The leaves dated from 1845 to 1982, and included specimens gathered during late blight epidemics in Ireland, continental Europe, and the United States.
Based on her analysis, Ristaino and her colleagues discovered three years ago that the Ib strain of P. infestans did not cause the Irish potato famine. The culprit had to be one of three different strains.
In their new study, the scientists conducted further analysis of 186 potato plant leaves from the 19th and 20th century. The researchers found that 90 percent were infected with the P. infestans pathogen and 86 percent were infected with the pathogen's Ia strain.
The pathogen's Ib strain, however, was only detected in leaves from more modern samples gathered in Central and South America.
The finding led the scientists to conclude that the Ia strain was the culprit behind the Irish famine. "Work is now progressing to figure out where it actually came from," Ristaino said.
Stephen Goodwin, a plant pathologist at Purdue University in West Lafayette, Indiana, said the data identifying the Ia strain of P. infestans as the culprit behind the Irish potato famine are surprising but convincing.
"It's certainly not what I would have predicted, and it doesn't make sense with other data, but it seems quite clear that Ia was the most common haplotype in those samples," he said.
South American Origin?
Earlier research by Goodwin based on analysis of nuclear DNA markers from modern strains suggests that the P. infestans pathogen is associated with the US-1 genotype, or type species. The US-1 genotype originated in Mexico and is associated with the Ib strain of P. infestans.
However, Ristaino says her recent findings makes it unlikely that Mexico was the source of the plant disease behind the Irish potato famine.
Ristaino and her colleagues are working on a theory that suggests the Ia strain of P. infestans originated in the South American Andesthe birthplace of the potatoand traveled to Europe and North America via exports of potato seed on steamships.
"In the 1840s, [farmers] were not growing commercial potatoes in Mexico," she said. But they were growing them in South America and shipping them to Europe and North America."
According to Ristaino, the Ib strain of the pathogen likely evolved from the Ia strain in the Andean region. The Ib strain subsequently dispersed to the U.S., Europe, and the rest of the world during the past century.
As evidence, Ristaino cites historical documents and letters of Spanish colonizers that refer to the blight of potatoes in regions that are now Colombia and Ecuador. Indigenous peoples were also familiar with the disease, she notes.
"The diversity of the potato germplasm in the Andean region probably kept the pathogen in check, unlike modern fields where [the] potato is grown as a monoculture and susceptible varieties are widely planted," Ristaino said.
Goodwin, meanwhile, said his analysis of the same historical records indicates there is no evidence for late blight in South America prior to the Irish potato famine. "The earliest one that could possibly be late blight is from about 1890," he said. Goodwin holds to his hypothesis that P. infestans originated in Mexico.
Forbes, of the International Potato Center, said that he has not seen any information that indicates a clear migration pathway from Mexico or South America. "It seems plausible that there would be more chance of introducing the pathogen from an area where potatoes were grown as a domesticated crop," Forbes said.
Ristaino's research is supported by a grant from the National Geographic Society's Committee for Research and Exploration. Additional support comes from the U.S. Department of Agriculture, the North Carolina State Agricultural Research Service, and North Carolina State University.
SOURCES AND RELATED WEB SITES