How did the potato blight happen?

Written and fact-checked by The Editors of Encyclopaedia Britannica

The Irish relied on one or two types of potato, which meant that there wasn't much genetic variety in the plants (a factor that usually prevents an entire crop from being destroyed). In 1845 a strain of water mold arrived from North America on accident, thriving in the unusually cool moist weather that year. It continued to destroy potato crops from 1846 to 1849.