Carlow, Ireland — Ewen Mullins is the face of modern Ireland: Young, cosmopolitan, highly educated, he is a plant scientist whose work on a genetically modified potato inherently looks to the future. But Mullins also must think back to one of Ireland’s darkest chapters, the Great Famine of the 1840s.

“It’s always there,” he said. “It’s not something we forget or something we should be allowed to forget.”

From his laboratory and greenhouse in a research farm outside Carlow, 42-year-old Mullins deals daily with a disease that not only afflicts his native land but haunts it: the potato blight, a pernicious rot caused by a fungus that still thrives in Ireland’s wet, cold climate....