88 DISEASES OF CROP-PLANTS 



of the indicated number of gallons should be used. The 

 formulae in this book are given in Imperial measures. 



The Copper Compounds. 



The fungicidal action of the copper compounds is principally 

 of value in preventing the successful germination of spores, and 

 for this purpose they have to be adapted for use on the tender 

 surfaces of leaves and growing fruits. In order to have avail- 

 able over as long a period as possible an amount of soluble copper- 

 salt sufficient to kill the fungus without damaging the almost 

 equally susceptible tissues of the plant, they are applied in 

 compounds which only slowly set free the actual fungicide. 

 Bordeaux mixture, which consists of copper salts formed by the 

 interaction of copper sulphate and lime, is for general purposes 

 the most convenient and effective. 



Bordeaux Mixture. 



The popular way of indicating the strength of Bordeaux 

 mixture is by formula, thus 6-4-50 is recognised to mean the 

 mixture obtained by the use of 61b. of copper sulphate, 41b. of 

 stone lime, and 50 gallons of water. 



For purposes of scientific discussion the strength is best 

 expressed as the percentage of copper sulphate used in prepara- 

 tion, and the Imperial gallon of water having a weight of lolb.*, 

 the use of ilb. of copper sulphate to each 10 gallons of water 

 makes a i per cent, mixture, which is about the standard strength 

 required for spraying and dipping. A 2 per cent, mixture is some- 

 times used, and 0.5 per cent, may be found to be strong enough for 

 a particular purpose or may have to be adopted to avoid injury. 

 These strengths are obtained by doubling or halving the pro- 

 portion of copper sulphate and lime in the i per cent, mixture, 

 taking therefore alb. or |lb. of copper sulphate for each 10 

 gallons of water. 



The amount of lime used, provided that it is of good quality, 

 admits of considerable latitude. According to S. U. Pickering, 

 the requirement to decompose the whole of the copper sulphate 

 is one-fifth of its weight of pure lime. In ordinary practice a 

 considerable excess of lime is used, commonly a weight equal to 

 that of the copper sulphate, and the mixture is in consequence 

 strongly alkaline. This gives a wide margin to cover defects 

 in the lime, and has advantages, but it should be noticed that in an 

 emergency, or where lime is scarce and dear, half this quantity 

 of good lime can be used without changing the nature of the 

 resulting mixture. Attempts are sometimes made to use air- 

 slaked lime, but this is quite unsafe and produces a worthless 

 mixture. Where stone lime is unprocurable soda should be used 

 instead. (See Burgundy mixture). 



* The American gallon of water weighs 8.345 lb. 



