ACTION OF FUNGICIDES 253 



account satisfactorily for the toxic properties of sulfur, it must be demon- 

 strated that the toxic agent is produced under the conditions which 

 prevail in the field in quantities sufficient to account for the observed 

 effects. Any evaluation of these hypotheses must take into account all 

 the variables involved. The caution of Wilcoxon and McCallan (1930) 

 is pertinent: 



In making comparisons of the toxicity of chemical substances to fungus spores, 

 there are two requisites for obtaining accurate results which, though quite 

 obvious, have not always received the consideration they deserve, (a) The 

 substance whose toxicity is to be measured must be available in a pure state 

 and of known concentration, and (6) the technique employed must be capable of 

 distinguishing between the toxicity of the substances it is desired to compare. 



It is agreed that elemental sulfur is not the toxic agent and that sulfur is 

 transformed into the toxic agent. There are three possible agencies for 

 such transformations: the atmosphere, the plant on which the sulfur is 

 dusted, and the fungus spores or mycelium. Sulfur acts at a distance, 

 and since sulfur is volatile at room temperature, this property offers an 

 explanation. Sulfur vapor is a gas, and in this state it should be more 

 easily transformed into the toxicant. 



Sulfur is slowly oxidized by the oxygen of the atmosphere to form sulfur 

 dioxide, but the rate at which this reaction occurs at ordinary tempera- 

 tures makes it impossible for this reaction to account for all the toxic 

 properties of sulfur, even though sulfur dioxide is toxic to fungus spores 

 (McCallan and Weedon, 1940). 



Young (1922) set forth the hypothesis that pentathionic acid is the 

 toxic agent formed from sulfur. It is agreed, even by those who do not 

 support Young's hypothesis, that this acid is formed on the surface of 

 sulfur dust. A considerable number of papers were published during 

 the next decade which gave support to this view (Liming, 1932). Wil- 

 coxon and McCallan (1930) investigated this theory thoroughly and 

 concluded that pure pentathionic acid had no toxic properties for the 

 spores of Sderotinia americana, Botrytis sp., Macrosporium sarcinaeforme, 

 and Uromijces caryopJujUinus. If sufficient pentathionic acid was used 

 to reduce the pH to about 4, spore germination was inhibited. Solutions 

 of sulfuric acid having the same pH were equally toxic. Neutral salts 

 of both acids were nontoxic. Roach and Glynne (1928) likewise found 

 pentathionic and sulfuric acids to have the same toxicity when tested 

 against the winter sporangia of Synchytrium endohioticum. Wilcoxon 

 and McCallan (1930) performed a decisive experiment when they washed 

 one lot of sulfur dust with alkali to remove pentathionic acid and com- 

 pared this pentathionate-free dust with the original sample, which con- 

 tained a trace of this acid. No difference in toxicity of the washed and 

 control samples of this sulfur dust was found. 



