GENERAL DISEASES 65 



means of soil disinfection applicable to fields is known. 

 Land once infected can therefore become again usable only 

 by eliminating the causal organisms by a long system of 

 rotation which is usually only partially effective, or by the 

 use of resistant varieties of plants, and these in most in- 

 stances are not yet known. 



In view of these facts it is especially necessary to stress 

 the importance of protection of healthy soils against these 

 diseases. To understand the means of protection the 

 modes of dissemination must be known. 



All plants affected with these diseases harbor immense 

 numbers of the reproductive parts of the causal organisms, 

 bacteria, spores, mycelium, etc. Upon the death and dis- 

 integration of the host plant they are liberated in the soil, 

 where they are able to live for considerable time. The 

 immensity of their number in a diseased plant is incon- 

 ceivable. Even a few diseased stems, roots, or leaves in 

 the field will stock the soil thoroughly with them. In- 

 fected plants, or soil in which infected plants have been, or 

 which bears parts of infected plants, can therefore convey 

 such diseases to healthy fields. 



Instances may be cited where the contagion has spread 

 by washing from higher land to lower; so, too, may it be 

 carried by any means which can convey soil from a sick 

 to a healthy field, notably through tools, which have been 

 used on diseased soil. Though apparently clean, such tools, 

 if they bear even minute particles of infected soil, may 

 convey hundreds of germs and thus start an epidemic in 

 an uninfected field. The hoofs of animals or the feet of 

 laborers may in a similar way bear disease-laden soil. 

 Wind passing over an infected field may pick up broken 



