The Irish Potato Famine (1845-1852) was caused by Phytophthora infestans, a fungal-like organism (oomycete) that causes potato late blight disease. This pathogen arrived in Europe from the Americas in 1844-1845, likely through infected seed tubers from Mexico, and devastated Irish potato crops.
The disaster was particularly catastrophic because Ireland was extremely dependent on a single potato variety called the "Lumper" and potatoes as their primary food source. According to CIP research, the blight destroyed approximately 75% of the Irish potato crop in 1845 and nearly 100% in 1846. While the same pathogen affected continental Europe, those regions experienced less devastating consequences due to more diversified agriculture.
The human toll was enormous. Approximately 1 million people died from starvation and disease between 1845-1852, while an estimated 1.5-2 million people emigrated from Ireland, primarily to the United States, Canada, Britain, and Australia. Ireland's population plummeted from about 8.2 million in 1841 to 6.5 million by 1851, continuing to decline to 4.4 million by 1901 — and has never recovered to pre-famine levels.
The Famine fundamentally demonstrated the catastrophic risks of crop monoculture and genetic uniformity. It directly motivated early plant pathology research, with Anton de Bary's 1861 study of P. infestans considered a founding moment of the field. Remarkably, late blight remains the most economically devastating potato disease today, causing an estimated $6.7 billion in annual global losses according to 2020 CIP data.