34 WILLIAM Q. LOEGERING 



interactions [Lit\ and Lit!) interact in a complementary manner to produce 

 an infection type neither produces alone and can be compared to Category II 

 interactions. Thus there is a parallel between Category I and II genetic 

 interactions of classical genetics and Category III and IV genetic inter- 

 actions peculiar to host :pathogen interactions. 



This gives the essentials of the gene-for-gene concept. Much more 

 could be written concerning problems of incomplete dominance of genes in 

 host and/or pathogen, reversal of dominance in reaction, cytoplasmic 

 inheritance, Lit's which singly are expressed as "high infection type", 

 reversal of models when we work with systems which include facultative 

 pathogens, the questionable existence of alleles for high reaction, the 

 vast confusion in the area of physiology of host :pathogen systems, tempera- 

 ture effects on expression of the aegricorpus, the relationship of the 

 gene-for-gene hypothesis to the physiologic race concept, progressive 

 increase in virulence, and many other areas. It perhaps need only be 

 said that the gene-for-gene concept is a potentially powerful tool in 

 shedding light on some of these question -- and that is its importance. 



SPECIFICITY IN WHITE PINE BLISTER RUST 



There is no reason to believe that the gene-for-gene relationship 

 does not hold for white pine blister rust; however, its application to 

 practical problems of protecting pine from this disease has peculiarities. 

 This is because (1) the non-repeating pycnial-aecial stage of the fungus 

 is the one which causes the economic damage which we wish to control, and 

 (2) we do not know whether some of the genes for pathogenicity in the 

 uredial host :pathogen relationship are the same as in the pycnial-aecial 

 host :pathogen relationship or whether these two relationships depend on 

 different genetic systems in the pathogen. 



In nature the pycnial and subsequent aecial stages of Cvonartium 

 ribiaola J. C. Fisch. ex Rabenh. do not normally spread from plant to 

 plant of the same species (are non-repeating) although under controlled 

 conditions this might be achieved. Thus, when a sporidium of C. vibioola 

 falls on a pine needle, germinates, and initiates infection, it must have 

 the right genes for high pathogenicity or it will be unable to carry on 

 through the life cycle. For this reason we can say there is absolute 

 selectivity for those sporidia which have all the genes for high pathogeni- 

 city corresponding to all the genes for low reaction which a particular 

 pine tree carries. Thus, in the ecological association of Pinus , Ribes , 

 and Cronartium, infection by a given sporidium developed from a teliospore 

 produced on Ribes might induce high infection type on one pine tree but 

 low infection type on another and vice versa. It is obvious that this 

 can't be tested directly, but it might be possible to obtain evidence by 

 grafting or "inoculation" with tissue cultures. 



Assuming that a gene-for-gene system does exist in the pycnial host: 

 pathogen relationship, only those genotypes of the pathogen will survive 

 which have the necessary genes for high pathogenicity corresponding to the 

 genes for low reaction in the host plant. This selectivity of certain 

 pathogen genotypes has been observed in the uredial host :pathogen relation- 

 ship for the wheat -.Pucoinia graminis Pers . f.sp. tvitiai Eriks. § E. Henn. 

 and flax:Af. lini relationships. Sears, Loegering, and Rodenhiser (1957) 

 discovered a gene in wheat for low reaction which was found to be almost 

 universally present in wheat varieties grown in the United States. As a 

 result, cultures of the pathogen are seldom found in the field which do 



