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 potatoes, which meant that there wasn't much genetic variety in the plants (diversity is a factor that usually prevents an entire crop from being destroyed). In 1845 a strain of water mold accidentally arrived from North America and thrived in the unusually cool moist weather that year. It continued to destroy potato crops from 1846 to 1849.