"I got chills the first time I held these," Jean Beagle Ristaino says. She gingerly spreads the shriveled leaves of five potato plants on a worktable. They look as if they’ve been moldering away under a refrigerator somewhere. But Ristaino, a plant pathologist at North Carolina State University in Raleigh, has been using them as Exhibit A in a demonstration of molecular detective work so deft one learned journal recently dubbed her "the Sherlock of Spuds."
Scientists have long assumed they knew what caused the potato blight that starved a million Irish in the mid-19th century: the 1b strain of the funguslike water mold called Phytophthora infestans, a disease that is believed to have originated in Mexico. Ristaino did not doubt that some strain of P. infestans was the cause, but she was not so sure about 1b. So she and her colleagues studied DNA from infected potato leaves collected in Ireland, Britain and France during the mid-1840s.
DNA, or deoxyribonucleic acid, is the double strand of molecules that carries a cell’s unique genetic code. Within the past decade or so, DNA analysis of criminal evidence has overturned the convictions of more than 100 inmates in U.S. prisons, including 12 on death row. Ristaino’s project marked the first time the technique had been used to investigate a historic plant disease, and last year she asserted that it proved 1b was not the culprit after all. "This was a fantastic application of a novel technique that told us we were on the wrong path," says Greg Forbes of the International Potato Center, a research facility in Lima, Peru.
But if 1b wasn’t responsible for the Irish famine, which strain was? Did it originate in Mexico or somewhere else? If we knew how the disease evolved, could modern outbreaks be prevented? Today, Ristaino is wrestling with these vexing questions.
The study of plant diseases is not for everyone. Those who do this specialized work—the American Phytopathological Society (APS), based in St. Paul, Minnesota, lists nearly 5,000 members—tend to be driven perfectionists. Typically a researcher inoculates healthy plants with fungi, a virus or bacteria to learn how a disease works. Breakthroughs like Ristaino’s are rare, but when they do occur they can change history.
In 1977 Eugene Nester of the University of Washington in Seattle discovered that a bacterium that produces plant tumors actually incorporates its own DNA into the DNA of a host plant’s cells. In time, that insight led scientists to be able to replace that bacterium’s "bad" genes with "good" ones, and use the technique to engineer disease- and herbicide-resistant cotton and soybeans. In 1993, Gregory Martin of the Boyce Thompson Institute for Plant Research in Ithaca, New York, became the first pathologist to clone a disease resistance gene. More than two dozen such genes have since been cloned, enabling breeders to grow hardier strains of tobacco, tomato, rice and flax.
Ristaino’s revelation that 1b did not cause the Irish potato famine is controversial—some researchers question her methodology—but her further suggestion that P. infestans could have originated in South America undercuts conventional wisdom. "There’s a big debate about this," cautions Stephen Goodwin, a U.S. Department of Agriculture (USDA) plant pathologist at Purdue University, whose own genetics studies a decade ago shored up the theory of a Mexican origin. "More work needs to be done." Meanwhile, fungicide-resistant new strains of the old disease are devastating potato fields all over the world.
A sixth-generation native of Washington, D.C., Ristaino entered the University of Maryland as a history major, then switched to biology. As a student at the USDA’s sprawling research center in Beltsville, Maryland, she worked part-time washing petri dishes and hauling sacks of alfalfa. One day she stumbled upon an enormous fungus collection squirreled away in the basement. "That’s where I discovered plant pathology," she says, her blue eyes glittering. "Lots of old specimens, things like rotten potatoes in bottles that were collected by famous pathologists."
While at Maryland, she married Andre Ristaino, and they soon headed west for the University of California at Davis, where she earned her doctorate in 1987. A few months later, she started her work in Raleigh. In the mid-1990s, Ristaino read about scientists who were analyzing DNA from such ancient materials as dinosaur bones and prehistoric insects preserved in amber. "I thought, 'Why not DNA from a plant pathogen?'" Her interest in the potato blight was underscored at a 1995 conference she attended in Ireland on the 150th anniversary of the Great Hunger. Two years later, she took a sabbatical. Poring through herbarium sheets containing infected potato plants, she collected the leaves from which she extracted the tiny bits of P. infestans DNA. She also came upon a trove of fascinating letters written by scientists during and after the famine. In the course of her study, Ristaino learned that when P. infestans began its fateful journey to Ireland, male laborers there typically consumed 12 to 14 pounds of potatoes a day. "People didn’t understand that pathogens cause disease," she says. "The Irish blamed bad weather, or the spirits, or acts of God." Today, in addition to figuring out how the blight mutates in infected modern potatoes to "outwit" fungicide, she continues a historic investigation into the disease’s true origins. "Gene genealogy," she calls it. Eventually it should lead to improved methods of diagnosing the disease, tracking its spread and controlling it in the field.
As for the resistance to her findings, the 45-year-old mother of two is undaunted. "I knew I was challenging major dogma," she says. "That’s part of the scientific process." And a small price to pay, she feels, for the knowledge that results. "This pathogen precipitated a major movement of people around the world and devastated an entire country," she adds. "What could be more exciting than trying to figure it out?"