Which crop failure in a genetically uniform monoculture killed a million people in the 1840s?

The Irish potato. The potato is propagated from tubers rather than seed, so a field is a set of clones, and the varieties grown across Ireland descended from a very narrow founding stock - which meant that when Phytophthora infestans arrived from the Americas around 1845, nothing in the population had any resistance and the crop failed in successive years.

The famine that followed was not simply a crop failure. Ireland continued to export food throughout - grain, butter, livestock - under a government whose relief policy was shaped by a conviction that intervention would corrupt the market and that the disaster was providential. Around a million died and a million emigrated, and the island's population has never returned to its 1841 level.

The genetic lesson is now standard agricultural policy and routinely ignored in practice. Almost every commercial banana is one clone, Cavendish, chosen after a fungus destroyed the previous single clone in the mid-twentieth century; a new strain of the same fungus is now spreading through Cavendish plantations. Coffee, wine grapes and much commercial wheat are similarly narrow, which is what the seed banks exist for.