May, 1922] RAINES VEGETATIVE VIGOR OF THE HOST 235 



The data presented by Thomas (1921) on the parallel relation between 

 health of the host and infection in the case of the leaf blight of celery, and 

 observations of similar occurrences in other diseases caused by non-obligate 

 parasites like the late blight of the potato (Jones, 1905) and the crown gall 

 of the beet (Levine, 1921) indicate that phases in which a symbiotic tend- 

 ency comes to the fore may occur in diseases of a predominantly destruc- 

 tive type caused by facultative parasites, and suggests the generalization 

 that the host-parasite relation in any given instance is not constant but 

 may vary with the state and condition of the organisms and with the prog- 

 ress of the relation. It is important to recognize that there may occur 

 mutualistic phases and stages in host-parasite relations of a violent and 

 destructive type, just as there are destructive phases in parasitisms of a 

 predominantly symbiotic tendency such as those of the mildews, the rusts, 

 and the smuts. 



CONCLUSION 



The inquiry initiated by the occurrence in rust literature of statements 

 of a relation between host vigor and susceptibility other than the inverse 

 relation commonly conceived as existing between these variables can be 

 considered as having brought forward evidence indicating that through 

 most of the course of certain infectious diseases such as the rust diseases 

 of the cereal grains, and in certain phases of other diseases like the leaf 

 spot of celery and the crown gall of the beet, the vegetative vigor of the 

 host and the virulence of the disease may be in direct relation. The demon- 

 stration of such a relation in diseases of large importance suggests, in turn, 

 emendation of current pathological concepts of the relation between host 

 vigor and pathogen activity into a form more in accord with our knowledge 

 of parasitological phenomena in general. A more catholic point of view 

 in pathologic thought, recognizing that, for longer or shorter phases in the 

 course of a disease, the relation between host and parasite may be highly 

 mutualistic, would be of material value as a working concept in the study 

 of disease and in defining the practical problem of disease prevention and 

 control. 



The work presented in this paper was done in the Botanical Laboratory 

 of Columbia University, under Professor R. A. Harper, to whom the writer 

 is greatly indebted for pointing out the problem and for constant con- 

 sultation and advice during the progress of the investigations. Acknowl- 

 edgment is also made of indebtedness to Dr. Michael Levine for taking the 

 photographs of the soil-culture experiments, and to Dr. H. E. Thomas for 

 helpful advice in devising the method used in the dosage studies on the 

 corn rust. 



LITERATURE CITED 



Adami, J. G. The principles of pathology. Vol. i. Philadelphia and New York, 1910. 

 Arthur, J. C. Problems in the study of plant rusts. Bull. Torr. Bot. Club 30: 1-18. 1903. 

 Bailey, L. H. Cyclopedia of horticulture. New York, 1914. 



