1904.] 



Potato Disease. 



285 



contact with it. This remedy is of especial value where 

 potatoes are grown under glass. 



Neither disease-resisting varieties nor cultivation can be 

 trusted entirely to ward off the disease in a wet season, but 

 spraying with various preparations of copper has been found 

 effective and is now part of the regular routine of cultivation 

 in humid districts like the West of Ireland. Even in dry 

 seasons when no disease is apparent the treatment is found to 

 be beneficial, producing a longer period of growth and an 

 increased yield. This is so generally recognised that spraying 

 has become usual in the potato-growing districts of the 

 Lothians, whatever the season promises to be. As the disease 

 does not, as a rule, make much headway before the end of 

 July, spraying is not wanted for the first and second early 

 sorts whose leaves will be dying down before any great harm 

 is done. The principle of the spraying is to coat the leaves 

 with a very slightly soluble compound of copper, the hydrate 

 or the hydrated carbonate. This will just dissolve sufficiently 

 to act as a fungicide without injuring the plant, and at the same 

 time it will not be removed as any more soluble substance would 

 be by the first shower of rain. 



The ordinary Bordeaux mixture should be made as follows : — 



Sulphate of copper (bluestone) ... ... ... 12 lb. 



Quicklime ... ... ... ... ... ... 6 lb. 



Water ... ... ... ... ... ... ... 100 gals. 



In purchasing the sulphate of copper care should be taken 

 to demand a product of 98 per cent, purity ; material offered 

 as " agricultural " sulphate of copper should be avoided. 



The usual adulterant of sulphate of copper is sulphate of iron, 

 which is much cheaper, but entirely ineffective for the present 

 purpose. An easy test for the presence of iron in the sulphate 

 of copper is to dissolve a little in water and add ammonia with 

 constant stirring until a deep blue liquid forms ; any quantity 

 of brown flocks floating about in this blue liquid indicates the 

 presence of so much iron that the material should be subjected 

 to a proper chemical analysis. 



The sulphate of copper should be dissolved in the bulk of 

 the water ; but, if it is thrown in lumps into the bottom of 

 the vessel it will take a long time to dissolve. Some people 



