What caused the Irish potato famine of the 1840's?

The Irish potato famine of the 1840s was caused by potato blight. The disease was caused by Phytophthora infestans, a potato pathogen that destroys leaves and rots tubers, making the crop inedible.

Blight reached Ireland in 1845 and returned with especially devastating force in 1846. Ireland was unusually vulnerable because millions of people, particularly poor tenant farmers, depended heavily on potatoes as their main food. Many farms also relied on genetically similar potato varieties, allowing the disease to spread rapidly.

The blight explains the crop failure, but the scale of the catastrophe also reflected poverty, land tenure, government policy, and Ireland's dependence on a single staple crop. The Great Famine lasted roughly from 1845 to 1852. About one million people died, while more than one million emigrated.

A common mix-up is to name British policy or landlordism as the direct cause. Those factors worsened the famine and shaped its consequences; blight was the immediate biological trigger.