Disease Relationships in Grafted Plants and Chimaeras

Abstract

SummaryDisease relationships in grafted plants and chimaeras are shown to have both theoretical and practical significance.Grafting experiments have been employed as a means of investigating the nature of resistance and susceptibility to diseases caused by pathogenic fungi and bacteria. The effects of grafting may be direct, owing to transmission through the graft union of the substance or substances actually responsible for the reaction concerned, or indirect, due to a change in the normal response to environmental conditions. Negative results, while necessarily inconclusive, 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.Inoculation experiments have also been used in the interpretation of graft hybrids and chimaeras. In the case of the artificially induced Solanum chimaeras, experiments with Septoria lycopersici have shown that the two components retain their characteristic reaction to infection unaltered. Unless this assumption can be made in other forms, whose mode of origin is unknown, it becomes impossible to distinguish the two components, as such, from the modified tissues of a true graft hybrid. Results with the Crataegomespili and Pirocydoniae are not altogether consistent with the periclinal chimaera theory.Examples are given of the practical importance of grafting in the prevention and control of disease and mechanical injury in fruit trees and other ornamental trees and shrubs. Choice of suitable stocks may entirely prevent leaf scorch and other physiological disorders, and a large body of information has accumulated concerning the influence of root‐stock on quality and storage life of the fruit.The importance of incompatibility between stock and scion is discussed, and examples are quoted in which grafting has led to the transmission of an unsuspected virus disease. Improper fitting and tying of the graft union is liable to result in the production of wound overgrowths, and also increases the danger of external infection.

Phytophthora Infestans (Mont.) Debary and Cladosporium Fulvum Cooke on Varieties of Tomato and Potato and on Grafted Solanaceous Plants

“…On the other hand, several stocks formerly in use have been discarded owing to the prevalence of scorch and other nutritional disord.ers i n d u d in the trees grafted upon them. The various practical and theoretical banes raieed by the disease relationships of grafted plants have been fully discussed in a previous paper (9). The purpose of the present communication is codned t o a single issue only, namely, the possibility of an induced or transmitted increase or decrease in susceptibility to invasion by pathogenic fungi.…”