DNA Study Sheds Light on Irish Potato Famine

John Roach
for National Geographic News
May 5, 2004

In the mid-19th century, a fungus-like disease that turned potatoes into black, inedible mush led to the fatal starvation of approximately a million people in Ireland. A team of DNA sleuths now believes they know the true identity of the killer disease.

The mystery began unraveling three years ago, when the researchers presented DNA evidence from samples of 150-year-old potato leaves. The scientists said the findings exonerated the previous prime suspect behind the Irish potato famine: a strain of the pathogen Phytophthora infestans known as the Ib haplotype. (The pathogen causes a plant disease known as late blight.)

Now the team, led by Jean Ristaino, a plant pathologist at North Carolina State University in Raleigh, says a different strain of P. infestans, the Ia haplotype, was the culprit. The scientists trace the pathogen's probable roots to South America's Andes Mountains.

Late blight continues to devastate potato and tomato crops around the world. In 2001, an epidemic wiped out Russia's potato crop, a major food source for many of the country's poor. Smaller outbreaks regularly occur in Mexico, Ireland, Ecuador, and the United States.

Ristaino and her colleagues hope to conclusively pinpoint the strain of P. infestans behind the Irish potato famine and trace the strain's place of origin. Doing so, the researchers say, will help scientists to control the plant disease.

"If you know the particular strain and where it came from, it will be of use in understanding how modern strains have evolved and aid in the search for sources of host resistance, which should lie in the center of origin," Ristaino said.

The researchers will describe their findings in the May issue of the science journal Mycological Research.

In addition, the scientists are compiling evidence that suggests South America was the probably source of the strain. The researchers plan to publish those results next year.

Greg Forbes, a research scientist with the International Potato Center in Lima, Peru, said the DNA research by Ristaino and her colleagues helps scientists to better understand the history of the late blight pathogen.

"The study of historical specimens is a novel and useful endeavor," Forbes said. "It is our only chance to actually get a glimpse of the past. Although we must remember that these analyses are only tiny snapshots from a large and complex series of events."

DNA Sleuthing

Ristaino's research is based on isolating and analyzing mitochondrial DNA from the P. infestans pathogen preserved in infected potato leaves. While on sabbatical in 1997, the scientist collected preserved leaves from university and museum collections in Europe and the Americas.

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