The Phytophthora fungus, which causes late blight…

The Phytophthora fungus, which causes late blight in potatoes, arrives accidentally in Ireland from North America in 1845.

This same year, Ireland has unusually cool, moist weather, in which the blight thrives.

The disease, which breaks out on September 9, 1845, destroys both the leaves and the edible roots, or tubers, of the potato plant.

Accordingly, much of this year's potato crop rots in the fields, beginning the Great Irish Famine.