Studies on Fungicides — III 25 



STUDIES ON FUNGICIDES 



III. THE SOLVENT ACTION OF SPORE EXCRETIONS AND 

 OTHER AGENCIES ON PROTECTIVE COPPER FUNGICIDES 1 



S. E. A. McCallan 



In the first paper of this series it was pointed out that protective fun- 

 gicides, or protectants, are in general insoluble in water. Kahlenberg 

 and True (1896) have shown that the toxicity of copper salts is due to 

 the ions in solution. Therefore, in the case of copper protectants it may 

 generally be assumed that copper ions effect the toxic action and hence 

 account for the protective efficiency of the fungicide. The question then 

 arises as to the manner in which the copper is rendered soluble, since 

 presumably it cannot exercise its toxic properties while in the insoluble 

 state. It is thus evident that, though the copper protectant must be 

 essentially insoluble in water, yet in the presence of pathogenic spores 

 it must become soluble to a sufficient extent to kill the fungus, or at least 

 to prevent infection of the suscept. 



This paper sets forth the results of some studies on the manner in which 

 the soluble copper is liberated from the nominally insoluble copper com- 

 pounds. No attempt has been made to study the actual toxic action, 

 or the manner in which the copper kills. 



Most investigations relating to the nature of the fungicidal action, or, 

 more specifically, the liberation of the copper from copper protectants, 

 have dealt with bordeaux mixture. This is to be expected, as bordeaux 

 mixture is our oldest and most important copper protectant. The studies 

 here described have likewise dealt with this protectant, and also with 

 copper-lime dust. 



The chemical and the physical nature of the compound or compounds 

 resulting from the reaction of copper sulfate and calcium hydroxide to 

 form bordeaux mixture, are of great complexity and variability. Despite 

 the extensive investigations on this subject, there is yet much to be clari- 

 fied. The first workers, Millardet and Gay on (1885, 1887 b), and other 

 early workers, such as Chester (1890), considered the reaction simple, 

 holding the active fungicidal constituent formed to be cupric hydroxide. 

 Sostegni (1890), Fairchild (1894), and Swingle (1896) state that the copper 

 is present as a basic sulfate in addition to the hydrate. More recent 

 workers — Pickering (1907, 1909), Bedford and Pickering (1910), and 



1 Also presented to the Faculty of the Graduate School of Cornell University, April, 1929, as a major 

 thesis in partial fulfillment of the requirements for the degree of doctor of philosophy. 



Author's acknowledgments. The writer desires to acknowledge his indebtedness to Professor II. H. 

 Whetzel, of the Department of Plant Pathology, Cornell University, at whose suggestion these studies 

 were undertaken, for the stimulating encouragement and helpful advice which he constantly afforded. 

 Sincere thanks are due also to Professors F, M. Blodgett and H. E. Thomas, of the same department, for 

 suggestions and criticisms. 



