INOCULATIONS WITH BACTERIA CAUSING PLANT DISEASE 295 



called ''single-cell cultures." This misleading use of a well-established 

 phrase provides both the investigator and the reader with a false sense 

 of security. 



Variations may be induced among plant pathogens by procedures 

 very similar to those employed on other bacteria. Some of the con- 

 siderations involved in such studies are discussed by Riker (1940). 



The pathogenicity of crown gall bacteria can be destroyed (Van 

 Lanen, Baldwin, and Riker, 1940) with certain amino acids and related 

 compounds added to common media. Attenuation was commonly 

 secured in 20-30 successive transfers. The rate of attenuation was 

 increased if bacterial growth was retarded by the strength of the com- 

 pound (e.g., 0.1-0.3 per cent glycine) and by an alkaline reaction (e.g., 

 pH 8.0). 



The virulence of partty attenuated cultures was restored by long 

 cultivation on suitable media and by ultraviolet irradiation (Duggar 

 and Riker, 1940). Likewise, when a virulent culture was inoculated 

 into a tomato stem above an inoculation with an attenuated (mlture, 

 the gall about the attenuated culture was approximately as large as that 

 about the virulent culture. A gall induced b}^ a plant hormone served 

 as well as that from a virulent culture (Riker, 1942). 



Pathogens acting together. Combinations of microorganisms some- 

 times induce symptoms different from those caused by any one alone. 

 As long as the pathogens can be cultivated on artificial media, the prin- 

 ciples in Koch's postulates can be apphed with two or more causal agents. 

 For example, a simple inoculation with one organism may involve a 

 series of susceptible plants growing in a suitable environment with the 

 living causal agent and a parallel control series. With two causal agents, 

 however, there should be four series of plants as follows: (1) with both 

 living pathogens, (2) with only one living pathogen, (3) with only the 

 other living pathogen, and (4) with neither living pathogen. Cor- 

 respondingly, three causal agents would require eight series of plants. 



One should not overlook the fact that in nature pure cultures seldom 

 exist except in the most advanced margin of the lesion. 



Cultures from another locality. The use of a culture of a pathogen 

 not already present on local plants requires critical consideration. The 

 progress of bacteriology calls for reasonable freedom in the movement of 

 cultures. This science, however, has a duty in the protection of local 

 plant populations and requires that cultures or strains brought into a 

 new locality should be handled with proper consideration of all the factors 

 involved. It must be insisted that cultures be secured and studied 

 only after both the investigators and their administrators have fully 

 considered and accepted the responsibilities involved. Younger research 



