Studies on Fungicides — III 29 



readily, though it acts to some extent on the basic sulfate of copper, pro- 

 ducing ordinary copper sulfate. The excess lime still present, however, 

 reconverts this soluble copper sulfate to the basic sulfate. This cycle 

 continues, finally resulting in only a part of the total copper being brought 

 into solution, and then not until nearly all of the excess lime has been 

 carbonated. 



In nature there is no appreciable delay in the fungicidal action of bor- 

 deaux mixture. Pickering explains this apparent contradiction by stat- 

 ing that in the laboratory the basic sulfates were constantly wet and any 

 soluble copper found would be immediately precipitated by the lime- 

 water. In nature the bordeaux mixture dries, and the particles of basic 

 sulfate and of lime become separated from each other. Hence, when 

 the former is attacked by the CO* there may be no lime particles sufficiently 

 near to decompose the copper sulfate formed. In view of the fact that 

 fungicidal protection is usually exerted only when the spores are on a 

 wet surface, this explanation seems hardly sufficient. Pickering considers 

 this theory established by a simple laboratory experiment, in which filter 

 paper is moistened with potassium-ferrocyanide solution and dried. When 

 a drop of bordeaux mixture is placed on the paper, there is no reaction 

 as long as it is wet. However, immediately on drying, the lime particles 

 no longer have access to the particles of basic sulfate, and soluble copper 

 appears as is shown by the formation of red copper ferrocyanide. 



Gimingham (1911) also passed C0 2 through suspensions of the various 

 basic sulfates similar to those used by Pickering (1907, 1909) and by Bed- 

 ford and Pickering (1910). In all cases the resulting soluble copper was 

 immediately reprecipitated on the removal of the C0 2 . This led Giming- 

 ham to believe that the soluble copper was probably in the form of a car- 

 bonate dissolved in the C0 2 solution. When the action of the C0 2 was 

 prolonged, the soluble copper did not easily reprecipitate. The action of 

 ordinary air brought little or no copper into solution. Gimingham con- 

 cluded that only in the presence of a large excess of C0 2 , such as could 

 hardly occur out-of-doors, is an appreciable amount of copper brought into 

 solution, and that " it appears impossible to assign the fungicidal action 

 to copper sulphate liberated by atmospheric carbon dioxide " (page 75 of 

 reference cited). 



Barker and ( liminghain (1911:86) slate: " The fungicidal action of the 

 bordeaux compound was evidently not in any way dependent upon the 

 presence of carbon dioxide." This conclusion is based on an experiment 

 in which conidia of Nectria ditissima and Sclerotinia fructigena were sown 

 on a fresh precipitate of no-excess-lime bordeaux (lOCuO, S0 3 ). In the 

 absence of C0 2 , there was very poor germination and any feeble growth 

 was soon halted. The response of the controls, exposed to the air, was the 

 same. 



