256 PHYSIOLOGY OF THE FUNGI 



which in turn may decompose by hydrolysis to yield hydrogen sulfide 

 and calcium hydroxide. At the same time some of the calcium poly- 

 sulfide is oxidized to calcium thiosulfate and sulfur. See Frear (1948) 

 for a discussion of the chemistry involved. 



It is known that lime-sulfur exerts an eradicant action on some fungi, 

 including Venturia inaequalis, when first applied. After a few days 

 this spray exerts only a protective action like that of elemental sulfur, 

 which probably depends on the elemental sulfur set free by the decom- 

 position of various constituents comprising lime-sulfur. The eradicant 

 action, then, depends upon either the calcium polysulfide or calcium 

 sulfide. We may consider that sulfide ion (S=) is the toxic agent. The 

 alkalinity of the spray may aid in penetration into the mycelium already 

 present. 



Lime-sulfur solution may be treated with ferrous sulfate or aluminum 

 sulfate in the spray tank to produce colloidal sulfur and hydrogen sulfide. 

 Aluminum sulfate, AI2 (804)3, hydrolyzes to form aluminum hydroxide 

 and sulfuric acid. A lime-sulfur spray so treated has only a protective 

 action. It has lost its eradicant value. We may assume, therefore, that 

 the decomposition of lime-sulfur in spray tanks when treated with acid 

 (aluminum sulfate) and the decomposition on the leaf follow a somewhat 

 similar pattern. This scheme of producing colloidal sulfur has the draw- 

 back that the added iron, when ferrous sulfate is used, is toxic to vegeta- 

 tion, and dangerous amounts of hydrogen sulfide are evolved. 



We may assume that hydrogen sulfide exerts its toxic action on fungus 

 spores by inactivating certain enzymes. Hydrogen sulfide is known to 

 inactivate many enzymes, including catalase, cytochrome oxidase, dopa 

 oxidase, lactase, and others. Generally, hydrogen sulfide and cyanide 

 inhibit the same enzymes. It is thought that these metalloenzymes are 

 inhibited by sulfide or cyanide because these agents react with iron or 

 copper to form highly insoluble or little-ionized compounds or complexes. 



ORGANIC FUNGICIDES 



The newer fungicides, with few exceptions, are either organic or organo- 

 metallic compounds. The organic mercury compounds were considered 

 with the inorganic compounds of mercury, since the mechanism of action 

 appears to be the same in both types of compounds. Many of the 

 organic fungicides exhibit greater specificity than the inorganic fungicides. 

 The possibilities of modification in the structure of organic compounds 

 are almost unlimited. The study of organic fungicides, therefore, offers 

 the opportunity of correlating structure with type and intensity of fungi- 

 cidal action. 



Aldehydes. The first organic fungicide to attain wide acceptance was 

 formaldehyde. At one time this compound was used for the surface 

 sterilization of grain and potato tubers, but at present formaldehyde is 



