SPRAYING PROBLEMS 145 



The role of Burgundy mixture is roughly analogous 

 to the use of a shrapnel screen put up to prevent 

 hostile aeroplanes from reaching the metropolis. The 

 barrage protects London: so the chemical " barrage' 

 protects the potato plant. 



The copper carbonate, or Burgundy mixture, is 

 insoluble in water and non-poisonous to plant life. 

 When once allowed to dry on, the heaviest rains will 

 not remove the deposit. To return to our illustration, 

 the more perfect the barrage the less chance there is 

 that the hostile machines will reach London. Again, 

 it is useless putting up the barrage after the machines 

 have reached their destination. So with Burgundy 

 mixture, if the plant is to be thoroughly protected, it 

 must be efficiently covered, and covered before the 

 blight makes its appearance. 



The question naturally arises: How does this non- 

 poisonous coat prevent or kill the germs of potato 

 blight? Barker has found that the fungus in ger- 

 minating excretes a vegetable acid which reacts with 

 the chemical film to produce a soluble salt of copper; 

 this soluble copper poisons the growing germ tubes. 

 In the case of the American gooseberry mildew, the 

 mycelium takes a green coppery stain, proving the 

 release of soluble copper. 



Spencer Pickering states that the carbon dioxide 

 present in the atmosphere reacts with the film of 

 Burgundy mixture to produce soluble copper. Be- 

 sides carbon dioxide, various impurities present in 

 the atmosphere in the neighbourhood of manufactur- 

 ing areas, chemical works, kilns, &c., when brought 

 down by rains, are probably capable of releasing 

 soluble copper, and producing it in sufficient quantity 

 to cause injury to the host plant. 



Recent cases of injury through spraying are chiefly 

 from manufacturing areas or allotments in the neigh- 



( C 948 ) 11 



