The potato was introduced to Europe in 1536, and subsequently by European mariners to territories and ports throughout the world. Thousands of varieties persist in the Andes, where over 100 varieties might be found in a single valley, and a dozen or more might be maintained by a single agricultural household. Once established in Europe, the potato soon became an important food staple and field crop. But lack of genetic diversity, due to the fact that very few varieties were initially introduced, left the crop vulnerable to disease. In 1845, a plant disease known as late blight, caused by the fungus-like oocmycete Phytophthora infestans, spread rapidly through the poorer communities of western Ireland, resulting in the crop failures that led to the Great Irish Famine.