288 MANUAL OF MICKOBIOLOGICAL METHODS 



observation of the host plant at the time of natural infection may reveal 

 the cause of the trouble. 



The simpler methods for preparing and using both ordinary and dif- 

 ferential media, for making isolations, and for studying the morphology 

 and physiology of such bacteria have been adequately described in other 

 chapters. This chapter, therefore, is concerned primarily with methods 

 of inoculation. 



In advanced research, investigators working with pathogens, whether 

 against plants, animals, or men, have many common interests. These 

 include, for example, (1) life cycles, referring to changes in the mor- 

 phology 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 gen- 

 erations; (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, location, exit, and transmission to a new host. 



Certain characteristics of plants not possessed by animals facilitate 

 basic research on pathogenicity. These include 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 

 costs and expenses of maintaining plants are relatively low. (3) The 

 species of plants studied frequently contain varieties or selections possess- 

 ing several degrees of resistance and susceptibility. (4) Plants are suited 

 to a wide range of experimental procedures, such as regulation of inter- 

 nal 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. Seeds from long lines of successively self-fer- 

 tilized parents often are available. When this is not sufficient, one 

 can commonly find or develop experimental units all genetically identi- 

 cal through vegetative propagation on their own roots. With such 

 material, any variations secured can be studied without concern that the 

 genetics of the host may have been obscuring pathogenicity. (7) Cer- 

 tain plant materials are being cultivated in vitro on media containing 

 only nutrients for which the chemical formulas are known. This tech- 

 nique offers many interesting possibilities for investigations on the 

 interactions of hosts and pathogens. 



SIMPLE REPRESENTATIVE INOCULATION METHODS 



The actual procedures for making inoculations vary with circum- 

 stances. Some simpler methods are considered briefly by way of illus- 



