REPORT OF THE SOCIETY 73 



THE IMMUNIZATION OF PLANTS 



G. P. McRostie, B.S.A., Ph. D. 



The term immunization is used in this paper as meaning the changing of 

 the nature of an individual plajit in such a way that it is no longer susceptible to 

 a given pathogene. The application of the principle of immunization prob- 

 ably offers the ideal method of disease control, but, unfortunately, our present 

 knowledge of the best means of bringing about such a desirable condition is as 

 yet rather unsatisfactor^^ In speaking of immunization in plants it is neces- 

 sary for us to keep in mind that actual immunity very seldom exists in the plant 

 world. What expresses nearer the truth and is now more commonly used is the 

 term resistance. This latter term is used to express the ability of a plant to 

 develop and function normally under conditions such that other plants of the 

 same species would fail to develop or be destroyed. 



The whole problem of the immunization of plants may be considered from 

 two stan'dpoints. First, artificial immunization obtained by introducing into 

 the host certain chemical elements or compounds, and secoTid, natural immun- 

 izatiqn by the isolation of resistant individuals by straight selection or bj'- hyb- 

 ridization followed by selection. 



Research on the first phase of this problem has been considerably stim- 

 ulated by the success that has followed the use of antitoxins for the prevention 

 of animal diseases. Many attempts to treat plants in a similar manner have 

 been made, u?uallj' without due recognition of the totally different structure and 

 mode of life of plants and animals. It is to be expected that due to the crudeness 

 of the methods employed, the majority of attempts to immunize plants by art- 

 ificial means have failed. The problem, however, offers an interesting and 

 profitable field of investigation. 



Efforts to immunize plants by artificial means have been conducted along 

 two definite lines, namely — soil applications aiid direct injection of serums, 

 toxin and chemicals. The first named line of attack has consisted chiefly in the 

 application to the soil of fertilizing elements or complete commercial fertilizers 

 Of the application of copper, iron or manganese sulfates. Laurent (l),in 1899 

 investigated the rotting of potatoes by bacteria not normally parasitic and f oujid 

 that their attack was either favored or hindered by the application of different 

 fertilizers. Pinchi (2) claimed to have decreased the severity of the attacks of 

 grape mildew by applying copper sulfate to the soil in close proximity to the 

 attacked vines. Marchal (3) reports being able to produice distinct resistance to 

 mildew, in lettuce, by the addition of copper sulfate to the nutrient solution in 

 which the plants were grown, Massee (4) was able to immunize tomatoes 

 against leaf mold, by watering them with a weak copper sulfate solution. Nor- 

 ton, (5) in determining the effect of various chemicals in solution applied to the 

 roots of tomato plants for the control of leaf diseases, obtained on the whole 

 negative results, although a few chemicals seemed to produce added powers of 

 resistance in the plants when inoculated with the disease producing organism. 



The method of direct injection of immunizing substances has been revived 

 in late years. Bolley (6) has made use of both nutrients and poisons to control 



