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: (1) The Fungous Diseases. (2) 
The Bacterial Diseases. 
THE FUNGOUS 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 
microorganisms. They do not therefore strictly belong to a 
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