The degree of host receptivity (disease potentia) of the potato to the late blight fungus can be measured by the Cp/R8-ratio (see text). In blight forecasting this ratio can be used to reveal the duration of a period of non-receptivity (the physiological barrier phase, Cp/Rs · 1,0) in early mid-growth. Periods of weather suitable for the fungus that occur during this phase are non-valid but the first weather period to occur after the CpRs-ratio of the host has risen above 1,0 is the valid one. It is, therefore, important to be able to determine the Cp/Rs-ratio quickly. Good correlations have been found between Cp and the fresh weight of all underground parts less the mother tuber, and between Rs and the fresh weight of the shoot. A table of estimated values is given, and a suitable field kit is described. Results from its use fit well with those obtained by analysis during the period of non-receptivity of the host, but are less precise for other stages of growth. Bright sun can delay or arrest the early stages of a blight epidemic, whilst the fungus Botrytis cinerea can combine with the blight fungus to accentuate an epidemic.