In 2013, an international team of scientists finally identified the strain of potato blight that caused the potato crops of Ireland to fail, spurring on genocidal negligence on behalf of the British and leading to a million deaths and mass starvation, and two million Irish migrants fleeing the country.
It has been known that Phytophthora infestans, a fungus-like organism that devastated potato crops, led to the famine in Ireland.
But the precise strain of the pathogen that caused the devastating famine from 1845 to 1852 was a mystery until 2013, when the strain of potato blight was christened HERB-1 by the biologists.
Study co-author Hernán Burbano, a researcher at the Max Planck Institute for Developmental Biology in Germany, said in a statement: “We have finally discovered the identity of the exact strain that caused all this havoc.”
The report says that a Phytophthora strain called US-1 was thought to have triggered the potato famine.
But, by sequencing the genomes of preserved samples of the plant pathogen, the researchers discovered that a different strain, new to science, was the real culprit.
Researchers studied 11 historic samples from potato leaves that were collected about 150 years ago in Ireland, the United Kingdom, Europe, and North America.
The ancient samples, preserved at the Botanical State Collection Munich and the Kew Gardens in London, still had many intact pieces of DNA.
The DNA quality was so good the researchers were able to sequence the entire genome of Phytophthora infestans and its host, the potato, within just a few weeks.
Johannes Krause, a professor of paleogenetics at the University of Tübingen in Germany and a study co-author, said: “The degree of DNA preservation in the herbarium samples really surprised us."
The decoded genomes were compared with modern Phytophthora strains from Europe, Africa, and the Americas and the results enabled the researchers to trace the evolution of the pathogen, including where and when the HERB-1 and US-1 strains likely diverged.
Their report determined that Phytophthora infestans originated in Mexico’s Toluca Valley.
When Europeans and Americans first came to Mexico in the 16th century, the pathogen experienced increased genetic diversity, and in the early 1800s, the HERB-1 Phytophthora strain emerged and was brought out of Mexico, the researchers said.
They added that by the summer of 1845, the HERB-1 strain had arrived at European ports, and the potato disease spread throughout Ireland and the United Kingdom, causing the Irish potato famine.
In the 20th century, as new varieties of potatoes were introduced, the HERB-1 strain was eventually replaced by the US-1 Phytophthora strain, the researchers said.
The study says the evolutionary change may have been spurred by the introduction of new crop breeding methods, which suggests that breeding techniques may affect the genetic makeup of plant pathogens.
Lead author Kentaro Yoshida, a researcher at The Sainsbury Laboratory in the United Kingdom, speculated: "Perhaps this strain became extinct when the first resistant potato varieties were bred at the beginning of the twentieth century.
“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.”
* Originally published in 2016, updated in Sept 2023.