On the Relation Between the Stage of Development of the Potato Crop and the Incidence of Blight (Phytophthora Infestans)

Abstract

Summary.The evidence for the common opinion that potato plants above a certain age are on that, account more susceptible to blight than younger ones is based on (1) general field observations, (2) dates of planting experiments, (3) artificial inoculation experiments. The results of such observations and experiments are discussed and summarised, and it is shown that they do not consistently support this opinion; further, it is pointed out that much more careful analysis than is usually employed is needed to interpret the results correctly.The difference in behaviour towards blight sometimes observed between assemblages of old and young plants appears in reality to be due to variation in “epidemic potentiality,” and not to change in inherent susceptibility with age. It is not necessary to assume the existence of differences in susceptibility between plants of different ages, for the differences in disease intensity and resultant rate of spread will follow from the increasing chances of the occurrence of primary infection foci as the plants become older, and from the greater chances of the environment (particularly the micro‐climate) being suitable for the development of the parasite amongst older than amongst younger plants. Accurate conclusions concerning inherent resistance or susceptibility are possible only from carefully controlled artificial inoculation experiments carried out under uniform optimum infection conditions. Seedlings are often readily attacked so that youthfulness does not confer any degree of resistance on the plant. The results of experiments hitherto carried out with older plants are conflicting, and further study of the problem on carefully planned lines is eminently desirable.