INOCULATIONS WITH BACTERIA CAUSING PLANT DISEASE x.^-li 



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). 

 When there seems to be a bacteriophage in the complex, the general 

 discussion by Krueger (1936) and the account by Thomas (1940) of a 

 precursor may well be consulted. 



The pathogenicity of crowngall bacteria can be destroyed (Van 

 Lanen, Baldwin, and Hiker, 1940) with certain amino acids and 

 related compounds added to common media. Attenuation was com- 

 monly secured in 20 to 30 successive transfers. The rate of attenua- 

 tion was increased if bacterial growth was reduced by the strength of 

 the compound (e. g., 0.1 to 0.3% glycine) and by an alkaline re- 

 action (e. g., pH 8.0). 



The virulence of partly attenuated cultures was restored by long 

 cultivation on suitable media and by ultra-violet irradiation (Duggar 

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

 into a tomato stem above an attenuated culture, the gall about the 

 attenuated culture was approximately as large as that about the 

 virulent culture. A chemical gall served as well as that from a 

 virulent culture (Riker, 1942). 



PATHOGENS ACTING TOGETHER 



Combinations of microorganisms sometimes induce symptoms 

 different from those caused by any one alone. So long as the patho- 

 gens can be cultivated on artificial media, the principles in Koch's 

 postulates can be applied 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 liv- 

 ing 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 patho- 

 gen. Correspondingly, three causal agents would require eight series 

 of plants, 



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 sci- 

 ence, however, has a duty in the protection of local plant popula- 

 tions and requires that cultures or strains brought into a new locality 

 should be handled with proper consideration of all the factors in- 

 volved. While reasonable freedom in the shipment of such cultures 

 from one laboratory to another is essential for certain work, it must 

 be insisted that they be secured and studied only after both the 

 investigators and their administrators have fully considered and 

 accepted the responsibility involved. Younger research workers 

 and particularly graduate students are advised to employ such cul- 

 tures only after detailed plans have been made in conference with 

 their advisors. 



