In this region over a period of several years one million people died and more than a million left to populate other countries, including the United States, with Irish immigrants. The terrible episode prompted the enactment of new laws, scientific thought, and literature. The white potato (Solanum tuberosum), known today as the Irish potato, is native to high elevations in the Peruvian Andes.  While it was an important food crop worldwide, in Ireland the potato was an essential source of calories among the poor.  Late blight of potato, which decimated potato crops, is caused by the oomycete, Phythophthora infestans,and it likely was introduced from the central uplands of Mexico to the continent of Europe and then to Ireland in 1845. The first symptoms of the blight included brown spots on plant leaves and damp spots on the tuber that spread quickly throughout the tissue. Potatoes rotted in the field and in storage. Late blight destroyed potato plants and was the principal cause of what came to be known as the Irish Potato Famine. The devastating plant disease wiped out potato crops in successive years of 1845, 1846, and again in 1848, the worst year being "Black ’47," but food shortages continued for several more years. Is the potato blight a worry of the past? In a way, yes, because most people have more varied diets that the Irish of the late 1840s. There is, however, renewed worry about the introduction of new strains of the fungus-like pathogen that make sexual reproduction possible. Recombination of genetic material during sexual reproduction may overcome the resistance of the many varieties of potatoes developed by plant breeders since the original outbreak. 

--S. N. George