Scientists believe they have finally identified the pathogen that caused the Irish potato famine. BBC News reports a research team led by The Sainsbury Laboratory in Norwich, England, used dried leaf cuttings — some of which are nearly 170 years old — to reconstruct the spread of the HERB-1 strain of Phytophthora infestans, a fungal disease that came to Ireland via Mexico in 1845. The disease destroyed potato crops and caused the deaths of a million people.

Researchers originally thought that the US-1 strain of the fungal disease was the cause of the famine. However, it is now credited with replacing the devastating HERB-1 variant and is now dominant around the world — having been helped by an evolution in crop breeding methods. By using advanced DNA sequencing techniques, UK, US, and German scientists were able to decode the genome of the plant pathogen and compare it with modern strains, identifying when it emerged, how it spread, and its extinction throughout the 19th century.

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