Path J 2 



the roots of a certain plant. For the experiment, the author screened 

 this soil and the nematodes, including some sapj.'ophytic forms, I assume, 

 were allowed to settle to the bottom of a beaker for a short time. It 

 was reasoned that the supernatant liquid could serve as a suitable con- 

 trol, since the bacteria and f\ingi of that soil would be present, whereas 

 the residue in the beaker could serve as the inoculum, since it contained 

 the nematode, as well as the bacteria and fiingi. 



Now, the author obtained stunting only when the residue was poured around 

 the roots of the test plants growing in steamed soil, and he concluded 

 that the pathogenicity of this nematode had been established. Although 

 the worker has made a valuable contribution, and I am not in any way try- 

 ing to discredit his work, I maintain he has not proven the pathogenicity 

 of this nematode at all. It is true that he has proven some type of re- 

 lationship between the host and this nematode, but, to prove pathogenicity, 

 one must fulfill certain prescribed criteria. If we are going to use the 

 concepts of the plant pathologist, we must use his rules in developing 

 these concepts. 



Let us look at the word pathogen — the word which first appeared in 1876 

 originated during the so-called golden age of bacteriology. The literal 

 meaning and original definition of a pathogen is simply an organism which 

 causes a disease in another organism. However, as our knowledge of dis- 

 ease and host-parasite relations in general has grown tremendously during 

 the twentieth century, it is now realized that a diseased plant frequent- 

 ly is the result of an extremely complex reaction, or series of inter- 

 actions, involving several organisms; and, consequently, the concept of a 

 pathogen, particularly in the case of root-rot, must become much more 

 specific. 



Perhaps in the case of a root-rot we should define a pathogen as a para- 

 sitic organism which, through its activities in relation to the host, 

 produces the full disease syndrome. This definition would exclude those 

 organisms which, although of great importance in the etiology, cannot by 

 themselves reproduce the full disease syndrome. Also the parasitic 

 organisms which have somewhat secondary roles in the etiology of disease 

 complexes must be excluded. 



We should also consider the role of the relatively unspecialized sapro- 

 phytic or weakly parasitic organisms which occur frequently in the infec- 

 tion court. Can we define the role of such organisms in the etiology of 

 a root-rot? VJhat I would like to discuss is how one can establish the 

 precise host-parasite relations of a nematode involved in a root-rot dis- 

 ease. 



I think the place to start is to return to the original concepts of 

 pathogenicity to see of what use they may be to us. Robert Koch, in 1882, 

 postulated his four laws of pathogenicity and gave to the bacteriologists 

 a sound and logical procedure for etiological investigations of the bac- 

 terial diseases of man and animals. Koch's so-called rules of proof of 

 pathogenicity, as rather loosely translated by Garrett, are as follows: 



