Abstract
This paper deals with the results of experiments and observations as to the pathogenicity of Phytophthora infestans which has changed after having passed through the potato varieties different in their pathogenicity.
1. A series of examinations was given to the tubers of 14 potato varieties in order to decide their susceptibility against the late-blight fungus. Consulting the results of these examinations, 42044-15 (Hybrid between Solanum demissum and S. tuberosum) and Danshakuimo (Irish Cobbler) were chosen as resistant and as susceptible varieties in the following experiments.
2. The late-blight fungus which had passed through the susceptible variety was proved to be more virulent than that which had grown on resistant variety.
3. The pathogenic fungus proved to be unstable in its infectivity even after having passed through the same variety three times in succession.
4. The pathogenicity of the fungus solely depends upon the kind of variety through which it passed finally.
