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....