Studies on Fungicides — III 73 



Titrations were made also with the indicator brom thymol blue. The 

 difference obtained in this case was from 4/20 to 7/20 cubic centimeter of 

 N/1000 NaOH. The two indicators thus agree well with each other. 



The actual acidity of the spore excretions of Sclerotinia americana 

 conidia from these preliminary tests appears to be very slight. It is 

 possible that some weak organic acid or acids are excreted by the fungus 

 spores, and that this condition accounts for the solvent action. 



SUMMARY 



1. An extensive review of the literature relative to the liberation of 

 copper from insoluble copper protectants is presented in this paper. 

 In the main, three hypotheses have been advanced by investigators to 

 explain this: (1) that of atmospheric agencies; (2) that of suscept 

 excretions; and (3) that of the dissolving action of spore excretions. A 

 less common hypothesis is that of the cumulative action of the toxic agent 

 in equilibrium solution. The present study is concerned primarily with 

 investigating the hypothesis of the dissolving action of spore excretions. 

 Some attention has been given also to the hypothesis of cumulative action. 



2. The experimental procedure employed was that of testing by spore 

 germinations, as described in the second paper of the series. In general, 

 conidia of Sclerotinia americana were used. 



3. The toxicity of copper-sulfate solutions toward conidia of Sclerotinia 

 americana was found to be positively correlated with total copper content 

 and negatively corrected with number of spores. Total copper content 

 must be calculated on the basis of concentration and volume of solution. 

 However, with increasing volumes, other factors being constant, the solu- 

 tions become relatively less toxic, presumably because of the factor of 

 diffusion. With increasing spore age, given solutions become more toxic. 

 Sclerotinia americana spores 5 to 10 days old, present to the extent of 

 about 1000 per 0.05 cubic centimeter of solution, are markedly sensitive 

 to 0.2 milligram of copper per liter of copper-sulfate solution. 



4. The toxicity of certain calcium compounds toward conidia of 

 Sclerotinia americana was studied. Chemically pure calcium carbonate 

 was found to be non-toxic, but C. P. calcium-hydroxide and C. P. calcium- 

 sulfate solutions were somewhat toxic. Calcium-hydroxide solutions 

 ceased to be toxic at dilutions of 0.1 milligram per liter of solution, while 

 calcium-sulfate solutions ceased to be toxic at 0.7 milligram per liter. 



5. The filtrates from C. P. copper carbonate and copper hydroxide in 

 distilled water contained quantities of soluble copper sufficient to kill the 

 spores of Sclerotinia americana. 



6. The action of atmospheric agencies was investigated. In no case 

 \v;ts copper brought into solution in toxic concentrations in distilled water 

 which had stood on glass surfaces coated with bordeaux mixtures or copper- 



