PREVENTIVES OF FUNGOUS DISEASES 267 



alternate from one host to another the destruction of one 

 form may prevent the development of the other. Thus, 

 the easiest way to prevent injury by apple rust is to cut 

 down the cedar trees on which the infecting spores develop. 

 In the case of others, the fungus may develop on wild 

 plants and the spores be carried to those cultivated. Thus, 

 in fighting black knot in plum and cherry orchards it is 

 essential that the infested wild cherry trees in the same 

 locality be destroyed. 



Remedial Measures 



The use of fungicides — or substances that kill fungi — 

 has recently become the most generally adopted method of 

 preventing these diseases. These substances are appUed 

 to the surface of the soil, stems, foliage, or fruit, where they 

 may kill the spores present at the time and may also re- 

 main for a considerable period in condition to destroy any 

 spores that subsequently alight. 



The various salts of copper are among the most impor- 

 tant fungicides. These are used in a great many ways, but 

 the most important combination is that of copper sulphate, 

 commonly called blue vitriol with lime and water. This 

 mixture was first used in Bordeaux, France, to prevent cer- 

 tain grape diseases. So it is generally called the Bordeaux 

 mixture. It is now the most widely used fungicide and is 

 recognized as a sure preventive of many plant maladies. 

 The usual formula is : Copper sulphate, 4 pounds ; unslaked 

 lime, 4 pounds ; water, 50 gallons. 



The application of Bordeaux mixture to apple trees often 

 causes a troublesome russeting of the fruit that lessens the 

 value of the crop. On this account, dilute lime-sulphur 

 washes are used in its place in inany orchards. 



Copper sulphate is also used in simple water solution. 



