100 GENERAL CONSIDERATIONS 



may be either animals or other plants. Plants that 

 are parasitic on and cause diseases of our cultivated 

 plants are to be found among the bacteria, the slime 

 molds, the fungi, and in a few cases even among 

 other flowering plants. In warm moist countries 

 damage is also sometimes caused by epiphytes. These 

 are air plants that do not enter the tissues of the 

 plants on which they grew, but they cause trouble by 

 crowding and keeping out light and air. Epiphytes 

 are found among the algse, the lichens, the mosses and 

 liverworts, and among flowering plants. Animals 

 causing injury to plants are principally found among 

 the nematode worms, the mites, and the insects. Slugs 

 and snails which belong to the molusca are also 

 troublesome, but their injuries, like those caused by 

 the grazing of the higher herbivorous animals, are 

 to be classed as mechanical injuries rather than as 

 diseases. 



Remedial measures against plant diseases may be 

 grouped under hygienic measures, including the 

 destruction of sources of contagion and rotation of 

 crops ; soil treatment ; seed treatment ; topical ap- 

 plications to the plant itself ; and the propagation 

 and selection of immune varieties. 



Bacterial diseases are exceedingly common and 

 difficult to combat. Pear blight, the bacterial blight 

 of potatoes, tomatoes, etc., the black rot of cabbage, 

 and the bud rot of the cocoanut may be mentioned 

 as typical examples. Hygienic measures, especially 

 the destruction of diseased plants or parts of plants, 

 and rotation for the annual crops, are as a rule the 

 only remedies, and these are far from satisfactory. 



