PROBLEMS AND PROGRESS IN PLANT PATHOLOGY 
99 
work remained nearly half a century after it was published as a standard 
writing in plant pathology.^ 
It required the plague of the potato disease and the example of 
the Irish famine finally to focus attention upon the fundamental 
problem — the relation of the mildew to the sick potato plant, of the 
smut and rust fungi to the infected grain — the problem of parasitism. 
True, they had been phrasing the term parasite much as we do, but so 
long as most held that the so-called parasitic fungus originated through 
the transformation of the sap or the degeneration of the diseased host; 
tissues there could be no real progress in plant pathology whether 
scientific or practical. To De Bary's master mind we owe the clear 
recognition of the parasitic relations of fungus and host plant, ^ and 
from his demonstration of this we date further progress. 
But although De Bary's work has settled for all time that the 
parasite is an independent plant entering the host from without and 
feeding upon it to its destruction, we must not forget that the more 
fundamental problems of parasitism remain with us. In biology, the 
definition is always dangerous, and the more complete and finished the 
more the danger. De Bary's classification of all fungi as parasites and 
saprophytes, obligate and facultative, is so complete and satisfying 
that it is constantly misleading. De Bary thought as the mycologist 
with attention focused upon the fungus. The first concern of the 
pathologist must ever be with the host plant, and chiefly with the 
host plant under conditions of culture. He must constantly be alert 
to the fact that parasitism is not a fixed but a fluctuating relation, 
dependent as to its occurrence and degree upon a complex of condi- 
tions, and these involving the reactions of not one but two widely 
different organisms. Although the fact of parasitism was settled 
and the modern science of plant pathology securely based upon it, 
there has been no time since when phytopathologists realized as 
clearly as today the importance of the problems yet to be solved in 
this field. We have scarcely begun the study of the intimate relations 
of parasite and host, the conditions and results of parasitism. 
^ The editor of Gardiner's Chronicle (1849, p. 211) prefaces the translation of 
Re's work with the statement that "it is the best work within our knowledge 
upon this subject. 
5 De Bary, A. Untersuchungen iiber die Brandpilze und die durch sie verur- 
sachten Krankheiten der Pflanzen mit Riicksicht auf das Getreide und andere 
Nutzpflanzen. 1853. 
