Abstract
SUMMARYExperiments are described which are concerned with the possibility of an increase or decrease in susceptibility to pathogenic fungi, induced or transmitted by grafting. Two diseases are investigated, namely, potato blight, caused by Phytophthora infestans (Mont.) dc By., and tomato leaf mould, caused by Cladosporium fulvum Cke.Preliminary inoculation experiments were carried out on eleven large‐fruited varieties of tomato, eight varieties of potato, various small‐fruited tomatoes, and other Solanaceous plants, with the following results: Phytophthora infestans. All the varieties of potato were equally susceptible, but certain varieties only of tomato, viz. Giant Red, Golden Queen. Detached leaves of the currant tomato were also infected. Cladosporium fulvum. Only tomato varieties and certain smail‐fruited tomatoes were susceptible. Giant Red and Golden Queen were particularly susceptible, Stirling Castle and Maincrop were relatively resistant. The currant tomato (Lycopersicum racemigerum Lange or L. pimpinellifolium Dun.) was immune. No support can be given to Schmidt's suggestion that immunity depended on the presence of a substance termed “prohibitin”, capable of inhibiting spore germination. It is suggested that the identity of Schmidt's cultures was mistaken.About fifty grafts were made involving various combinations of resistant and susceptible plants. The grafts were inoculated, in most cases, after new growth had developed from stock and scion. Without exception, the results of these experiments were the same as on the ungrafted material, both stock and scion retaining their characteristic reaction to infection unaltered. Although negative results cannot be conclusive in themselves, they indicate that resistance and susceptibility are either genotypic properties of the protoplasm or else are due to some factor that is not, as such, transmissible from stock to scion or vice versa.
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