ECONOMIC IMPORTANCE OF FUNGI 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) to America (1896), and the potato blight from Chili 

 to Colorado and across North America to Europe (1845). 

 The downy mildew of the grape is an example of a disease 

 probably originating in North America, where it has been 

 known from the earliest times, and travelling thence to 

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

 as Greece by 1881, and to Brazil by 1890. 



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 all 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 year only from the immune or most resistant 

 individuals, a crop may sometimes be obtained which not 

 only withstands the disease itself, but interferes with or 

 finally stops entirely its spread. No phase of plant breed- 

 ing is more important than this. 



5. Vaccination. When bacteria produce poisons (toxins) 

 in the system, the cells affected are stimulated to secrete 



