The menacing strain of fungus long blamed for the Irish Potato Famine was not the culprit after all, according to genetic research.
During the 1840s, a blight raced through Ireland's potato crop, leading to famine that killed at least 1 million people and spurred a wave of emigration to the United States.
Scientists have long known that the blight was caused by the fungus Phytophthora infestans. And they have long assumed that the fungus was of the same strain that is most widespread today, US-1.
But that assumption is wrong, said Jean Ristaino, a plant pathologist at North Carolina State University who reported her findings in Thursday's issue of the journal Nature.
Ristaino found no sign of the US-1 strain when she examined DNA from more than two dozen lesion-covered potato leaves collected in Ireland and England between 1845 and 1847.
While researchers could not pinpoint what strain of the fungus is to blame, the study suggests that one of three other known versions is the culprit.
It is also quite possible the disease did not arrive in western Europe from Mexico, as previously thought, the researchers said.
The question of what caused the blight is more than just a matter of historical curiosity; it could help scientists fend off similar epidemics.
Ristaino's team is now looking at DNA of preserved leaves from several other countries to try to trace how the fungus got to Europe.
Identifying the precise origin could help scientists find potato plants that have developed natural resistance to the fungus. Those plants, in turn, could be used to breed new, more resistant potato varieties, she said.
The issue is timely. In recent years, strains of the fungus have again begun to spread aggressively around the globe, with attacks most notable in Russia, but also present in Mexico, Ecuador and the United States.
Ristaino said her study marks the first time genetic analysis has been used to look at old plant specimens to trace a disease. Similar methods have been employed to study human history.
DNA fingerprinting could be used to study other crop diseases, she said.
"We have all these archives of epidemics of the past sitting on shelves in herbariums. What I'm working with is just one example," she said.
Nicholas Money, a botanist at Miami University in Ohio, called Ristaino's work a "remarkable piece of molecular detective work" given the difficulties in separating DNA from older plant specimens and making sure it is not contaminated.
Ristaino and Money said they hope the work encourages governments and universities to pay closer attention to their natural history collections - particularly old plant specimens - since they could supply clues for the future.