Studies on Fungicides — III 31 



Butler (1914) carefully reviews the literature relative to the action of 

 C0 2 on bordeaux mixture. He appears, however, to have performed but 

 few experiments himself. In one case he found that carbonated bordeaux 

 mixture containing 0.5 per cent of cupric sulfate was not toxic to spores 

 of Phytophthora infestans or of Plasmopara viticola in tap water, but that 

 when distilled water was used, regardless of the ratio of cupric to calcic 

 oxide, the mixtures were toxic at 0.0625 per cent. This difference, he 

 explains, is due to the fact that tap water contains calcic bicarbonate in 

 solution, and " calcic bicarbonate is a precipitant of soluble copper in the 

 electro-positive condition " (page 139 of reference cited) ; hence, because of 

 this, the tap water is unable to dissolve the copper in bordeaux mixture. 

 Butler therefore concludes, from this experiment, that bordeaux mixtures 

 after carbonization are slightly soluble in pure water, and that they will 

 dissolve readily in water containing C0 2 in solution — the latter presum- 

 ably a conclusion drawn from the literature. Finally, he holds that these 

 mixtures are soluble also in water containing ammonic salts in solution. 

 No experimental proof is presented for this last conclusion. 



Pantanelli (1920), from his investigations, is inclined to the view that 

 copper sprays are efficient for long periods, as the hydroxide is slowly acted 

 upon by the C0 2 and rendered less basic and more soluble. 



Cerasoli (1926) advances the hypothesis that many of the components 

 of bordeaux mixture, such as the basic sulfates of copper, are hydrolyzed 

 by water to form gelatinous copper hydrate. The copper hydrate dis- 

 sociates very slightly, giving copper and hydroxyl ions. Exposed on the 

 leaf, the carbonic acid derived from the C0 2 of the atmosphere and of 

 plant respiration furnishes hydrogen ions. These hydrogen ions combine 

 with the hydroxyl ions of the base to form water which is practically undis- 

 sociated. The product of the copper and hydroxyl ions then becomes too 

 small, and the equilibrium is destroyed by a further dissociation of the 

 copper hydrates. This production of copper and hydroxyl ions will con- 

 tinue as long as there remains any undecomposed copper hydrate. In 

 such a manner the sprayed plant parts are coated with an infinitesimal 

 amount of soluble copper, which is sufficient to inhibit the germination of 

 fungus spores. Cerasoli offers no experimental evidence. 



Schmidt (1924) reviews the literature briefly, and states that of some 

 six theories only two can now be considered as probable; namely, that of 

 Millardet on the action of atmospheric C0 2 , and that of the solvent 

 action of the spores as advanced by Ruhland (1903) and others. With 

 these two hypotheses in mind, Schmidt undertook various experiments. 

 Excess-lime bordeaux mixture was made, potassium ferrocyanide was 

 added, and leaves of apple, pear, and grape were sprayed. Only after a 

 lapse of time did the red color appear to show the presence of soluble 

 copper. Similar results were obtained on filter paper. A little potassium 



