422 MYCOLOGY IN RELATION TO PLANT PATHOLOGY 



During the latter half of the nineteenth century the overwhelm- 

 ing interest in plant diseases centered around etiology. In the 

 textbooks of this period, as in nearly all recent textbooks, plant 

 diseases are arranged on the basis of the classification of the etio- 

 lo^ic agent. The reasons for this situation are numerous and in- 

 clude the following: 



1. The period of 50 years after the overthrow of the theory of 

 spontaneous generation, now regarded as the "golden age of dis- 

 covery" in bacteriology, was also the golden age of discovery in 

 fields pertaining to bacteria and fungi as causes of disease in plants. 

 In this period the cause of a disease and the disease itself were all 

 too commonly regarded as synonymous. The connotation host- 

 parasite, which indicates a food relationship, became a common- 

 place and was used instead of pathogen-suscept, which indicates 

 the disease relationship. Writers spoke of "spread of disease" and 

 "spread of infection" when they meant spread of inoculum or of 

 pathogenic agents. These examples indicate the confusion of 

 ideas that have been carried over from mycology to phyto- 

 pathology. 



2. The emphasis in studies of plant-pathogenic fungi has re- 

 mained so overwhelmingly etiologic that even at the present time 

 too little recognition is being given to the influence of "predispos- 

 ing factors," as stressed by Sorauer, and to the morbid anatomy 

 of diseased plants, as stressed by Kiister (1925) in his first edition 

 of Phytopathologische Pflanzenanatomie, which appeared in 1903. 

 That plant diseases should be classified on the basis of the disease 

 processes themselves is cogently argued by Whetzel (1929). It is 

 becoming increasingly apparent that instead of stating that a 

 given fungus is the cause of a particular disease one should state 

 that it is one of the causes, because environmental factors may 

 exert a controlling influence. It is also apparent that the classifica- 

 tions of disease by Kiister are fundamental, and future develop- 

 ments must be built on his scheme. 



3. The investigators of this period lacked training in phyto- 

 pathology, and in consequence their attention was centered pri- 

 marily on the pathogen, with only passing consideration being 

 given the diseased plant. In their scientific writings they em- 

 ployed terms from fields of knowledge with which they were 

 familiar. As soon as interest shifted, a distinctive terminology, 

 applicable only to plant pathology, began to develop, as exempli- 



