Irish potato famine mystery solved

The Great Famine of Ireland reduced the population by an estimated 20-25%; approximately 1 million people died and a million more emigrated from the country. The potato blight responsible for the famine effectively changed the course of Irish history: even today, the Irish population has reportedly still not recovered to pre-famine levels.

While the fungus-like oomycete Phytophthora infestans has been identified as the culprit, an international team of scientists has recently found that a unique strain of this potato blight - which they call HERB-1 - was specifically responsible for the famine.

This is the first time scientists have decoded the genome of a plant pathogen and its plant host from dried herbarium samples, opening up a new area of research in understanding how pathogens evolve and how human activity impacts the spread of plant disease.

To uncover the mystery strain, the researchers deciphered the entire genomes of 11 samples of Phytophthora infestans from potato leaves collected from Ireland, the UK, Europe and North America. The samples had been preserved in the herbaria of the Bavarian State Collection for Botany in Munich and the Kew Gardens in London. Although the samples were up to 170 years old, they had enough intact DNA for the scientists to analyse.

A strain of Phytophthora infestans called US-1 had previously been named as the cause of the potato blight that caused such destruction in Ireland. However, by comparing the historic samples with modern strains, the scientists were able to estimate with confidence when the various Phytophthora strains diverged. The HERB-1 strain emerged in the early 1800s; the US-1 strain only emerged in the 20th century, after new potato varieties were introduced.

In addition, the scientists found several parallels between historic events and divergences in Phytophthora strains. The first contact between Europeans and native Americans in Mexico in the 16th century coincided with a considerable increase in Phytophthora’s genetic diversity. The spread of the pathogen from its centre of origin in the Toluca Valley, Mexico, would have accelerated its evolution, the scientists say.

The research team’s success in deciphering entire genomes from dried samples could open the way for future research into plant pathogens.

“Herbaria represent a rich and untapped source from which we can learn a tremendous amount about the historical distribution of plants and their pests - and also about the history of the people who grew these plants,” said Kentaro Yoshida from The Sainsbury Laboratory in Norwich, a lead author of the study.

“The degree of DNA preservation in the herbarium samples really surprised us,” said Johannes Krause from the University of Tübingen, a co-author of the study. Because of the DNA quality and quantity in the herbarium samples, the research team could evaluate the entire genome of Phytophthora infestans and its host, the potato, in just a few weeks.

“Perhaps this strain became extinct when the first resistant potato varieties were bred at the beginning of the twentieth century,” Yoshida said. “What is for certain is that these findings will greatly help us to understand the dynamics of emerging pathogens. This type of work paves the way for the discovery of many more treasures of knowledge hidden in herbaria.”

The research was published in an article, The rise and fall of the Phytophthora infestans lineage that triggered the Irish potato famine, in the journal eLife.