POWDERY mildew on potato was first recorded in Great Britain in 1932, when the oidial stage was found on the leaves of seedling potatoes in a glasshouse at Cambridge1. It was again observed at Cambridge in the late summer of 1945, and a field survey was made to determine its prevalence. The mildew was found not only on seedling plants in the greenhouse, but also on potatoes in trial plots in the open. A search was then made for it in the immediate neighbourhood, and later this was extended to cover an area within a ten-mile radius of Cambridge.