288 Gardening 
prepared. Some of these plants, such as the mushrooms 
and bread mold, use dead plant or animal material for 
food. Others feed directly on living plants or animals. 
These are called parasites, and the plant or animal on 
which the parasite feeds is called the host. 
Parasitic diseases of plants. Many diseases of gar- 
den plants are due to the attacks of parasitic plants 
such as the rusts, smuts, and mildews. Most blights 
and rots are caused by fungi; but some of them, and 
also many other plant diseases, are due to bacteria. 
It is only within the last forty or fifty years that the 
cause of these diseases has ceased to be a mystery. The 
host plants become sickly, and even die suddenly ; but 
because of their small size, the parasites are not even 
seen with the naked eye. But the invention of the 
microscope enabled man to see these small parasitic 
plants; consequently much is now known of the 
various parasites that cause plant diseases and how to 
control them. 
The gardener can learn to recognize many of these 
diseases by such signs as spots or blotches on the leaves, 
by the occurrence of powdery or moldy growth, or by the 
decay or rotting of parts. Just as the physician, without 
seeing the germs, recognizes whooping cough or measles 
from the symptoms of the patient, so the gardener can 
learn to recognize plant diseases by the condition of the 
host plants. 
The diseases discussed below are selected to illustrate 
the various sorts of parasites that are likely to appear 
on crops in the home vegetable garden, and to give also a 
knowledge of the diseases that are most destructive to 
