252 



PHYSIOLOGY OF THE FUNGI 



(S=) forms excellent fungicides, while sulfites (SOa^) are only slightly- 

 toxic and sulfates (SO 4=) are nontoxic. 



According to Large (1940), elemental sulfur has been used to control 

 powdery mildew for slightly over 100 years. The effectiveness of sulfur 

 increases as the particle size diminishes. Finely divided sulfur adheres 

 to plant surfaces much better than larger particles. In addition, the 

 distance between particles tends to be decreased when fine particles are 

 used, and the infection court is thereby better protected. The odds are 

 increased that a fungus spore falling upon a treated leaf will be within the 

 range of action of a particle of sulfur. An example of the effect of particle 

 size of sulfur on toxicity is given in Table 44. The greater toxicity of 



Table 44. The Relation between the Particle Size and Toxicity of a Sulfur 

 Dust to the Conidia of Sclerotinia americana 

 (Wilcoxon and McCallan, Phytopathology 20, 1930.) 



the finely divided sulfur is due to the fact that sulfur enters the spore in 

 the form of vapor. The amount of vapor formed from a given amount of 

 sulfur in a given time depends upon the area of the exposed surface, as 

 well as upon temperature. Therefore, the fineness of the sulfur particles 

 governs the effective concentration of sulfur vapor and its effectiveness 

 as a fungicide. 



McCallan (1946) estimated the yearly consumption of sulfur in the 

 United States alone to be 142 million pounds. Of this amount 110 

 million pounds is used as sulfur dust, 5 million pounds as wettable sulfur, 

 and 27 million pounds as lime-sulfur. Approximately 62 per cent of 

 this is used primarily for the control of apple scab alone. 



Since elemental sulfur is insoluble, its action upon fungi cannot be 

 attributed to the sulfur in this form. Two general theories have been 

 proposed to explain the action of sulfur. One theory holds that the 

 action is due to oxidized sulfur, such as SO2 or SO3 (which form sulfurous 

 and sulfuric acid, respectively, with water) or pentathionic acid, H2S5O6. 

 According to the second theory, the reduced form of sulfur, H2S, is the 

 active toxic agent. Both these theories are supported by published 

 experimental evidence. All these compounds are toxic to fungi under 

 certain conditions, if in high enough concentrations. However, to 



