In North America there is a belt of wheat cultivation running from Texas in the south, up the Mississippi valley, to the Canadian prairies in the north. Winter wheat is grown in the south, and spring wheat in the north, the spring wheat area straddling the Canada/US border. Wheat is subject to epidemics of stem rust caused by the fungus Puccinia graminis. This fungus does not survive the winter in the north, except in now rare barberry bushes which do not come into my story. It overwinters in the south, in Texas and Oklahoma largely, and moves northwards in spring and summer. In 1953 and 1954 it caused severe epidemics in the spring wheat area. Spores of the fungus blew up from the south in quantity long before the spring wheat ripened. These spores infected the spring wheat; and because the fungus starts a new generation every 7-10 days, it built up to destructive levels before the ears were full. Yields were down, and the grain shrivelled with a low bushel weight. Most of the spring wheat varieties at that time contained the stem rust resistance gene Sr9d. Here Sr stands for stem rust, and the 9d means that it was the fourth resistance allele to be assigned to locus 9. This was not the only Sr gene in spring wheat at that time, but it is the only one relevant to my story of that particular epidemic. Sr9d had held the fort during the 1940's when stem rust caused little damage in the spring wheat area. But in the early 1950's it failed to protect. The change was not in the wheat, but in the fungus population. During the 1940's the proportion of the stem rust population with virulence on Sr9d was small. In other words, the fungus population coming northwards was not