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) an d 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 
