Studies on Fungicides — III 



51 



Spores from 5 to 10 days old, present to the extent of less than 1000 

 to a drop of solution, are markedly sensitive to 0.2 milligram of copper 

 per liter of solution. This is one hundred times more sensitive than the 

 potassium-ferrocyanide test. 



THE TOXICITY OF CALCIUM COMPOUNDS 



Since a number of the common copper protectants contain calcium in 

 various forms, a few preliminary tests were made to determine the relative 

 toxicity of certain of these calcium compounds. The calcium compounds 

 present in excess-lime bordeaux mixtures and copper-lime dust are calcium 

 hydroxide, calcium sulfate, and calcium carbonate. Calcium hydroxide 

 and calcium sulfate are both moderately soluble in water at 25° C, the 

 former to the extent of 1.541 grams per liter expressed as Ca(OH) 2 , and 

 the latter 2.755 grams per liter expressed as CaS0 4 • 2H 2 (Cameron and 

 Bell, 1906). Calcium carbonate is soluble in distilled water only to the 

 extent of 19.6 x 10~ 5 gram mols per liter at 25° C. (Askew, 1924). 



Four experiments were performed with calcium hydroxide, the first two 

 in triplicate and the third and the fourth in duplicate. A saturated 

 solution of Ca(OH) 2 was obtained by slaking a high grade of commercial 

 stone lime with distilled water, making milk of lime, and filtering. From 

 this filtrate various dilutions were made. This source of Ca(OH) 2 was 

 employed in the first three experiments; for experiment 4, Baker's Chemi- 

 cally Pure Ca(OH) 2 was used. The toxicity of all was determined in the 

 usual manner. The results are presented in table 4. 



TABLE 4. Percentage Germination of Sclerotinia Americana Conidia in Calcium- 

 Hydroxide Solutions 



Similar tests were made with calcium-sulfate solutions. The calcium 

 sulfate was obtained from three sources — Bausch and Lomb C. P., 

 Merck precipitated, and Baker technical. The results are shown in table 5. 



