30 WILLIAM Q. LOEGERING 



not to a plant or variety and a culture or race. The two words are used 

 as a hyphenated adjective of the word "interaction". The interaction is 

 the subject of the paper, not the host or the parasite. This idea is the 

 core of the gene-for-gene concept. Specifically Flor says in his title 

 that the interaction he will discuss is "flax rust" not Melampsora lini 

 (Pers . ) Lev. or Linvm spp. The rest of the title says that the "genetics 

 and other implications" of the interaction will be discussed and not the 

 genetics and other implications of the host or parasite. While the 

 genetics of host and parasite are discussed in the paper, they are not 

 considered as independent systems. 



No attempt will be made to review the data from which the gene-for- 

 gene hypothesis was derived. The reader is directed to selected reviews 

 which summarize some of the evidence (Flor, 1956, 1959b; Moseman, 1966; 

 Person, 1959; Williams, Gough and Rondon, 1966). There are between 50 

 and 100 illustrations of the validity of the gene-for-gene hypothesis - 

 the exact number depends on what degree of proof is desired. It seems 

 reasonable to assume that exceptions to the hypothesis will be found, 

 but no one, insofar as I am aware, has demonstrated a single instance 

 where the hypothesis does not hold. Questions have been raised regarding 

 the validity of the expression "gene-for-gene" but not of the concept 

 itself. 



There is much concern with generalized resistance of the type van 

 der Plank (1963) called "horizontal", or more recently, "uniform" (van 

 der Plank, 1969). Within the structure of Flor's concept of host-parasite 

 relationships the word "non-specificity" should be applied to this idea 

 of van der Plank's to contrast it with the specificity of the gene-for- 

 gene relationship. In our present stage of knowledge, "non-specificity" 

 can only be defined as a host -pathogen relationship for which specificity 

 has not been demonstrated. While this is a circular definition it is 

 needed to indicate our current degree of ignorance. Non-specificity is 

 very likely a valid concept and should be studied more intensively. It 

 is not the purpose of this paper to discuss non-specificity; however it 

 is important that in discussing the gene-for-gene concept we do not lose 

 sight of the fact that a host-pathogen relationship perhaps involves 

 more than specificity. 



Current definitions of "disease" in plant pathology as well as in 

 animal and human pathology consider disease a character of the host. 

 Disease is variously defined as a malfunctioning process of the host, a 

 deviation from normal in the host, and economic damage to the host. These 

 concepts are valid and useful; after all we are interested in a healthy 

 plant, animal, or self. However, these definitions cannot apply to the 

 host -pathogen relationship which is a natural and, therefore, a normal 

 relationship. It must not be forgotten that sometimes the relationship 

 is more detrimental to the pathogen than the host. 



The idea of disease is usually expressed in diagrams in such a way 

 that the pathogen infects the host, both are influenced by the environ- 

 ment, and the net result is disease which is expressed by symptoms. These 

 symptoms express the resistance and susceptibility of the host according 

 to whether the infection type is low or high. 



