relationship (regardless of whether the so-called primar:/ parasite is a 

 fiingus, bacterimn, virus, nematode, etc.). Some day the role of these 

 aggravators will have to be investigated. 



A slightly more precise relationship will be found in the role of a 

 Vector . The definition of a vector in the flow-sheet is quite straight- 

 forward. Many workers do suspect that nematodes are acting as vectors 

 in certain vims diseases, but such a relationship has yet to be proven. 

 It is possible that saprophytes could act as vectors, but a highly 

 specialized vector-virus relationship would almost certainly require a 

 plant parasitic nematode. 



As far as we are concerned, the more highly specialized relationships 

 between a nematode-parasite and a host are more interesting and important 

 at the present time. The definition of a pathogen on the flow-sheet is 

 essentially Koch's concept of a pathogen , i.e., the parasitic activities 

 of the organism are the direct and sole cause of the disease. On this 

 basis, many of our nematode parasites have been proven to be pathogens, 

 as for instance, with species of Ditylenchus , Meloidogyne , and a few 

 others. We would certainly be wise to restrict the word pathogen to the 

 meaning on the flow- sheet. 



In many cases, especially with root-attacking nematodes, we will find 

 that the host-parasite relations are not sufficiently precise to call 

 them pathogens. In many instances, we will find that fungi, bacteria, 

 and perhaps other nematodes have a necessary role in the etiology, and 

 I think perhaps the concept of Incitant should be used here. If an 

 axenic culture of the nematode cannot reproduce the full disease syndrome, 

 then we should not speak of a pathogen. Perhaps in many cases, we will 

 find that endoparasitic, root-attacking nematodes, although capable of 

 acting as pathogens, invariably act as incitants, and the same organism 

 could, therefore, act in either capacity. 



To conclude, I would like to stress that the various host-parasite rela- 

 tionships which I have suggested by no means illustrated all of the pos- 

 sible relationships which might develop between a plant and a parasite. 

 However, in the case of root-rots I am not certain that we shall ever be 

 able to define such relationships with great precision especially, as 

 they likely will over-lap. 



You might ask of what value is all of this theory in the development of a 

 nematode control program. This is a valid question, because it is likely 

 that so long as there is any type of host-parasite relationship, there 

 will be some growth response to the control of the nematode. This is why 

 soil fumigation alone can never be used experimentally to detenuine the 

 exact host-parasite relationship. However, in defense of this discussion, 

 there are at least two important reasons why a nematologist should try to 

 establish the host-parasite relations as precisely as possible. 



First, it Tfd.ll lead to a much clearer -understanding of the disease, 

 especially of its epidemiology and potentiality. Also, it appears to me 



