Path»3 



(1) constant association of the organism with the disease 



(2) isolation of the organism in pure culture 



(3) reproduction of the disease by inoculation with the pure 



culture of the organism 

 (U) re-isolation of the organism from the inoculated diseased 

 host, and identification of it with the original in- 

 oculant . 



It is apparent that there are at least two difficulties which make a 

 literal application of these postulates to root-rot diseases unrealistic 

 and perhaps unattainable. The first problem is that of obtaining the 

 organism in pure culture. To my knowledge, plant parasitic nematodes have 

 not yet been cultured on a chemically defined mediiim. Nematologists can 

 share this problem with virologists and workers investigating the role of 

 obligately parasitic fungi. It is perfectly obvious that if one cannot 

 culture the organism, it is impossible to fulfill the second of Koch's 

 Laws. 



A second problem is that the literal application of Koch's postulates to 

 plant pathological research is not always as simple or as logical as was 

 first believed. Koch was a bacteriologist, and the environment with 

 which he was concerned was the mammalian bloodstream, which normally is 

 essentially axenic. The host-parasite relation which Koch defined as 

 pathogenicity was relatively simple. It was merely the result of intro- 

 ducing a parasitic bacterium into a normally sterile system where compli- 

 cations resulting from the interaction of other microorganisms were 

 normally absent. 



Contrast with this the problem of defining the host-parasite relations 

 in a root-rot disease where the activities of a primary parasite may be 

 almost immediately obscured due to the colonization of the infection 

 court by numbers of other microorganisms. A logical question is whether 

 Koch's original concept of pathogenicity has any application in root-rot 

 investigations. I believe that it has, but in a relatively restricted 

 sense, as I shall try to develop a little later. 



Returning to Koch's postulates, I should like you to notice how logically 

 they were developed. Within the four postulates, one can discern three 

 distinct stages or steps, and these are exactly the same three stages and 

 occur in exactly the same order that a research project in plant pathology 

 should follow. The stages are as follows: 



(1) establishing the association of the organism to the disease 



(2) establishing the parasitic capabilities of the organism 



(3) establishing the host-parasite relations- 



At this point we have separated parasitic capability from host-parasite 

 relation. Such a separation is necessary because a plant parasite should 

 never be considered of necessity a pathogen. For instance, a variety of 

 tobacco tolerant to meadow nematodes may harbor within its roots many 

 hundreds of the parasite without the development of any discernible host- 



