SPECIFICITY IN PLANT DISEASE 1 



William Q. Loegering 



Genetics Department, University of Missouri, 



Columbia, Missouri, U.S.A. 



ABSTRACT 



The development of the gene-for-gene hypothesis has served a 

 useful function in that it has focused attention on the host- 

 pathogen relationship (aegricorpus) . As a result it is now 

 understood that specificity is the result of the "gene-for-gene* 

 relationship in host-pathogen associations. Specificity has 

 been demonstrated only in cases where both host and pathogen 

 can be propagated as clones or pure breeding populations. In 

 general the pycnial and aecial stages of rust fungi cannot be 

 used in studies of specificity since they are non-repeating 

 and cannot be maintained as clones or pure breeding populations 

 Thus sporidia, which initiate the pycnial stage, must have the 

 genes for high pathogenicity corresponding to any genes in the 

 host plant for low reaction in order to produce an infection 

 which survives. 



INTRODUCTION 



There is a great difference between the classical view of plant 

 disease and the view resulting from the development by H. H. Flor of the 

 gene-for-gene concept. Both points of view are valid and useful, and 

 one should not be used to the exclusion of the other when approaching 

 problems in plant pathology. Some problem areas in plant pathology remain 

 unresolved because they have been approached only from the classical 

 standpoint. The most prominent example is the failure to resolve the 

 "nature of plant disease resistance". If we approached this problem by 

 studying the "nature of the host-parasite interaction", more progress 

 might be possible. This paper discusses the genetic nature of the host- 

 parasite interaction which is commonly referred to as the "gene-for-gene 

 hypothesis" and suggests how it might be utilized in studies of white 

 pine blister rust. 



The gene-for-gene hypothesis was developed by H. H. Flor from studies 

 which he started in the 1930's. He summarized the philosphy in the title 

 of a paper he wrote in 1955 (Flor, 1955) : "Host-parasite interaction in 

 flax rust - its genetics and other implications." In this title, the 

 words "host" and "parasite" refer to two taxonomic groups of organisms, 



1 Contribution from the Missouri Agricultural Experiment Station, 

 Journal Series Number 5723, approved June 16, 1969. 



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