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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