vi PREFATORY NOTE 



century for ever dispelled these views among en- 

 lightened scientists and gave the first impetus 

 toward the study of plant diseases from a rational 

 standpoint. We now know that aside from the 

 injuries caused by insects or other animals, nearly 

 every cultivated plant is subject to attack by 

 few or many parasitic fungi or bacteria. In 

 addition to these diseases and to those caused by 

 unfavourable climatic or soil conditions, there are 

 also some diseases, well marked in their character 

 and so far as we can determine, absolutely non- 

 parasitic in origin, the cause of which appears to 

 be spontaneous to the plant. Such diseases offer 

 a fine field for investigation by the thoroughly 

 trained student. 



As the causes of plant diseases became known, 

 attempts were made to develop rational methods 

 of combating them. Thus the mycologists were 

 succeeded by the plant pathologists who attempted 

 to determine not only the cause of each disease, 

 but the conditions favourable and unfavourable 

 to its development. In the case of diseases caused 

 by parasitic organisms, it became necessary to 

 follow out in detail every step of the life history 

 of these organisms in their relation to their hosts, 



