384 



action of the atmosphere, more especiiiUy owing to the presence of 

 the carbon dioxide in it; (2) that the insokible compounds are dis- 

 solved by some substance or substances secreted by the sprayed 

 leaves ; (3) that the copper is rendered soluble by some substance 

 secreted by the fungus itself, which consequently brings about its own 

 destruction. 



The first of these theories was supported by Pickering (see 

 Elevc^nth Report on the Woburu Experimentnl Fruit Farm, 1910); but 

 recently, further work carried out by Gimingham, and by Barker and 

 Gimingham, discredits this theory and lends support to the last, 

 namely that of the action of the fungus itself. { Journal of Agricul- 

 tural Science, Vol. IV, pp. 69 and 76. 



Pickering found that the insoluble substances containing copper 

 formed in the preparation of Bordeaux mixture were partly dissolved 

 in water containing a large amount of carbon dioxide and that copper 

 sulphate was formed in the solution. This naturally led to the idea 

 that the fungicidal action of the mixture was due to the effect of 

 atmospheric carbon dioxide on the insoluble copper precipitate. 

 Furthermore, he observed that when the mixture contained excess of 

 lime, no copper appeared in the solution until all the lime had been 

 converted into chalk by the action of the carbon dioxide. Conse- 

 quently, he recommended that care should be taken to prepare 

 Bordeaux mixture v;ithout excess of lime, since this substance would 

 only delay the action of the mixture as a fungicide. 



Gimingham, however, found that if the excess of carbon dioxide 

 was removed from the liquid, the copper was again precipitated in an 

 insoluble form ; while as the result of several experiments he finally 

 concluded that it was unlikely that the copper was rendered soluble 

 by the action of the atmiosphere. 



The possibility that the copper is rendered soluble by secretions 

 from the sprayed leaves was examined by Barker and Gimingham. 

 They found that a certuin amount of soluble copper sulphate was 

 produced by the substances secreted through minute punctures or 

 abrasions on the surfaces of the leaves. This quantity, however, was 

 not sufficient to account for the fungicidal action of the Bordeaux 

 mixture; on the other hand, it was enough to cause scorching of the 

 leaves in spots, and explains why older leaves which have been longer 

 subject to possible damage are more liable to scorching than younger 

 foliage, since undamaged leaves do not appear to secret the necessary 

 substances. 



The same workers then turned their attention to the possibility 

 that the copper is rendered soluble by substances secreted by the 

 spores or germ tubes of the fungi themselves. They found that spores 

 possessing thin walls, and also the tips of young germ tubes, do 

 actually appear to secret small quantities of substances sufficient in 

 amount to dissolve enough copper to cause their death. The amount 

 of copp.'r dissolved depended, however, on the distance of the spore 



