ON THE NATUKE OF DISEASE RESISTANCE IN PLANTS 
J. C. Walker 
Introduction 
Since the pioneer work of DeBary (26) and Ward (87) upon 
the nature of parasitism, numerous contributions to our knowledge 
of the subject have been made. On the whole, however, we are 
still very much in the dark as to the nature of the complex rela¬ 
tions which exist between the parasite and the host. Duggar (28) 
in 1911 remarked that, ‘‘in general, the physiology of penetration 
is poorly understood and it will reward investigation. . . . 
There is now at hand ample material for a thoroughgoing study 
of some of the factors governing resistance and susceptibility. The 
problem is doubtless extremely difficult, but it is believed by 
some the work may yield results. ... At present there is no 
information respecting the cause of this difference in behavior as 
regards resistance. ’ ’ In connection with the writer’s studies upon 
the nature of disease resistance in the onion (86), a review of 
previous work upon the general subject was found necessary. The 
results of this survey of the literature are presented at this time. 
Types of Disease Resistance 
Orton (61), followed by Freeman (32) and Butler (16), calls 
attention to the distinction between avoidance of disease, endur¬ 
ance of disease, and true resistance to disease. It is the last of 
these three classes with which we are concerned in this paper. 
We shall include in this class, cases where through some inherent 
quality in its composition, the plant is capable of successfully 
resisting, to a greater or less extent, the attack of a given parasite. 
A survey of the studied cases of disease resistance in plants 
brings out at once the fact that all degrees are to be found, varying 
from complete immunity to a high degree of susceptibility. We 
have, for instance, in certain varieties of the potato complete 
immunity to the wart disease (Synchytrium endohioticum (Schil.) 
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