450 ELEMENTARY STUDIES IN BOTANY 



The list of fungicides is a long one, and would not be 

 appropriate here. Their names and their composition can 

 be obtained easily if needed. The most famous and the 

 most generally useful is called " Bordeaux mixture," which 

 was discovered in connection with the ravaging of European 

 vineyards by mildew. It is a mixture of copper sulphate and 

 lime. 



When it is discovered that a fungicide is appropriate, the 

 next thing is to know when to apply it. This can be made 

 plain by a few illustrations. The grape mildew infects the 

 grape-vine by means of its spores falling upon young leaves. 

 Accordingly, the young leaves are sprayed with the fungi- 

 cide, and this treatment has proved to be completely effective 

 in controlling the disease. 



The case of potato disease (" potato rot ") is somewhat 

 different. Here also the disease is spread by wind-blown 

 spores, which infect young leaves. Therefore, early spray- 

 ing of potato plants with Bordeaux mixture checks the 

 spread of the disease. But the more serious trouble comes 

 from the infected tubers which pass the disease on from 

 generation to generation. The fungicide treatment in this 

 case, therefore, does not eliminate the disease, but simply 

 checks its spread. 



In the brown rot of stone fruits, the infecting spores are 

 lodged on the bark and leaf buds. It follows that these 

 spores should be destroyed by the application of a fungicide 

 in late winter or early spring. In brown rot, in addition to 

 spores lodged on bark and leaf buds, there is danger of infec- 

 tion from mummied fruits hanging on the twigs or fallen on 

 the ground, and it is evident that all such fruits should be 

 destroyed. 



Of course the powdery mildews, such as attack grapes and 

 induce a skin disease of the fruit, are easily reached and 

 killed by a fungicide while the fruit is young. 



These illustrations will serve to indicate what is meant 



