6o2 BACTERIA IN RELATION TO PLANT DISEASES 



diseases appear time limited, since they appear only in the spring and early summer 

 when the plant possesses a considerable amount of growing tissue. On the other hand, 

 there are many illustrations of direct correlation between susceptibility and maturing 

 or ripening tissues (various potato- tuber rots). 



There have been innumerable attempts to invoke simple explanations for these 

 phenomena of susceptibility-resistance, and to use them in generaUzations. None of 

 them, as is to be expected, has been successful. Juiciness of tissues, presence or ab- 

 sence of food or antagonistic substances such as acids, sugars, amino acids, tannins, 

 anthrocyanins, flavones, and various volatile substances, are some that have been con- 

 sidered. Recently, the factor of acidity of the cell sap has been given much attention. 

 Gardner and Kendrick' report that Bad. exitiosum, which does not tolerate a pH 

 less than 5 in culture media, readily attacks seedlings, leaves, and green fruits which 

 have a pH varying from 6.3 to 5, but does not infect mature and ripe fruits whose 

 pH usually is about 4.6. Berridge,^ in testing out twenty-eight species of bacteria, 

 fourteen of which are parasites, determined the points of maximum agglutination of 

 the organisms and their acid limits for growth. She found no consistent direct correla- 

 tion of these with acidity of the cell sap of plants. Smith and Quirk^ were not able to 

 demonstrate that tissues with pH much less than 5.7, which represents the acid toler- 

 ance of Bad. tumefaciens in culture media, are necessarily resistant to attack by the 

 organism. They conclude that possibly the preponderant kind of acid of the tissues 

 is a more important factor than the total acidity. Gillespie^ reported that certain 

 soils with a pH of about 5.2 or less generally produced a scab-free crop, and the as- 

 sumption became generally current that acidity of the soil was the limiting factor for 

 the pathogenicity of Adinomyces scabies. Sanford,^ however, reports that moisture 

 content of the soil is a more important factor than H-ion concentration. It is highly 

 probable that the mechanisms of immunity lie deeper, in the very constitution and 

 activities of the protoplasm itself. 



ACQUIRED IMMUNITY 



Attempts to demonstrate an acquired immunity in plants have failed. Brown^ 

 unsuccessfully attempted to develop resistance to Bad. tumefaciens in the Paris daisy. 

 Riker' was unable to demonstrate agglutinins or precipitins in plants having large 



' Gardner, M. W.,and Kendrick, J. B.: "Bacterial S^oi oiToma.X.0," J . Agr . Research, 21, 123-56. 

 1921. 



^ Berridge, E. M.: "The Influence of Hydrogen-ion Concentration on the Growth of Certain 

 Bacterial Parasites and Saprophytes," Ann. Appl. Biol., 11, 73-85. 1924. 



3 Smith, E. P., and Quirk, J.: "A Begonia Immune to Crown-Gall: with Observations on Other 

 Immune or Semi-immune Plants, Phytopath., 16, 491-508. 1926. 



1 Gillespie, L. J.: "The Growth of the Potato-Scab Organism at Various Hydrogen-Ion Concen- 

 trations as Related to the Comparative Freedom of Acid Soils from the Potato Scab," ibid., 8, 257-69. 

 1918. 



5 Sanford, G. B.: "The Relation of Soil Moisture to the Development of Common Scab of 

 Potato," ibid., 13, 231-36. 1923. 



' Brown, N. A. : "Experiments with Paris Daisy and Rose to Produce Resistance to Crown Gall," 

 ibid., pp. 57-99- 1923- 



7 Riker, A. J.: "The Influence of Temperature and of Previous Infection in the Development of 

 Crown Gall," ibid., 15, 45. 1925. 



