PLANT DISEASES 311 



resemble closely a slime, while at another time they appear mould-like; 

 hence, the name. The dreaded club root disease of Cabbage and aUied 

 plants, is caused by a sHme mould as is also powdery scab of 

 Potatoes. 



Bacteria are perhaps better known because of their connection 

 with disease in man. This has been emphasized so much that there is 

 a general behef that most bacteria are harmful, although actually most 

 of them are beneficial. Bacteria are either rod-shaped, spherical or 

 spiral like. Those which cause diseases in plants belong to the rod- 

 shaped groups. At the present time, there are over 150 plant diseases 

 known to be caused by bacteria, including fire bhght, one of the most 

 serious diseases of Pears, Apples, and Quinces. 



Fungi make up a large group of lower plants of great diversity 

 of form and life history. The well known mushrooms and puifballs 

 belong to this class as well as countless microscopic forms, like rusts, 

 mildews, etc.; some are parasites while many are not. Probably the 

 largest number of diseases of plants is caused by fungi. 



A knowledge of the life history of these organisms is especially 

 important if one is to devise and use methods of preventing the diseases 

 which they cause. We try to determine the point in the life history 

 at which the pathogene (agent that causes disease) is most vulnerable 

 and strike there. 



Concerning the Control of Diseases 



All methods of controlling plant diseases are based upon one of 

 four principles: exclusion, eradication, protection and immunization. 

 Exclusion is a matter of preventing the introduction of disease pro- 

 ducing fungi, bacteria and the like into a given country or locaUty. 

 This is most commonly attempted by legislation, exclusion laws and 

 quarantines. Eradication refers to the removal of a pathogene from 

 an area when once it has gained a foothold, and involves many opera- 

 tions, such as sorting out and destroying diseased seed, plants and plant 

 parts, soil sterilization, crop rotation and seed disinfection. This 

 principle may be apphed in the control of a large number of diseases. 

 In practicing protection methods we work from the standpoint of a 

 plant and place some barrier between it and the parasites which attack 

 it. In the operation of spraying which is a protective measure, we 

 place a layer of poison called a fungicide on the leaves of a plant and 

 when the fungus comes in contact with the poison it is killed. The 

 addition of certain substances to the soil to prevent organisms from 

 attacking the roots of plants, as in the liming of soil to prevent club 

 root of Cabbage, may be classed as a protective measure. Immuniza- 

 tion involves the selection or breeding of varieties or strains which are 



