The late blight disease of potatoes, which triggered the great Irish famine of 1845-1849, remains one of the most feared and intractable plant diseases today. Decades of dispute about the cause of the disease followed the outbreak of 1845, and the scientifi c controversy illustrates the uneasy historical relationship among farmers, scientifi c agronomists, and plant pathologists. Consensus fi nally emerged that the fungus Phytophthora infestans was the true cause of the disease, but that organism's full life cycle remained obscure. Its sexual oospores could not be readily obtained by mycologists, despite sporadic reports that had been observed. The 20th century opened with great optimism that resistant varieties could be developed using dominant R-genes obtainable from some wild species, and this optimism led to a proliferation of public breeding programs between 1925 and 1935. But these hopes had foundered by the early 1950s with the inexplicable appearance of new fungal races that could overwhelm the most blight-resistant germplasm. The Rockefeller Foundation's postwar agricultural initiative in Mexico led during the 1950s to dramatic and unexpected solutions to some of the late blight puzzles. But even then the fungus remained obscure, and effective, non-chemical control methods have never been forthcoming. This article examines the historical frustrations of late-blight science and advances that history as a case study illustrating the rise and fall of an ““heroic age”” of resistance breeding and plant pathology in the first half of the 20th century.