122 



SCIENTIFIC AGRICULTURE 



March, 1921. 



The Immunization of Plants 



G. v. McROSTIE, B.8.A., Ph.D.. Associate Professor of 

 Cereal Husbandry, Macdonahl College, P.Q. 



The term immunization is used in this paper as mean- 

 ing the changing of the nature of an individual plant in 

 such a way that it is no longer susceptible to a given pa- 

 thogene. The application of the principle of immuniza- 

 tion probably offens the ideal method of disease con- 

 trol but unfortunately our present knowledge of the best 

 means of bringing alxint such a desirable condition is as 

 yet rather unsatisfactory. In speaking of immuniza- 

 tion in plants it is necessary 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 com- 

 monly used is the term resistance. This latter term is 

 used to express the ability of a plant to develoi> 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 standixiints. First, arti- 

 ficial immunization obtained by introducing into the 

 host certain chemical elements oi- compounds, and se- 

 ond, natural innnunization by tlie isolation of resis- 

 tant individuals by straight selection or by hybridiza- 

 tion followed by selection. 



Research on the first phase of this problem has been 

 considerably stimulated by the success that has follow- 

 ed the use of anti toxins for the prevention of animal 

 diseases. Many attempts to treat plants in a similar 

 manner have been macle, usually without due recogni- 

 tion 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 artificial means have 

 failed. The problem, however, offers an interesting and 

 profitable field of investigation. 



Efforts to immunize jdants by artificial means have 

 been conducted along two definite lines, namelj' — soil 

 applications and direct injection of serums, toxin and 

 chemicals. The first named line of attack has consisted 

 chiefly in the application to the soil of fertilizing ele- 

 ments or complete commercial fertilizers or the applica- 

 tion of copper, iron or manganese sulfates. Laurent 

 (1) in 1899 investigated the rotting of potatoes by bac- 

 teria not normally parasitic and found that their attack 

 was either favored or hindered by the application of 

 different fertilizers. Pinchi (2) claimed to have de- 

 creased 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 

 produce distinct resistance to mildew, in lettuce, by the 

 addition of copper sulfate to the nutrient solution in 

 which the plants were gro^ra. Massee (4) was able to 

 immunize tomatoes against leaf mold, by watering them 

 with a weak copper sulfate solution. Norton (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 prod'-cing organism. 



The method of .direct injection of immunizing sub- 

 stances has been revived in late years. Bolley (6) ^las 

 made use of both nutrients and poisons to control disea- 

 se by injections of these substances into ti-ees. Potter 

 (7) used injections of a toxin from soft rot to exhibit 



the growth of the causal organism of this disease on 

 turnips. Campbell (8) working in Italy claims that a 

 wild .scion grafted on a cultivated stock i-enders shoots 

 from iieach and apple stocks resistant to peach leaf curl 

 and mildew respectively. Extraradicate introduction of 

 weak solutions of tailaric, citric aiid malic acids ren- 

 dered cultivated apjiles immune to mildew and to cer- 

 tain insects. 



These few instances of reported success in artificial 

 immunization are culled from a long list of experiments 

 in which the results were largely negative. 



Considerably more progress has been made in secur- 

 ing di.sease resistance in plants by natural means. This 

 pliase of investigation calls for the combined efforts of 

 the iilant pathologist and plant breeder. The plant pa- 

 thologist must search the ranks of our different species 

 for individuals that siiow more resistance than their 

 fellows to a given pathogene. That plants of a given 

 variety and similar in external appearances differ in 



Figure 1. Two varieties of the white pea bean varying in 

 their resistance to bean mosaic. Row A is badly attacked by 

 mosaic, and in consequence only a very few pods have set. 

 Row B. is resistant to this disease and has an excellent set of 

 pods. Row A. would only yield from two to three bushels 

 per acre, while Row B would yield from twenty to twenty- 

 five l.)ushels per acre. 



their ability to resist the attacks of various pathogenes 

 is now common knowledge. Of even more importance 

 from a practical standpoint is the fact that these varia- 

 tions in degree of resistance are, in most cases, definitely 

 inherited. Plants possessing resistance to any particular 

 pathogene can therefore be used as a starting point for 

 the production of resi.stant strains. 



Because of the fact that the isolation of individuals 

 already resi.stant offers the easiest and least complicat- 

 ed method of obtaining resistant varieties or strains, 

 it is to be expected that the majority of such varieties 

 or strains now in existence should have originated in 

 this manner. This accounts also for the fact that a few 

 of our disease resi.stant strains have little else to recom- 

 mend them. If what we desire in this connection al- 

 ready exists it is obviously unnecessary to seek any 

 farther. However, it often happens that a disease re- 

 sistant plant may be undesirable in other respects either 

 because of undesirable growth characteristics or a very 

 limited adaptability. Under such conditions it becomes 

 necessary for the plant breeder to make use of hybridiza- 

 tion with other types possessing the desired cpialifica- 

 tions but lacking in their ability to resist the pathogene 

 in (juestion. 



