests inedible with blackened leaves and shrivelled tubers. While the blight spread across much of northern Europe, it was most tragically felt in Ireland -- a million of the island's eight million inhabitants died of starvation and disease, while as many again are estimated to have fled abroad.


We know that the pathogen that caused the famine was Phytophthora infestans, a fungus-like microorganism that infects potato plants through their leaves, but the full story has always been a mystery. Historians trying to trace where the specific strain of Phytophthora infestans came from have had to rely on shipping records and the knowledge that it originated in the Toluca Valley in Mexico, sometime after Europeans began colonising the Americas. We've never been sure if the strain that caused the famine has died out though, or if it's still alive today, causing harvest losses (albeit on a smaller scale thanks to modern agricultural technology).

Samples of 11 dried leaves, between 170- and 120-years-old, from Ireland, the UK, Europe and North America, were found in the collections of the Botanical State Collection in Munich and Kew Gardens in London. Although these plants are now extremely old, a team from the Sainsbury Laboratory, Norwich was successfully able to find extant DNA in them and sequence their genomes to track the spread of the blight over the course of the 1800s. It's the first time that an ancient plant genome has been sequenced successfully like this.

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The researchers compared the historical samples to modern strains from Africa, America and Europe, alongside two other closely-related species of Phytophthora. It turned out, contrary to expectations, that the strain that caused the Irish Potato Famine isn't US-1, which still causes blights today around the world -- it's an entirely new strain called HERB-1.

Looking through the genomes they recovered, the researchers found clear evidence that Phytophthora infestans had undergone rapid diversification shortly after the arival of Spanish invaders in the 1500s. It spread rapidly beyond the Toluca Valley to the rest of the Americas, and to Europe. HERB-1 appears in the early 1800s, spreading to Europe in 1845 to begin the Famine. It only died out in the early 1900s, to be replaced by the US-1 strain that continues to destroy crops around the world to this day.

Plant scientist Kentaro Yoshida said: "Perhaps this strain became extinct when the first resistant potato varieties were bred at the beginning of the twentieth century. What is for certain is that these findings will greatly help us to understand the dynamics of emerging pathogens. This type of work paves the way for the discovery of many more treasures of knowledge hidden in herbaria."

The Famine was arguably the defining moment in Ireland's modern history. The island's population dropped by between 20 and 25 percent, those that didn't die emigrating to the New World, England or Australia. By the end of the period the Irish constituted as much as a quarter of the population of Liverpool and the major cities of the American east coast.


Importantly, the popular perception was that the British government had left the Irish to die through either malice or (at the very least) negligence. Many of the major actors in the Irish independence movement were first- and second-generation emigrants who felt that the British had forced them from their ancestral home, and the decades of struggle for independence that followed the famine owe their tone to this legacy. Ireland's population has still not recovered to pre-Famine levels. The character of modern Ireland -- and the two countries that share it -- has been shaped in incalculable ways by Phytophthora infestans.

The research will be published in eLife.

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