Potato Famine culprit identified from 166-year-old dried leaves

An international team of researchers at the Max Planck Institute for Developmental Biology have identified the strain of Phytophthora infestans that caused the Irish Potato Famine of 1845-1852. In the thick of blight, botanists classified it as a mildew-causing fungus of the Botryotinia genus. In the 20th century it was reclassified as Phytophthora but was thought to be a strain called US-1 which is still widespread today. By analyzing dried specimens collected between 1845 and 1896 that have been kept in herbaria at Kew Royal Botanical Gardens in England and the Botanische Staatssammlung Munchen in Germany, researchers were able to find trace amounts of Phytophthora infestans DNA, map its genome and identify a previously unknown strain they’ve named HERB-1. (Full pdf study here.)

“Both herbaria placed a great deal of confidence in our abilities and were very generous in providing the dried plants,” said Marco Thines from the Senckenberg Museum and Goethe University in Frankfurt, one of the co-authors of this study. “The degree of DNA preservation in the herbarium samples really surprised us,” adds Johannes Krause from the University of Tübingen, another co-author. Because of the remarkable DNA quality and quantity in the herbarium samples, the research team could evaluate the entire genome of Phytophthora infestans and its host, the potato, within just a few weeks.

They found that HERB-1 is related to US-1 more than it is to any other modern strain, but it is unique. Phytophthora infestans originated in Toluca Valley, Mexico, among the potatoes that grow wild there. It was already endemic when Europeans arrived in America and brought the potato back, and yet, hundreds of years would pass before any Phytophthora strain made its way across the ocean. Scientists believe the US-1 and HERB-1 strains diverged in the Americas in the early 1800s. The newly individual HERB-1 hitched a ride on a trading ship and landed in Europe in Antwerp, Belgium, in the summer of 1845 before rapidly spreading to the Low Countries and other countries in Western Europe. Then it made the sea voyage to England and, most disastrously, Ireland.

Ireland was hit the hardest because more than a third of its population was dependent on potatoes as the sole source of nourishment. Irish Catholics were prohibited by law from owning land. Instead, the became tenant farmers who paid rent and worked the property of absentee English or Anglo-Irish landlords producing crops and cattle for export. This was a hand to mouth existence. Potatoes had the most bang for your caloric buck and could grow in the marginal land which was all the tenant farmers had left once the export crops and cattle pastures got the choicest bits.

By the early 1800s, the potato was the sole staple of the Irish farmer. Not only was it their only food, but almost all of the potatoes grown in Ireland were one breed: the Irish Lumper. The profound dependence on the potato coupled with a lack of genetic variety geometrically expanded the impact of the late blight when it arrived. HERB-1, used to the challenges of tough wild varieties, just slaughtered the cultivated potato crop. Author and scientist E.C. Large wrote in his seminal work The Advance of the Fungi that the blight “spread faster than the cholera amongst men.” Over the seven years of the Famine, HERB-1 destroyed crops so thoroughly that the Irish Lumper breed was almost driven to extinction. (It’s back now as an heirloom potato.)

The population of Ireland was more than decimated. In 1845 the population was more than eight million. By 1852, there were only five million people left in Ireland. One million of them died from starvation and the diseases that ravage the hungry. Two million emigrated to the United States, Canada, Australia, New Zealand and other countries. Ireland’s population today at just 4.5 million has yet to recover from the devastation of The Great Hunger.

HERB-1 may have been defeated by human-directed evolution, namely the breeding of new blight-resistant potato strains in the early 20th century. US-1 is probably the strain that replaced HERB-1. Now that the genome of HERB-1 has been decoded, researchers can see if it really was driven to extinction or if it still exists in secluded pockets somewhere around the world that the genetically resistant potato varieties haven’t reached yet.

This is the first time the genome of a plant pathogen has been decoded from dried samples. It brings new life to musty old specimen collections that aren’t exactly exciting to most museum visitors and have been relegated to storage. Scientists studying the evolution of pathogens, how they develop in concert with human breeding of crops will find a rich new vein to mine in the back rooms of Victorian museums.