Incredibly, actual leaves from diseased potato crops that devastated Ireland during the Great Famine still exist.
These leaves were originally taken from diseased potato plants at the Botanic Gardens in Glasnevin Dublin in 1846 and 1847. Today they are stored securely in The Herbarium at the Royal Botanic Gardens Kew, London.
177 years ago, the sight of these leaves and the smell of rotting potato plants struck fear into the heart of Irish farmers and labourers across Ireland.
The pathogen that caused the blight was Phytophthora infestans, a fungus-like microorganism that infects potato plants through their leaves, but the full story had always been a mystery.
In 2013, using DNA from these leaves, Researchers identified the specific pathogen responsible for the blight behind the Irish Potato Famine by sequencing its genome.
Phytophthora infestans originated in the Toluca Valley in Mexico, sometime after Europeans began colonising the Americas.
It turned out, contrary to expectations, that the strain that caused the Irish Potato Famine isn't US-1, which still causes blights today around the world -- it's an entirely new strain called HERB-1.
Looking through the genomes they recovered, the researchers found clear evidence that Phytophthora infestans had undergone rapid diversification shortly after the arrival of Spanish invaders in the 1500s. It spread rapidly beyond the Toluca Valley to the rest of the Americas, and to Europe.
HERB-1 appears in the early 1800s, spreading to Europe in 1845 to infect the potato crops. It only died out in the early 1900s, to be replaced by the US-1 strain that continues to destroy crops around the world to this day.