77^ 



Summary of Agricultural Experiments, [dec, 



Weeds and Plant Pests. 



The Action of Carbon Dioxide on Bordeaux Mixtures (C. T. Giming- 

 ham, Jour. Agric. Science, Vol. iv., No. i, May, 191 1 : Cambridge 

 University Press).— Mr. Pickering has shown that when carbon dioxide 

 is passed through Bordeaux mixture the basic sulphates contained in 

 it are decomposed with the formation of copper sulphate. To the solu- 

 ble copper sulphate liberated in this way he attributes the main fungi- 

 cidal action of Bordeaux mixtures. 



In the investigation described in this paper, however, Mr. Giming- 

 ham found that the whole of the copper thus brought into solution by 

 the action of CO a for a short time was reprecipitated in an insoluble 

 form on the removal of the C0 2 by allowing the liquid to stand exposed 

 to the air or by passing air through it. It appears therefore that 

 though copper sulphate is formed by the action of CO a on Bordeaux 

 mixture, it is immediately changed into basic copper carbonate, which 

 is only soluble in the presence of C0 2 . This, though quite consistent 

 with Pickering's work, is difficult to reconcile with the view that 

 atmospheric carbon dioxide is the important factor in bringing into 

 play the fungicidal properties of Bordeaux mixtures. To test this 

 under more practical conditions, Bordeaux mixture was allowed to 

 stand in a shallow layer exposed to the air, or had a current of air 

 drawn through it, for long periods, but though a portion of the copper 

 could be changed into the carbonate by atmospheric carbon dioxide, no 

 more than a trace was kept in solution. The conclusion drawn from 

 these experiments is that, although the action of carbon dioxide in 

 excess on the compounds present in Bordeaux mixtures brings copper 

 into solution, yet it does not appear that sufficient copper is held in 

 solution by carbon dioxide derived from the atmosphere to be of 

 importance as a fungicide. It is difficult to apply experimental results 

 directly to practical conditions, because we have no accurate knowledge 

 of the state of affairs in a film of dew or rain on the surface of a 

 leaf; possibly traces of carbonate of copper are present in solution 

 under some conditions, but that there should ever be enough carbon 

 dioxide to dissolve more than very minute amounts seems most un- 

 likely. 



The Fungicidal Action of Bordeaux Mixtures (B. T. P. Barker and 

 C. T. Gimingham, Jour. Agric. Science, Vol. iv., No. 1, May, 191 1 : 

 Cambridge University Press). — The problem of the manner in which 

 Bordeaux mixture exerts a fungicidal action, apart from its physio- 

 logical effect on plants, is generally accepted to be equivalent to the 

 question of how the copper, which is present in an insoluble form, is 

 rendered soluble. 



A purely chemical explanation — that copper is brought into solution 

 by the action of the carbon dioxide of the air — has been rejected by 

 one of these authors in the paper noticed above. Two other theories 

 have been advanced, viz., that either the leaves, or the fungus itself, 

 convert some of the copper into the soluble form by which the fungus 

 is poisoned. The supposition that excretions from the leaves exert a 

 solvent action was examined by some experiments, in which jars 

 containing distilled water and Bordeaux mixture precipitate were fixed 

 round growing shoots of fruit trees. In all cases but one, using 



