The organism which causes late blight of potato is Phytophthora infestans. The potato is native to North America, but once it was introduced to Europe, it quickly became an important food crop. Late blight did not follow its host plant across the Atlantic until much later. The disease organism grows into the stem and leaf tissues, causing death, and may also infest the tubers, which are the part of the plant that is eaten. The disease spreads rapidly under cool and damp conditions, which are common in western Europe.
In one week during the summer of 1846, this disease wiped out almost the entire potato crop of Ireland, a crop which was the primary food of the poor at that time. Nearly a million Irish died in the Great Famine, and an additional one-and-a-half million emigrated to other countries, including America.