Using molecular tools only recently available, Ristaino has decyphered a fragment of DNA she has collected from blighted leaves stored as specimens at the Royal Botanic Gardens in Kew, England, and other famous herbariums. Her findings should help historians and epidemiologists identify, once and for all, the source of the potato late blight pathogen that decimated Irish potato harvests from 1845 to 1849, killing by starvation more than 1 million people and forcing several million more to flee the island nation.

This will mark the first time a disease pathogen has been identified using DNA from a historic dried plant sample.

“There are so many unsolved mysteries this DNA should finally help us answer,” she says. “Where did the late blight pathogen originate? How did it spread around the world? How has it evolved over the past 150 years? Are its new genotypes, or genetic forms, different from the old ones?”

The answers to those questions have more than just historical significance, she says, because the modern form of the late blight pathogen, Phytophthora infestans, is still wiping out potato crops worldwide, despite modern fungicides and biosafety measures. Outbreaks occur each year in Mexico, Ireland, Ecuador, North Carolina and other potato-growing regions of the United States.

“Ultimately, if we can learn how the late blight pathogen has evolved, we can develop new control measures that could help eradicate future outbreaks,” Ristaino says.

To conduct her research, Ristaino extracted and amplified ribosomal DNA fragments, 100 base pairs long, from 20 blighted potato leaf specimens. The amplified DNA from the eight oldest specimens known to be infected with the late blight pathogen was sequenced, allowing Ristaino to identify for sure that the pathogen involved was late blight. She also amplified mitochondrial DNA from the dried potato leaves and plans to identify its genotype and track its historic route of migration around the world.

She has devised new diagnostic tools, using recombinant DNA techniques, that allow more rapid and accurate detection of the pathogen’s presence in potatoes before they are stored or planted. N.C. State University has been issued a patent on Ristaino’s technique.

“This research opens a window to the past,” Ristaino says. “We’re excited about our results and believe these kinds of studies of epidemics could be constructed for a range of plant pathogens.”