A microscopic "plant destroyer" not only helped trigger one of the deadliest famines in modern history but also reshaped global agriculture and gave birth to an entirely new scientific discipline. Now, nearly 180 years after the Irish Potato Famine devastated Ireland and altered the course of human history, researchers are tracing the remarkable scientific journey of the organism behind it: Phytophthora infestans and the larger genus, Phytophthora.