Once thought to be a fungus, the protist Phytophthora infestans—an oomycete—is of the potato plant’s most lethal pathogens. It causes what is known to many farmers and gardeners as the “late blight” of potato—a disease that has popped up in numerous places throughout the centuries. It was first noticed in the 1500s in Europe; a few centuries later, it was responsible for the Irish potato famine of 1845 to 1849 (it’s estimated that 400,000 people perished during the famine due to malnutrition).

P. infestans thrives and produces spores under humid and moist environments; thus, each year, the temperature and rainfall affect the timing and severity of the late blight disease. It decays the leaves and stem of the potato plant, eventually causing the tuber to stop growing; from there, the tubers rot after they are attacked by the pathogen.