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 



