8 FUNGOUS DISEASES OF PLANTS 



which have been placed upon the damage caused by prevalent 

 plant diseases during a single season amount frequently to a very 

 considerable per cent of the total value of the crops. In the United 

 States alone the destruction wrought by fungous diseases is some- 

 times not far from half a billion dollars. 



The diseases of plants induced by other biological, physical, 

 chemical, or mechanical agencies are not included. The lack of 

 plant nutrients, or the presence of particular nutrients in quanti- 

 ties sufficient to cause injury, the phenomena commonly termed 

 sunscalds, wind effects, abrasions due to contact, etc. are all dis- 

 turbances which demand attention, but they may have no def- 

 inite relation to epidemic fungous diseases, and would therefore 

 be fundamentally considered only in a general treatise on plant 

 pathology. On the other hand, it is felt that in connection with 

 any account of the fungous diseases of plants it is desirable to 

 place within easy reach of the student certain related information. 

 In isolated chapters, therefore, there is presented a brief review 

 of culture methods, histological technique, and such facts of physi- 

 ological significance as seem requisite. 



Culture methods are here concerned with the essential steps in 

 preparing important nutrient media and means for the isolation 

 and study of fungi in artificial cultures. Such cultures are important 

 in morphological and physiological study, and they afford in the 

 majority of cases the only proper source of spores or mycelium for 

 inoculation purposes. 



Histological technique is requisite not merely to insure a proper 

 morphological study of a fungus and its distribution in the host, 

 but also in order to make possible a more comprehensive analysis 

 of the tissue modifications in the host. 



A discussion of special biological or physiological relations has 

 been limited to a few topics. The germination of spores is from 

 the outset one of the investigational or routine duties of the pathol- 

 ogist ; the relations of the fungi to chief environmental factors 

 cannot be disregarded ; artificial infection is required in determining 

 the causal organism ; and the principles of disease control are con- 

 cerned with the immediate application of pathological study to 

 economic purposes. 



