1 84 AMERICAN JOURNAL OF BOTANY [Vol. 9 



Among plant pathologists, Jones (1905, p. 23) writes: 



Disease resistance and vegetative vigor are closely associated, although the factors 

 involved are not necessarily identical. ... So far as the evidence goes it seems to suggest 

 that high vegetative vigor enables the plant to ward off in some degree the fungus attack. 



In direct contrast with this view it must be noted that workers in the 

 field of the rust diseases of higher plants have on several occasions been 

 prompted to generalize to directly the opposite effect that they found host 

 vigor and susceptibility to disease not antithetic, and not independent, but 

 parallel variables. Arthur (1903, p. 13), in a presidential address before 

 the Botanical Society of America, stated from his long experience with 

 plant rusts that 



So intimate is the association of parasite and host that as a rule the vigor of the parasite 

 is directly proportional to the vigor of the host. Every culturist soon learns that to have 

 success in his work he must employ strong, rapidly growing plants. Even if he succeeds in 

 infecting weak plants, the fungus will rarely come to satisfactory fruitage. 



Sheldon (1903, p. 74), summarizing his illuminating studies on the 

 asparagus rust, concludes: 



Lowered vitality does not favor infection. . . . Whatever affects the growth of the 

 asparagus has a like effect on the rust. 



Stakman, in his extensive studies on the cereal rusts, on several occasions 

 expresses similar views. Thus (1914, p. 40) he concludes that 



These experiments show that whatever is conducive to the vigorous development of 

 the host is ordinarily conducive to the vigorous development of the parasite also. 



And in another place (Stakman and Levine, 1919, pp. 75 and 76): 



Deficiency of soil moisture and sunlight and other ecological factors affecting the host 

 plant unfavorably appear to be equally unfavorable to the rust parasite. . . . Adverse 

 environmental conditions unfavorable to the host are also unfavorable for the parasite, 

 affecting the virulence and spore size of the latter. 



While the authors make their point incidentally and in no instance dis- 

 cuss their findings from the point of view of the general question of the 

 relation between host vigor and susceptibility in -infectious disease, it is 

 evident that the question is suggested as to the possibly special and, from 

 the point of view of the relation observed in the larger number of infectious 

 diseases of plants and animals, peculiar relation between vegetative vigor 

 of the host and susceptibility to infection in the rust diseases of the higher 

 plants. The demonstration of such a peculiar relation would be of the- 

 oretical interest as limiting and qualifying the universality of the commonly 

 accepted dictum that host vigor and virulence of disease are in inverse 

 relation and would be of profound import in defining the practical problem 

 of the prevention and control of the diseases concerned. 



