The object is to join a susceptible scion to a rootstock that is either resistant to the attacking organism or that encourages resistance in the scion. The graft can be between plants of the same species or even between different species.
The review cited here examines all recent research on grafting for the control of soil-borne pest and disease organisms, as well as foliar diseases such as tomato spotted wilt and pepino mosaic virus. The main approach has been the selection and breeding of resistant rootstocks. However, in addition to selecting rootstocks for their resistance to disease, they can also be selected for their ability to encourage disease resistance in the scion.
Many resistant rootstocks also possess a vigorous root system and this probably enables them to take up more nutrients, thus boosting the growth of the scion. Roots are also the source of cytokinins, a group of naturally-occurring growth-promoting chemicals, and any increase in their production will benefit scion growth. A further indirect benefit can be gained by enhancing the uptake of silicon, for example, because silicon can enhance plant resistance to some foliar diseases.
Dr Ken Cockshull, Associate Fellow, Warwick Crop Centre, University of Warwick
Defence Mechanisms Involved in Disease Resistance of Grafted Vegetables by Guan, Zhao, Hassell and Thies (2012). HortScience 47 (2): 164-170. The author's abstract can now be seen in full at http://hortsci.ashspublications.org but ISHS members can view HortScience on the website www.ishs.org.