8 BULLETIN 866, U. S. DEPARTMENT OF AGRICULTURE. 



To prepare Pickering (C) sprays containing in the finished spray 

 the equivalent of — 



6 per cent copper sulphate: Mix enough of the stock solution of copper sulphate 

 to obtain 1 pound. 8 ounces of crystallized copper sulphate with 38.98 U. S. gallons 

 of the stock limewater, and make up the total volume to 50 IT. S. gallons. 



O.SS per cent copper sulphate: Mix enough of the stock solution of copper sulphate 

 to obtain 15.3 ounces of crystallized copper sulphate with 18.52 U. S. gallons of the 

 stock limewater, and make up the total volume to 50 U. S. gallons. 



0.115 per cent copper sulphate: Mix enough of the stock solution of copper sulphate 

 to obtain 7.7 ounces of crystallized copper sulphate "with 9.26 U. S. gallons of the stock 

 limewater, and make up the total volume to 50 I T . S. gallons. 



Samples of water used in the various localities for the preparation 

 of Pickering and Bordeaux sprays, analyzed by the Water Laborator}^ 

 of the Bureau of Chemistry, contained very little lime or any other 

 constituent that might interfere with the preparation of sprays, 

 such as the Pickering sprays, which are made according to definite 

 formulas and are said to depend for their activity on the presence of 

 definite compounds, the basic sulphates of copper. According to 

 the tests of Bedford and Pickering (3, 4), slight variations in the 

 amounts of limewater employed result in the formation of different 

 basic sulphates of copper, each of which functions as a fungicide in 

 a characteristic way. 



In slaking lime it is important to add just enough water to make 

 it heat, after which water is added slowly to keep the lime from 

 burning. When the reaction nears completion more water is added 

 to make a paste. FinaUy, when the total amount of water required 

 has been added, the solution is stirred to form saturated limewater. 



RESULTS OF INVESTIGATION. 



potatoes. 



Blight Control and Yield, 

 pickering and standard bordeaux sprays in 1916. 



Six acres of Green Mountain potatoes in northern Maine (near the 

 Aroostook Farm at Presque Isle) were selected for these experiments. 

 As the sprayer was made to spray four rows at a time, the field was 

 divided into four-row plats which, in turn, were subdivided into 

 200-foot lengths. Since the field was 800 feet long, four subdivisions 

 were made in each group of four rows. A certain spray was applied 

 to the first and third 200-foot plats, another one to the second and 

 fourth 200-foot plats, etc. Thus each spray was applied to one plat 

 in the front and to one in the rear of the field. The plats were so 

 arranged that each four-row plat where any particular spray was 

 being tested had a four-row plat of Bordeaux-sprayed potatoes on 

 one side and a four-row check plat on the other. 



Seven Pickering sprays were tested. Four (those used on plats " 

 1, 2, 3, and 4) were made according to Formula A, while three (those 



