66 Stevens — Science of Plant Pathology. [June 



the development of epidemics* by furnishing abundant mater- 

 ial for the parasitic organisms to attack, abundant nutriment 

 upon which thej may multiply, and abundant opportunity for 

 them to reach new hosts and spread the contagion. With 

 potatoes, for example, raised merely as garden crops, the 

 probability of an epidemic affecting the majority of gardens 

 is not so great as when potatoes are raised in vast fields. A 

 single field crop, once infested, so contaminates the air with 

 spores that other fields are almost sure to become infected. 

 The contagium becomes sufficiently multiplied to break the 

 quarantine, and a general epidemic results. Any factor 

 which tends to increase the occurrence of epidemics may 

 quickly raise a given disease from obscurity to a position of 

 commanding importance. So, too, dees the increase in value 

 of hitherto comparatively insignificant crops. The pecan and 

 cranberry are at present objects of particular solicitude by the 

 plant physician. 



With the importation of plants from foreign countries and 

 the transportation of plants from one part of the country to 

 another comes the possibility of increased disease transfer- 

 ence. Recent years have seen the San Jose scale spread from 

 the Pacific to the Atlantic; the asparagus rust from the 

 Atlantic to the Pacific; the hollyhock rust has invaded us 

 from Europe; the chrysanthemum rust from the Orient; the 

 watermelon wilt is now moving northward and the peach yel- 

 lows southward. In nearly all cases where the soil is dis- 

 eased the affected region is annually enlarging, so that soil 

 diseases a decade ago insignificant in the territory of their 

 occupation are fast assuming control of alarmingly large 

 regions. The growing of plants in larger quantities also 

 increase the amount of germ-bearing refuse to the ultimate 

 end that the very air and soil become germ laden. 



Civilization, higher culture and community life, especially 



*The use of the word epidemic in relation to plant diseases while ety - 

 mologically incorrect, seems justified since no other word conveys the 

 desired meaning and the meaning of this word is clear to all. 



