Rapid determination of host receptivity in potato blight forecasting

The degree of host receptivity (disease potentia) of the potato to the late blight fungus can be measured by the C_p/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, C_p/R_s · 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 C_pR_s-ratio of the host has risen above 1,0 is the valid one. It is, therefore, important to be able to determine the C_p/R_s-ratio quickly. Good correlations have been found between C_p and the fresh weight of all underground parts less the mother tuber, and between R_s 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.