CHAPTER XX. 

 THE PARASITIC DISEASES OF PLANTS. 



It is by no means easy to draw a sharp line between plant disease 

 and the phenomenon of decay. If the tissues of a living plant show 

 signs of decay it is called a disease; if the decay occurs in fruit or 

 vegetables after they are harvested we speak of it as decay. But 

 there are some parasitic organisms that may grow in the living plant 

 and thus find access to the fruit so that the fruit will decay after 

 harvesting. Should this be called a disease? In some cases the 

 parasites seem to do no injury to the living plant, but live in its tissues 

 to injure the stored fruit or vegetable later. In such cases it is 

 manifestly difficult to say whether the phenomenon should be 

 called a disease. In the types given in the following pages the 

 parasites in most cases do injury to the living tissues, although one 

 or two are exceptions. 



While most parasitic diseases of animals are due to bacteria, 

 with a considerable number caused by animal parasites and 

 almost none by the higher fungi, a different condition of things 

 is found among plants. The larger majority of plant diseases are 

 caused by the higher fungi, a considerable number by bacteria, 

 while, so far as known, none are caused by microscopic animal 

 life. In our brief survey of this important field we may best divide 

 the subject into two divisions: i. The Fungoid Diseases. 2. 

 The Bacterial Diseases. 



THE FUNGOID DISEASES OF PLANTS. 



This is by far the largest class of plant diseases, but they can only 

 be touched upon in this work. The Fungi that cause this class of 

 diseases are mostly of some size and can hardly be called micro- 

 organisms. They do not therefore strictly belong to a discussion of 



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