Pathi6 



This represents a flow-sheet prepared with the idea of showing how we 

 might develop the host-parasite relations by following Koch's original 

 idea of moving from the association to the parasitic capabilities and 

 finally to the host-parasite relations. 



First then, a suspect organism is established on the basis of the asso- 

 ciation. To determine whether an organism is suspect is relatively 

 simple. It is based mainly upon field studies; distribution of the 

 organism in relation to distribution of the disease, the intensity of 

 the disease in relation to the inoculiim level, control of the disease by 

 control of the suspect organism, and so on. Now this is a most important 

 phase in any etiological study. It is at this stage that I believe we 

 are justified in criticizing some of the other branches of plant path- 

 ology in addition to our own. There is a tendency to bring our studies 

 of a disease into the laboratory at far too early a stage, to isolate 

 any organism wMch might be present and to trj'^ to define the host-para- 

 site relations. The results of this type of research often are meaning- 

 less. We should always be certain of the existence of a real and con- 

 sistent association of the organism to the disease before attempting to 

 define its role in the etiology. 



The next step is to try to establish the parasitic capabilities of the 

 suspect organism. VJe can refer therefore to the concepts of parasite 

 and saprophyte. I admit that my basis of separating these is rather 

 naive. We can perhaps define a parasite as I have done, but insofar as 

 nematodes are concerned, we are on rather dangerous ground when we dis- 

 cuss saprophytes. Our concpots of saprophytism and our knowledge of 

 nematode bionomics are too fragmentary at this stage, and many saprophytic 

 nematodes are so-designated on practically no scientific evidence at all. 



Assuming that we have established the suspect organism as a parasite, we 

 should now make an attempt to define, with some precision, the host-para- 

 site relations involved. As far as host-parasite relations are concerned, 

 I have attempted to make the flow-sheet travel from completely unspecial- 

 ized relations (involving even saprophytes theoretically) to the very 

 highly developed host-parasite relationship of pathogenicity. For the 

 first stage, we might postulate the concept of Aggravator . 



Metabolic break-down products excreted by animals are said to be (by and 

 large) phytotoxic. vlhen one examines a root lesion, relatively large, 

 so-called saprophytic nematodes frequently occur in the more advanced 

 portion of the lesion which is characterized by cellular debris, fungi, 

 bacteria, etc. Of course, these nematodes might be feeding on bacteria 

 or fungi and are scarcely saprophytes, yet, insofar as the host-plant is 

 concerned, they have no parasitic capabilities. Nevertheless, they ai^ 

 present in the lesion, feeding in some manner and excreting products 

 which are very likely phytotoxic. The products are capable of diffusing 

 across the lesion to its periphery and conceivably so affect uninvaded 

 plant cells that they become predisposed to invasion by the parasite 

 responsible for the disease. If this is true (and I have no evidence 

 that it is) thP5n these organisms do play a part in the host-parasite 



