Scientists have identified a unique strain of blight which they believe triggered Ireland's potato famine in the mid-19th century.
They have called the strain HERB-1.
The deadly famine has been linked to a fungal disease called potato blight, which came to Ireland from Mexico.
A team of molecular biologists from Europe and the US reconstructed the spread of the potato blight pathogen from dried plants.
Many of the materials, despite being between 120 to 170 years old, contained intact pieces of DNA, according to the details published in the journal eLife.
A strain called US-1 was thought to have been the cause of the fatal outbreak but the new study suggests a different but closely-related strain may have been the cause.
Hernan Burbano from the Max Planck Institute for Developmental Biology said: “Both strains seem to have separated from each other only years before the first major outbreak in Europe.”
Potato blight is caused by the microorganism, Phytophthora infestans, which destroys the leaves of potato crops.
The researchers compared the historic samples with modern strains from Europe, Africa and the Americas as well as two closely-related strains of Phytophthora which are destructive parasitic fungi species which cause brown rot in plants.
The scientists believe the destructive strains diverged from each other.
It was claimed: “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 twentieth century, after new potato varieties were introduced, was HERB-1 replaced by another Phytophthora infestans strain, US-1.”
Potato blight in Ireland in the 1840s caused a potato shortage which saw more than one million people die.
The international team of scientists looked at 11 historical samples of Phytophthora infestans from potato leaves collected over more than 50 years.
These came from Ireland, the UK, Europe and North America and had been preserved in the herbaria of the Botanical State Collection Munich and Kew Gardens in London.