One of history’s worst agricultural catastrophes was the great Irish potato famine of 1845–1849, which killed more than a million people in Ireland, forced the emigration of 1.5 million others, and fueled resentments between the Catholic and Protestant factions of the country for generations. The precise pathogen responsible for that catastrophic failure of the Irish potato crop was long a matter of speculation among agricultural researchers. In 2013, however, using DNA-based analytical techniques on samples of old potato leaves preserved in herbariums, an international team of scientists pinned the blight to a single strain of a common fungus. Beyond the value of this discovery in illuminating what triggered the potato famine, it marked the first time that researchers were able to study the genome of an extinct botanical pathogen from traces in old cuttings of its host plant. See also: Deoxyribonucleic acid (DNA); Disease ecology
The edible tuber of the genus Solanum, known as the Irish potato, is, contrary to its name, not a plant native to Europe. South American people began cultivating it more than 7000 years ago, and Spanish conquistadors brought it back with them from Peru before 1570. It became a popular food staple, and eventually potatoes were adopted as the mainstay of nineteenth-century Ireland’s agricultural system. Yet the potato crops grown throughout Europe and North America had a weakness in that they had very little genetic variation, with little of the robust diversity that helps natural populations endure blights. When a particularly virulent potato pathogen emerged in the mid-nineteenth century and was carried to Ireland by boat, it became a pandemic that swept through farms and devastated the harvests, with fatal consequences for the large Irish populations that depended on them. See also: Potato, Irish; Solanales
For years, agricultural researchers guessed that the organism responsible for the Irish potato blight was the US-1 strain of Phytophthora infestans, a fungus of the class Oomycota, which has been known to damage potato crops around the world for more than 150 years. Genetic analysis of potato leaves from the nineteenth century revealed that conclusion to be incorrect, however. Instead, the Irish famine was started by a different, closely related strain that Hernán Burbano of the Max Planck Institute for Developmental Biology and his colleagues named HERB-1. Their studies suggest HERB-1 evolved in the Americas in the early 1800s, rampaged through vulnerable crop fields, and then went extinct in the early twentieth century as farmers began to rely on less susceptible varieties of potatoes.