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disease of potato and tomato plants that is caused by the fungus Phytophthora infestans. The disease occurs in humid regions with temperature ranges of between 40 and 80 F (4 and 29 C); hot, dry weather checks its spread. Potato or tomato vines that are infected may rot within two weeks. The Irish potato famines of the mid-19th century were caused by late blight. The disease destroyed more than half of the tomato crop in the eastern United States in 1946, leading to the establishment of a blight-forecasting service in 1947.