In spite of more active fungicides and improved methods of application, including the use of aircraft, the control of potato blight is not very much better than it was thirty years ago. The basic weakness appears to be the impossibility of achieving and maintaining anything like a complete fungicide cover on the foliage of a growing crop of potatoes. In every season in which the weather favours the parasite a time comes when the number of blight spores is so great that many of the gaps in the fungicide defence cover are bound to be penetrated. Once a number of infections have thus been established in a crop, the progress of the disease is only a matter of time and weather, for each of the new infections normally becomes an ineradicable centre for further spread. If this diagnosis is essentially correct, the chances of major improvements in blight control with our present methods are small. It is therefore desirable to examine the history of blight epidemics in detail, to see if there is any possibility of delaying the build-up of the spore inoculum. This would give the protective fungicides a better chance of holding off the attack until the crop had produced its full potential yield.