44 PROTECTION OF PLANTS, 1920-21 



appearance in solution. If this occurs on such tender foUage as peach or apple, 

 copper injury results. The decomposition of the copper compound may be 

 represented somewhat as follows : 



2(10Cu.0.S03.3Ca0)+15 C02=9(CuO) 2.CO2+6CaC03+2CuS04. 



Thus free copper sulphate is liberated. It is apparent that this reaction 

 cannot take place while any free lime remains in the spray film. For this reason 

 large excesses of lime are necessary on tender foliage. 



While discussing Bordeaux mixture, it is appropriate to mention the copper 

 dust recently brought forward by Mr. Sanders, and which is being so extensiv- 

 ely used in Nova Scotia. This dust is really a Bordeaux mixture. The novel 

 feature being the use of the copper sulphate mono-hydrate, CUS04H2O, This 

 form of copper sulphate is a fine white powder very suitable for dusting, while 

 the ordinary blue copper sulphate, CUSO4.5H2O, does not lend itself to the same 

 purpose. 



Sulphur 



Finely ground sulphur is extensively used for dusting, and has a fairly high 

 fungicidal value. For want of more knowledge on the subject, it is assumed 

 that the fungicidal action depends upon a slow oxidation of the sulphur to 

 sulphur dioxide, SO2, which material is known to have a powerful toxic action 

 on all lower forms of life. 



In order to reduce the cost, hydrated lime has sometimes been mixed with 

 sulphur. When this is done, sulphur dioxide would be taken up by the lime, and 

 it might be supposed the fungicidal value thereby lessened. However, with 

 lime and sulphur in the presence of water a reaction occurs somewhat as follows : 



3Ca(0H)2 +4S=2CaS+CaS203+3H2 and soluble calcium sulphide is 

 formed. Does this calcium sulphide have as high fungicidal value as the sul- 

 phur dioxide from free sulphur ? This is a point on which work is needed. 

 Mixtures of sulphur and lime do have a definite fungicidal value, for many 

 people use it with success. Our own experiments in Nova Scotia would strongly 

 indicate that mixtures of lime and sulphur were inferior to free sulphur. 



