Studies on Fungicides — III 43 



fungicidal action, with a substance showing a minimum of solubility, though the possibility 

 of this action depends entirely on the substance passing into solution. 



Barker and Gimingham (1914 b: 223), as a result of their work on the 

 localized action of solvent excretions from fungus spores, say: "Pickering's 

 explanation that death is due to absorption of minute traces of copper in 

 solution in the continuous film of liquid covering the preparation abso- 

 lutely fails to explain such cases." 



There are a number of other hypotheses related to fungicidal action. 

 They are not concerned directly with the solution of the copper, however, 

 but primarily with the toxicity of copper. Since a study of the principles 

 of toxicity is not within the scope of this paper, they are not discussed 

 here. 



Some workers, notably the Villedieus (1920a, 1920b, 1924), and G. 

 Yilledieu (1921, 1924), have maintained that bordeaux mixture owes 

 its protective action almost entirely to its basicity. There is little experi- 

 mental evidence in support of this hypothesis. 



Hooker (1924), studying the fungicidal properties of colloidal copper 

 hydrate, suggests that the action of the copper is a catalytic one promot- 

 ing oxidation. This is similar to the view of Dubois (1923), that copper 

 and its salts appear to destroy fungi by an action comparable to that of 

 oxidases and peroxidases and not by direct toxic action. No experimental 

 proof was offered in either case. 



The view, especially held by a number of German investigators, that 

 the copper is absorbed by the leaf and is thus present to protect the sus- 

 cept from invasion, is not dealt with here. In any case the copper must 

 presumably be in a soluble form in order to enter the leaf. We are thus 

 still faced with the question of the solution of the copper. 



EXPERIMENTAL WORK 

 PROCEDURE 



In the studies conducted at this laboratory, various methods of approach 

 have been used in an effort to detect the exertion of solvent action by 

 tin' spores of fungi. The methods are similar, however, in that spore 

 tests have been universally employed; that is, spores have been used 

 directly to detect the presence of soluble toxic materials. We are here 

 concerned with such minute quantities of the toxic substances that the 

 usual chemical tests have been found inadequate. However, certain 

 types of spores will give a very definite reaction to the presence of dele- 

 lerious substances in extraordinarily small amounts. This is especially 

 so in the case of copper. Furthermore, in nature it is the spore itself 

 which in the final analysis must be the indicator of toxicity. 



The general method and technic of testing by spores have been dis- 

 cussed in detail in the second paper of this series. The procedure there 



