90 



DISEASES OF CROP-PLANTS 



For insecticidal purposes lead arsenate paste 1-2 ounces to the 

 gallon (or powder ^-i ounce), or Paris green i ounce to 7 gallons 

 (added to the milk of lime before mixing), as stomach poisons, 

 or Black-leaf 40 (nicotine sulphate) i ounce to 4-6 gallons or i 

 teaspoonful to the gallon, as a contact poison, are frequently 

 added to Bordeaux mixture. 



G. E. Sanders and A. Kelsall have worked out a method of 

 combining white arsenic, which is relatively cheap, with Bordeaux 

 mixture. One pound of superfine white arsenic (which should 

 pass through a screen of 250 meshes to the inch), mixed with one 

 pound of dry hydrated lime, is first added to each 10 gallons of 

 the water used in preparing the usual 10 per cent, stock solution 

 of copper sulphate. The latter, thoroughly stirred before dilu- 

 tion, is then used in the ordinary way. 



Preparation. 



The standard method for making the usual alkaline Bordeaux 

 mixture will now be described. The proportions are indicated 

 in the following table : — 



It is usual to have two tubs each holding 25 gallons and one 

 barrel holding 50 gallons. The copper sulphate is dissolved in 

 25 gallons of water in one tub. When left standing it dissolves 

 best if it is suspended near the surface tied up in a piece of 

 sacking. The lime is slaked in a bucket by adding water a little 

 at a time and at intervals of 5-10 minutes until it heats, swells 

 up, and crumbles, after which water can be added freely. Time 

 and trouble can be saved with some limes by using hot water. 

 The resulting milk of lime is turned into the second tub and made 

 up to 25 gallons. The contents of the tubs are now run together 

 at the same rate into the barrel, or baled out bucket for bucket. 

 The mixture is then ready for use, and the more quickly it is 

 used the better. If required, the preservatives, adhesives or 

 insecticides previously described can now be added. In filling 

 sprayers the mixture should always be passed through a strainer 

 of fine copper or brass gauze. 



When Bordeaux mixture is needed from time to time, it is 

 convenient to prepare stock solutions of each material. These 

 are usually of 10 per cent, strength, i lb. of copper sulphate to 

 each gallon of water in the one case, and i lb. of lime to the 



