What caused the potato crops to fail during the Great Famine?

The failure of the potato crops during the Great Famine was primarily caused by a lack of genetic diversity in the potatoes grown in Ireland, which made the plants highly susceptible to disease.12

In 1845, a strain of Phytophthora infestans, a funguslike water mold, was accidentally introduced to Ireland from North America. This pathogen causes a disease known as late blight, which thrives in cool, moist conditions. Because the Irish rural population relied on only one or two high-yielding varieties that were propagated from tubers rather than seeds, the crops were genetically identical.12

The unusually wet summer of 1845 allowed the mold to spread rapidly, rotting the crops in the fields. This lack of genetic variation meant the blight could easily destroy the potato harvest, leading to repeated failures between 1846 and 1849 that devastated the population.12