74 PROTECTION OF PLAISiTS, 1920-21 



disease by injections of these substances into trees. Potter (7) used injections 

 of a toxin from soft rot to inhibit the growth of the causal organism of this di- 

 sease on turnips. Campbell, (8) working in Italy, claims that a wild scion grafted 

 on a cultivated stock re^ders shoots from peach and apple stocks resistant to 

 peach leaf curl and mildew respectively. Extraradicate introduction of weak 

 solutions of tartaric, citric and malic acids rendered cultivated apples immune to 

 mildew and to certain 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 securing disease resistance 

 in plants by natural means. This phase of investigation calls for the combined 

 efforts of the plant pathologist and plant breeder. The plant pathologist must 

 search the ranks of our different species for individuals that show more resis- 

 tance than their fellows to a given pathogene. That plants of a given variety 

 and similar in external appearances differ in their ability to resist the attacks of 



Figure 1. — Two varieties of tlie white pea bean varying in tlieir 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 bushels 

 per acre. 



various pathogenes is now common knowledge. Of even more importance 

 frorn a practical standpoint is the fact that these variations in degree of resistance 

 are, in most cases, definitely inherited. Plants possessing resistance to any par- 

 ticular pathogene can therefore be used as a starting point for the production of 

 resistarit strains. 



Because of the fact that the isolation of individuals already resistant offers 

 the easiest and least complicated 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 resistant strains have little else to recommend them. 

 If what we desire in this connection already exists, it is obviously unnecessary 

 to seek any farther. However, it often happens that a disease resistant plant 

 may be undesirable in other respects either because of undesirable growth cha- 



