268 DISEASES OF TROPICAL PLANTS CM. 



have been greatly advanced. If it is possible to select 

 or develop varieties which produce improved fruits, 

 grains, etc., it is also possible to secure immune varieties 

 in the same manner. In a state of nature, many species 

 of plants have been able to resist disease and maintain 

 themselves, while under cultivation, varieties of these 

 same species suffer considerable loss and are sometimes 

 destroyed. 



In fact, in recent years certain wild species known 

 to be resistant to diseases have been selected and 

 grown with the idea of improving them and sup- 

 planting closely -related cultivated varieties which 

 were subject to disease, only to find that after a few 

 generations the progeny were as great sufferers from 

 the disease as the varieties that had been cultivated for 

 many generations. This is because in a state of nature 

 those wild individuals or varieties which were non- 

 resistant have been destroyed, while the resistant ones 

 have survived. For many generations cultivated 

 plants have been selected with reference to the market 

 values of their products and without regard to their 

 power to resist diseases. In recent years the plant 

 breeders have demonstrated that it is as possible to 

 develop and select varieties which are immune to 

 certain diseases, as it is to develop and select varieties 

 which possess any other character. Notable instances 

 of this is the development in the United States of a 

 variety of cotton which is resistant to root rot and a 

 variety of clover resistant to anthracnose. 



Much of this work can be done by the individual 

 grower. If a particular crop is seriously diseased, the 

 grower should examine it carefully for healthy or com- 

 paratively healthy plants, which should be reset or 

 from which the seed should be kept for use the following 

 year. These plants or this seed should be used on the 

 same land as the preceding crop, so that it may be 

 grown as nearly as possible under the same conditions 

 and have all possible opportunity to contract the 

 disease. This new crop should be carefully examined, 



