Phytophthora infestans

Phytophthora infestans is an oomycete that causes the serious potato disease known as late blight or potato blight. (Early blight, caused by Alternaria solani, is also often called "potato blight".) Late blight was a major culprit in the 1840s European, the 1845 Irish and 1846 Highland potato famines. The organism can also infect tomatoes and some other members of the Solanaceae. At first, the spots are gray-green and water-soaked, but they soon enlarge and turn dark brown and firm, with a rough surface.