X4S-4 MANUAL OF METHODS FOR PURE CULTURE STUDY 



agents on artificial media has not yet been worked out, their patho- 

 genicity is established in other ways (e. g.. Rivers, 1937). When 

 causal relations are being worked out, one may well differentiate be- 

 tween predisposing, inciting, and continuing causes. Various 

 factors that influence the physiology of the plant may also affect 

 pathogenicity. 



The simpler methods for making isolations, for preparing and using 

 both ordinary and differential media, and for studying the morphol- 

 ogy and physiology of such bacteria have been adequately described 

 in Leaflets II, IV, V, VI, and elsewhere (e. g., Rawlins, 1933; Riker 

 and Riker, 1936; and Smith, 1905-1914, 1920). This Leaflet, there- 

 fore, is concerned primarily with methods of inoculation. 



To insure against erroneous conclusions, the environmental con- 

 ditions for experimental inoculations should be maintained as nearly 

 as possible like those occurring in nature at the time of natural infec- 

 tion. When difiiculty is experienced in artificial inoculation, careful, 

 continued observation of the host plant at the time of natural infec- 

 tion may reveal the cause of the trouble. 



In advanced research it appears that investigators working on 

 pathogens, whether with plants, animals, or men, have many common 

 interests. These include, for example, (1) life cycles, referring to 

 changes in the morphology of individual cells and the relation of these 

 different forms to virulence; (2) changes in colony characters and 

 physiology, including particularly changes in pathogenicity; (3) 

 factors attending changes, such as the time, frequency, and conditions 

 of origin, as well as the influence of environment, and relations to 

 earlier and succeeding generations; (4) statistical analyses to classify 

 the origin and frequency of the variations observed; and (5) life 

 histories of the pathogens in relation to entrance into the host, loca- 

 tion, exit, and transmission to a new host, the well-known essentials 

 of studies in epidemiology which are vitally influenced by variations 

 in the pathogens. 



Certain characteristics of plants not possessed by animals facilitate 

 basic research on pathogenicity. Among the advantages in experi- 

 mental work are the following: (1) Large numbers of hosts are easily 

 available. The number used, whether 10 or 10,000, is selected on 

 the basis of experimental needs. (2) The initial cost and expense of 

 maintaining plants are relatively low. (3) The species of plants 

 studied frequently contains varieties or selections possessing several 

 degrees of resistance and susceptibility. (4) Plants are suited to a 

 wide range of experimental procedures, such as regulation of internal 

 temperature and moisture, that are not feasible with animals. (5) 

 Epidemics^ are induced with relative ease and without concern for 

 the health of the technician or the public. (6) The genetic purity of 

 the host can be assured. Seed from long fines of successively self- 



i"Epidemic," in the original Greek meaning "on the people," was early applied to 

 plant diseases, together with many other medical terms. It is an old and common 

 word in plant pathology, although on etymological grounds its use for human disease 

 alone is preferred by some medical authorities. In this paper, however, the broad 

 definition from Gould's Medical Dictionary is followed, "Epidemic: of a disease affect- 

 ing large numbers or spreading over a wide area." 



