Great Famine | Definition, Causes, Significance, & Deaths (2024)

Great Famine, famine that occurred in Ireland in 1845–49 when the potato crop failed in successive years. The crop failures were caused by late blight, a disease that destroys both the leaves and the edible roots, or tubers, of the potato plant. The causative agent of late blight is the water mold Phytophthora infestans. The Irish famine was the worst to occur in Europe in the 19th century.

Cause of the Great Famine

In the early 19th century, Ireland’s tenant farmers as a class, especially in the west of Ireland, struggled both to provide for themselves and to supply the British market with cereal crops. Many farmers had long existed at virtually the subsistence level, given the small size of their allotments and the various hardships that the land presented for farming in some regions. The potato, which had become a staple crop in Ireland by the 18th century, was appealing in that it was a hardy, nutritious, and calorie-dense crop and relatively easy to grow in the Irish soil. By the early 1840s almost half the Irish population—but primarily the rural poor—had come to depend almost exclusively on the potato for their diet. Irish tenant farmers often permitted landless labourers known as cottiers to live and work on their farms, as well as to keep their own potato plots. A typical cottier family consumed about eight pounds of potatoes per person per day, an amount that probably provided about 80 percent or more of all the calories they consumed. The rest of the population also consumed large quantities of potatoes. A heavy reliance on just one or two high-yielding types of potatoes greatly reduced the genetic variety that ordinarily prevents the decimation of an entire crop by disease, and thus the Irish became vulnerable to famine.

In 1845 a strain of the water mold Phytophthora infestans, which causes late blight in potatoes (as well as tomato plants), arrived in Ireland accidentally from North America. When plants become infected with it, lesions appear on the leaves, petioles, and stems. A whitish growth of spore-producing structures may appear at the margin of the lesions on the underleaf surfaces. Potato tubers develop rot up to 15 mm (0.6 inch) deep. Secondary fungi and bacteria often invade potato tubers and produce rotting that results in great losses during storage, transit, and marketing. Hot dry weather checks the spread of Phytophthora, but in 1845 Ireland had unusually cool moist weather, which allowed the blight to thrive. Much of that year’s potato crop rotted in the fields. That partial crop failure was followed by more-devastating failures in 1846–49, as each year’s potato crop was almost completely ruined by the blight.