114 THE PESTS OF CULTIVATED PLANTS 



Spraying, dusting, and controlling the conditions under 

 which the plants are grown and removing the affected parts 

 are some of the methods employed to save our economic 

 plants from destruction from the various diseases that prey 

 upon them. It is evident that certain diseases as well as 

 certain insects cannot be controlled by spraying, and also 

 that several sprays are needsd to control the different kinds 

 of diseases. 



The fungous diseases can usually be controlled or at least 

 held in check by the application of fungicides to the plant. 

 The fungicide which is a chemical compound of some kind 

 is either placed in water and sprayed on the plant or dusted 

 on by some method. 



The bacterial diseases cannot be controlled by the applica- 

 tion of any fungicide and the only means of control is to cut 

 away the affected part or destroy the entire plant. 



The following troubles are a few that cannot be controlled 

 Ijy spraying, but must be held in check by other means: 

 peach yellows, little peach, potato scab, melon wilt, pear 

 blight, apple-twig blight, and dry rot of the potato. 



Bacterial Diseases. — Bacterial diseases are caused by the 

 action of small microscopic organisms commonly known as 

 bacteria. These are \'ery small, usually one-celled plants 

 that live for the most part in the cell sap or in the juices 

 found in the plant tissue. From the nature of their attack 

 it is apparent that they are responsible for a disease that 

 from its nature is extremely difhcult to control. vSince the 

 bacteria are internal, either precautionary or A-ery drastic 

 means must be devised to hold them in check. 



Bacteria ordinarily are extremely simple in form, and are 

 usually confined to three types, namely spheres, cylindrical 

 rods of greater or less length, and spiral rods. In size they 

 are Aery minute, being by far the smallest living organisms 

 known, and demanding the highest power of the microscope 

 for their study. 



Many bacteria ha\'e the power of motion, which is produced 

 by slender, motile hairs arising from their bodies. By mo\-ing 

 these little hairs back and forth the bacteria produce loco- 

 motion and change their position. These hairs are called 



