184 Principles of Plant Culture. 



328. Fungi that Infect their Host from Spores 

 Deposited On the Aerial Parts of the plant, as the 

 scab of the apple* and pear, and the downy grape-vine 

 mildewt may be held in check by applying a fungicide 

 (321) to the host plant, to destroy the spores as they 

 alight upon it. Various compounds of copper and of 

 sulfur are destructive to the spores of fungi, and when 

 properly applied, are harmless to the plant. The cop- 

 per compounds are more generally satisfactory, since 

 they have the greater adhesive power. 



329. The Bordeaux Mixture, which consists of a 

 compound of copper sulfate (324) and lime, is now ex- 

 tensively used to prevent many fungous diseases of 

 this class. A standard formula for the Bordeaux mix- 

 ture is: 



Dissolve 5 pounds of copper sulfate in 25 gallons of 

 water by suspending it in a bag of coarse texture near 

 the surface of the water ; slake 5 pounds of fresh quick- 

 lime in sufficient water to form a paste and dilute to 

 25 gallons. Pour the two solutions together. 



Metal vessels, other than those of brass or copper, 

 should not be used. 



Prepared by the above formula, the Bordeaux mix- 

 ture often contains more lime than is needed for the 

 chemical action that occurs. To avoid this excess of 

 lime, a chemical test may be used, as follows: Pour 

 only half of the slacked lime and water into the cop- 

 per-sulfate solution, stir well, put a few teaspoons of 

 this liquid into a bottle and add a few drops of a 20 

 per cent solution of potassium ferrocyanide. If a rich, 

 reddish-brown color is produced, more lime is needed. 



Venlnria I'oml. f l^lasmopara viticola. 



