Scientists have used plant samples collected in the mid-19th Century to identify the pathogen that caused the Irish potato famine.
A plant pest that causes potato blight spread to Ireland in 1845 triggering a famine that killed one million people.
DNA extracted from museum specimens shows the strain that changed history is different from modern day epidemics, and is probably now extinct.
Other strains continue to attack potato and tomato crops around the world.
The fungus-like infection causes annual losses of enough potatoes to feed hundreds of millions of people a year.
A team led by The Sainsbury Laboratory, Norwich, traced the global spread of potato blight from the early 1800s to the present day.
Until now, it has been unclear how early strains of Phytophthora infestans are related to those present in the world today.
High-tech DNA sequencing techniques allowed them to decode ancient DNA from the pathogen in samples stored as early as 1845.
These were compared with modern-day genetic types from Europe, Africa and the Americas, giving an insight into the evolution of the pathogen.
"This strain was different from all the modern strains that we analysed - most likely it is new to science," Prof Sophien Kamoun of The Sainsbury Laboratory told BBC News.
"We can't be sure but most likely it's gone extinct."