FUNGICIDES 363 



were used, namely Sclerotinia frudicola (cause of brown rot of stone fruits), 

 Botrytis sp. {cinerea type) (cause of marigold blight), Macrosporium sar- 

 cinaeforme (cause of leaf spot of red clover), and Uromyces caryophyllinus 

 (cause of carnation rust) . In all four cases the toxicity of the pentathionic 

 and sulfuric acids was found to be identical and apparently was due to the 

 acidity. Hydrogen sulfide was found to be from 6 to 200 times as toxic as 

 the pentathionic or sulfuric acid. Neutral salts of pentathionic acid were 

 non-toxic except when decomposed with alkali to give fungicidally active 

 colloidal sulfur. Most samples of sulfur gave water extracts containing 

 pentathionic acid, but these were found to be non-toxic. Finally a commer- 

 cial sulfur dust was treated with alkali to remove the pentathionic and 

 sulfuric acids and a comparative toxicity test with the same material before 

 treatment showed no difference. Thus it was concluded that pentathionic 

 acid is not a factor of importance in the fungicidal action of sulfur. 



Hydrogen sulfide. As early as 1875, Pollacci in Italy noted that grape 

 leaves treated with sulfur evolved H2S, which he believed was thus respon- 

 sible for the fungicidal action of sulfur. Marsh " in England had showTi 

 that certain leaves, when sidfured, evolved H2S and also that H2S was 

 toxic to fungus spores. 



In view of these findings and the high toxicity of H2S observed in the 

 pentathionic acid w^ork, a detailed examination of this theory was under- 

 taken.^^ It w^as found that all species of plants tested evolved H2S when 

 in association ^vith sulfur. The plants included the attached leaves of 

 26 species of higher plants, the spores of 16 species of fungi, the sporophores 

 of 3 species of Agaricaceae, and the expressed and filtered juice of Pleurotus 

 ostreatus. A quantitative determination on a potted strawberry plant 

 showed at 35° C (95° F) an evolution of 0.002 mg of H2S per hour per sq 

 dm of leaf surface. 



Quantitative determinations were also made on the spores of eight dif- 

 ferent species of fungi, namely: Venturia inaequalis, Uroynyces caryophylli- 

 nus, Puccinia antirrhini, Sclerotinia frudicola, Macrosporium sarcinaeforme, 

 Pestalotia stellata, Glomerella cingulata, and Botrytis sp. {cinerea type). The 

 spores w^ere mixed with an aqueous paste of sulfur in a small stoppered 

 bottle with a strip of lead acetate paper suspended from the stopper, and 

 the time was determined for the lead acetate paper to attain a known degree 

 of blackening. Thus rate of production curves for H2S w^ere obtained and 

 the total amount produced calculated. This amount was found to be 

 directly proportional to the number of spores, but varied with the different 

 species. In 12 hours at 30° C (86° F), the most active fungus, Glomerella, 

 produced an amount of H2S equivalent to 9.8 per cent of the weight of the 

 spores, while the least active fungus, Macrosporium, produced only 0.14 

 per cent of its weight of H2S. The effect of temperature was marked with 

 a well-defined maximum at about 35° C (95° F), but at 60° C (140° F) the 

 reaction was entirely inhibited. These temperature relations indicate an 

 enzymatic reaction. Within a pH range of 4.0 to 8.0 the rate of evolution 



