FUNGICIDES 



365 



Table 38. Comparison between the Toxicity and the Production of Hydrogen Sulfide, 

 Expressed in Units Equal to the Amount of Hydrogen Sulfide Required to Reduce 



Germination 50 Per Cent. 



and Sclerotinia), which can be controlled in the field with sulfur, produce 

 more than one unit, while the four resistant species not controllable by 

 sulfur produce considerably less than one toxic unit of H2S. Very recently 

 Horsfall ^ has demonstrated that when spores of Macrosporium and Sclero- 

 tinia are placed together in the same drop of water in the presence of 

 sulfur, the sulfur "resistant" Macrosporium is killed, presumably because 

 of the excess H2S produced by Sclerotinia. 



As a result of these studies the fungicidal action of sulfur is explained 

 thus: ^^' P-^^ "Sulfur in the vicinity of fungous spores, by reason of its vapor 

 pressure, gives off sulfur vapor which diffuses into the spores. Here reduc- 

 tion takes place ^nthin the spores with hydrogen sulfide as a final product. 

 The reaction is enzymatic in nature and is probably concerned with — SH 

 compounds. The toxic product, hydrogen sulfide, being produced in inti- 

 mate contact ^vith the living cell, is able to exert its maximum effect. It 

 is not believed that the hydrogen sulfide produced from the leaves in the 

 open affects the spores. . . . Each individual spore, therefore, by reason 

 of its ability to reduce sulfur to hydrogen sulfide, is thus instrumental in 

 bringing about its own death." 



In order to throw further light on the mode of action of sulfur as a 

 fungicide, a study was made of the comparative toxicity of sulfur and its 

 analogous elements, selenium and tellurium. ^^ These were found to be much 

 less toxic than sulfur to spores of Sclerotinia fructicola, Pestalotia stellata, 

 and Uromyces caryophyllinus. Even in colloidal form, selenium showed no 

 appreciable toxicity. The toxicity of hydrogen selenide, however, while 

 difficult to measure accurately because of the instability of this compound, 

 appeared to be comparable to that of H2S. But the formation of hydrogen 

 selenide and hydrogen telluride by yeast cells or by glutathione took place 

 less readily than the reduction of sulfur to H2S under similar conditions. 

 Thus the lesser toxicity of selenium and tellurium is readily understood, and 

 is in accord ^vith the explanation offered for the fungicidal action of sulfur. 



