SULFUR 



473 



was earlier devoted to proving that some more soluble derivative is the 

 active agent; the history of these investigations has been reviewed else- 

 where (174, 244). It is now agreed that the form in which sulfur 

 reaches the fungus spore is the vapor phase of elemental sulfur (246). 

 Fungitoxicity in any particular situation of use is dependent upon tem- 

 perature (459), the particle size of the sulfur deposit (370, 442), and, to 

 some degree, the allotropic form of the sulfur (103). 



For some time following a series of critical studies, the details of 

 which are reviewed by McCallan (244, 246) and Horsfall (174), ele- 

 mental sulfur was considered to be reduced by the fungus spore and the 

 effective toxic agent was taken to be the hydrogen sulfide formed by the 

 biological reduction. Hydrogen sulfide is fungistatic (442, 443) and 

 somewhat fungicidal (248, 249). It appears now, however, that the 

 toxicity of elemental sulfur on a weight basis is greater than that of an 

 equivalent amount of hydrogen sulfide, as shown in Table 2. Conse- 



Table 2. The Relative Toxicities of Sulfur and Hydrogen Sulfide* f 



Species 



Monilinia fructicola 

 Cephalosporium acremonium 

 Aspergillus niger 

 Glomerella cingulata 

 Neurospora sitophila 

 Rhizopus nigricans 

 Alternaria oleracea 

 Stemphylium sarcinaeforme 



* From Miller, McCallan, and Weed (289) , by permission of the Boyce Thomp- 

 son Institute for Plant Research, Inc. 



f Toxicity is expressed as the dose, in parts per million, required to kill 50 

 per cent of spores in an exposure of 24 hours; concentrations are of the external 

 solution or suspension. 



% Highly significant difference between colloidal sulfur and hydrogen sulfide. 



quently the second part of the older theory — that hydrogen sulfide is 

 the ultimate toxicant — is untenable. 



Attention has therefore been drawn to the process of sulfur reduc- 

 tion itself as a possible primary event in toxicity. The reduction of 

 sulfur to hydrogen sulfide by fungus spores is a general phenomenon 

 and has been shown to be brought about by spores of many different 

 fungi, under both anaerobic and partially aerobic conditions (253, 

 289). The activity of species varies widely: Cephalosporium acremo- 

 nium under anaerobic conditions forms 6.39 mg hydrogen sulfide per 



