Scientists Reveal What Triggered Irish Potato Famine

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

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Phytophthora infestans changed the course of history. Even today, the Irish population has still not recovered to pre-famine levels. “We have finally discovered the identity of the exact strain that caused all this havoc,” says Hernán Burbano from the Max Planck Institute for Developmental Biology in Germany.

For research to be published in eLife, a team of molecular biologists from Europe and the U.S. reconstructed the spread of the potato blight pathogen from dried plants. Although these were 170 to 120 years old, they were found to have many intact pieces of DNA.

“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,” according to Kentaro Yoshida from The Sainsbury Laboratory in Norwich.

The researchers examined the historical spread of the fungus-like oomycete Phytophthora infestans, known as the Irish potato famine pathogen. A strain called US-1 was long thought to have been the cause of the fatal outbreak. The current study concludes that a strain new to science was responsible. While more closely related to the US-1 strain than to other modern strains, it is unique. “Both strains seem to have separated from each other only years before the first major outbreak in Europe,” says Burbano.

The researchers compared the historic samples with modern strains from Europe, Africa, and the Americas as well as two closely related Phytophthora species. The scientists were able to estimate with confidence when the various Phytophthora strains diverged from each other during evolutionary time. The HERB-1 strain of Phytophthora infestans likely emerged in the early 1800s and continued its global conquest throughout the 19th century. Only in the 20th century, after new potato varieties were introduced, was HERB-1 replaced by another Phytophthora infestans strain, US-1.

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