LEAFLET X 



INOCULATIONS WITH BACTERIA CAUSING 

 PLANT DISEASE 



Introduction 



The methods for studying the pathogenicity of bacteria in plants, 

 and for making a few selected cognate investigations are briefly 

 treated in this Leaflet. The procedures, in relation to handling certain 

 organisms and to studying the diseases they induce, vary so widely 

 that no given directions apply to the group as a whole. The selected 

 representative methods included are thus to be considered primarily 

 as guides to the beginner, and are to be modified as circumstances war- 

 rant. 



Difficulty in interpretation is frequently encountered from varia- 

 tions in results, depending on the methods used. A given bacterial 

 character may sometimes be positive when measured by one method 

 and be negative when measured by a slightly different technic. Stu- 

 dents should employ a known positive and a known negative as 

 controls when making critical determinations. The method used 

 should always be given or cited when a character is listed, so that 

 the validity of the character can be correspondingly estimated by 

 the reader. Some of the technical pitfalls to be avoided have been 

 listed by Frobisher (1933). 



A number of topics discussed in Leaflet VII regarding bacteria 

 pathogenic on animals are applicable to bacteria pathogenic on plants. 

 These include particularly: (1) identification of the active agent as 

 the bacterial cell or its products; (2) distinction between invasion 

 and the power to cause disease after entry; (3) variability in virulence 

 of the pathogen, which requires single-cell cultures, and in suscepti- 

 bility of the host, which frequently calls for plants with known genetic 

 constitution, when critical studies are involved; and (4) relations 

 between reactions induced in the test tube and in the host. 



The pathogenicity of a microorganism may be proved by fulfilling 

 Koch's postulates, which have been stated and modified in various 

 ways, and which are so important that they are repeated here. One 

 summarized statement follows: (1) The causal agent must be associ- 

 ated in every case with the disease, and conversely the disease must 

 not appear without this agent. (2) The causal agent must be isolated 

 in pure culture and its specific characters determined. (3) When the 

 host is inoculated under favorable conditions with suitable controls, 

 the characteristic symptoms of the disease must develop. (4) The 

 causal agent must be reisolated, usually by means of the technic 

 employed for the first isolation, and identified as that first isolated. 

 Obviously, the demonstration of pathogenicity is made only after 

 repeated trials, preferably with a number of different isolates which 

 are of unquestioned purity. When the technic for cultivating causal 



