426 



General Botany 



Rotating crops on farms so that there is small chance of the 

 disease-producing organisms being carried in the soil from one 

 season to the next has been found an efficient deterrent of certain 

 diseases. The root rot of tobacco, wilt of potatoes, and smut of 

 corn are some examples of soil-borne diseases. 



Sterilizing soils and treating seed with fungicides before plant- 

 ing are used to control diseases that attack seedlings. Finally, 

 diseases of one state or country may be excluded from another by 

 the enforcing of a strict quarantine. At the present time nursery 

 stock grown in Europe is excluded from the United States. 

 Nursery stock that is to be shipped is also carefully inspected by 

 government experts for symptoms of certain diseases, and only 

 healthy stock may be shipped from one state to another. 



Symptoms of disease. The external signs of diseases are of 

 many kinds, among which the following classes may be noted : 



Erwin F. Smith, U.S.D.A. 

 Fig. 264. Tumor on a leaf of Bryophyllum inoculated with Bacterium iumifaciens. Roots 

 are developing from the tumor at r. This disease is commonly known as "crown gall." 



