ECdN'OXIIC IMPORTANCE OF FrXGI 313 



Immigrants coming to America from foreign countries are 

 carefully examined, and if they have a contagious disease 

 they are isolated (placed in quarantine) until well. 



The United States Government maintains a stringent 

 quarantine against the shipment of diseased plants from 

 one state to another, or from foreign countries into the 

 United States. Some, if not all, of our worst plant diseases 

 have been imported. The map (Fig. 227) shows how the 

 rice smut travelled from Japan to South Carolina in 1898, 

 the chrysanthemum rust from Japan through England 

 (1895 1 to America (1896), and the potato bhght from ChiU 

 to Colorado and across Xorth America to Europe (1S45). 

 The do\vn}" nuldew of the grape is an example of a disease 

 probably originating in Xorth -\merica, where it has been 

 knowTi from the earUest times, and travelling thence to 

 France (1873) and other parts of Europe, reaching as far 

 as Greece by iSSi, and to Brazil by 1S90. 



These brief references indicate the importance of main- 

 taining a strict quarantine on plants at all our ports of 

 entry. 



4. Breeding of resistant varieties. This is one of the 

 most important of aU prophylactic measures. Just as 

 some persons are immune to certain contagious diseases, 

 so certain plants in a crop are less susceptible than others, 

 or even entirely immune, to a given disease. By choosing 

 seed each Aear only from the immune or most resistant 

 indiAiduals, a crop may sometimes be obtained which not 

 onlj- withstands the disease itself, but interferes vrith. or 

 finally stops entirely its spread. Xo phase of plant breed- 

 ing is more important than this. 



5. Vaccinution. ^Mien bacteria produce poisons (/o.v/h5) 

 in the svstem, the cells affected are stimulated to secrete 



