266 DISEASES OF TROPICAL PLANTS OH. 



becoming more and more abundant and more de- 

 structive, and this belief is in a great measure true. 

 While the species of organisms which cause the diseases 

 may not have increased in number, they have become 

 more widely distributed. This wide distribution is due 

 to the improvements in the facilities for travel, and to 

 commerce, immigration, increased agricultural interests, 

 increased commerce in agricultural products, the intro- 

 duction of plants from one part of the world to another, 

 and the development of new varieties of plants. 



The emigrant carries the plants which he has been 

 accustomed to use for food and ornamentation into his 

 new home, and on some of these plants are introduced 

 the organisms which cause plant diseases. Improved 

 facilities for transportation have developed a commerce 

 in perishable fruits and vegetables on which organisms 

 of disease, especially those which cause decay, are intro- 

 duced from one place to another. The increase in 

 transportation, commerce, and population has been 

 accompanied by an increase in agriculture throughout 

 the world. This increase in agriculture has necessitated 

 the introduction of useful plants from one part of the 

 world to another, and these introduced plants have 

 carried with them the various organisms of disease to 

 which they were subject in their original habitat. 

 These diseases may have proved more or less destructive 

 in their new environments than in their old homes, or 

 they may have spread to other plants, on which they 

 have become more destructive than on their original 

 hosts. This is well illustrated by the mildew of the 

 American gooseberries, which has been introduced into 

 Great Britain and become very destructive on the 

 English varieties, although it is of little consequence 

 on the American varieties, and by the mildew of the 

 American grape which has been introduced into 

 Southern Europe and proved so destructive, although 

 of little consequence on the American grapes. 



It is also true that the introduced plant may meet 

 with organisms of disease on related plants in its new 



