Phytophthora infestans, the fungus that invades the potato plant and causes its rapid decay, struck for the first time in the eastern United States in the summer of 1843. The invisible fungus spores were transported to Belgium in a cargo of apparently healthy potatoes, and in the summer of 1845 the fungus revived and reproduced, devastating the potato crop in Flanders, Normandy, Holland and southern England. By August of 1845 the blight was recorded at the Dublin Botanical Gardens, and a week later, a total failure of the crop was reported in County Fermanagh. By October, there was panic in the west of Ireland as the blight destroyed healthy potatoes harvested in August.

In 1845, the blight had been localized and variegated, but from early reports in 1846 it was obvious that blight had affected the potato crop throughout Ireland. While it had been difficult to obtain an accurate estimate of the damage caused by the blight in 1845, as its appearance was spread over a number of months, in 1846 the destruction of the potato crop was as rapid as it was comprehensive. The nutritious potato had been the mainstay of the agricultural laborers and cottier class and dominated the diets of at least two-thirds of the population. No other country in Europe depended on the potato as extensively as Ireland.

When the blight hit the first year it was a disaster for those who depended on the potato. When the blight returned in the following years it meant death for many of those who were already living precariously at subsistence level, and emigration for those who had the resources to flee disease, death and poverty.