Phytophthora infestans is an oomycete or water mold 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.