Harold Henry Flor (1900-1991) was born and raised in St. Paul, Minnesota. He received his M.S. in 1922 and his Ph.D. in 1929. He became a USDA employee assigned to study flax diseases and based on the North Dakota State University campus at Fargo. His seminal scientific work was with flax and its rust disease fungus Melampsora lini. Working from Stakman's discovery of physiological races in stem rust fungi, Flor made crosses of both the host plant flax, and its rust fungus to determine Mendelian genetic interactions. In the simplest cases Flor's data suggested that for every gene for rust resistance in flax there was a corresponding gene for pathogenicity in the flax rust fungus. This led to his famous "gene-for-gene" interaction hypothesis. Gene-for-gene theory became a critically important paradigm in plant pathology and other scientific fields. This theory had both extraordinary utility in breeding of disease resistant cultivars and as an intellectual platform for molecular genetic understanding of host/parasite interactions.
Harold Flor received many awards for his pioneering genetic studies in host-parasite interactions, and also served as President of the American Phytopathological Society.