CHAPTER 11 



Fungicide Investigations 



S. E. A. McCallan 



The work of the fungicide laboratory has been along three main lines. 

 These are: (a) studies on the nature or mechanism of fungicidal action; 

 (6) development and improvement of laboratory and greenhouse methods 

 of evaluating the effectiveness of fungicides; and (c) research on new chemi- 

 cals as possible fungicides. Since these three approaches are closely in- 

 terdependent on one another, advances made in any one have been of 

 considerable help in the furthering of the others and in our general un- 

 derstanding of fungicides. 



Fungicidal Action of Sulfur 



The initial research on fungicides was begim in 1929 by Dr. Frank 

 Wilcoxon and the author under a grant from the Herman Frasch Founda- 

 tion for Research in Agricultural Chemistry, and coincidentally but very 

 appropriately the nature of the fungicidal action of sulfur was first chosen 

 for study. 



The earliest use of sulfur as a fungicide probably is shrouded in the mists 

 of antiquity. About the beginning of the 19th century various intelligent 

 gardeners were advocating it for the control of mildews. Today sulfur is 

 one of our most important fungicides and annually about 150,000,000 

 pounds are consumed for this purpose in the United States. Because of such 

 a long history and extensive use, there have been many theories to account 

 for the manner in which insoluble sulfur acts as a fungicide. 



Pentathionic acid. A recent theory wliich had received some publicity 

 was first advanced in 1922 by Young, '*<' who contended that traces of pen- 

 tathionic acid, associated with sulfur and formed from it, constituted the 

 active fungicidal agent. Because of the relative newness of this theory, a 

 critical study of it was undertaken. ^^ Potassium and barium pentathionates 

 were prepared for the first time in sulfur toxicity studies and from these, 

 solutions of the uncontaminated pentathionic acid were obtained. 



Comparative toxicity tests were made of pentathionic acid, sulfuric 

 acid, and hydrogen sulfide by means of the slide-germination method. In 

 the case of hydrogen sulfide it was necessary to work in a closed system in 

 order to prevent its escape — a precaution ignored by some earlier workers. 

 Four representative fungi exhibiting different degrees of sulfur sensitivity 



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