Studies on Fungicides — III 



49 



fairly satisfactory germination here. For the medium volumes, that is, 

 from 1 to 25 cubic centimeters, petri dishes were employed. The smallest 

 volumes were placed directly on slides and a drop of spore suspension was 

 added. With the large volumes, the drops of spore suspension were 

 first placed on the bottom of empty petri dishes and allowed to stand for 

 about thirty minutes before the solution was added. The spores had 

 then settled to the surface of the dish, to which they adhered, and were 

 not readily dislodged. They were thus easily found for the purpose of 

 making germination counts. 



In this manner two sets of tests were performed. A copper-sulfate 

 solution of a concentration of 0. 1 milligram of copper per liter, was employed 

 throughout. The volumes of solution are expressed as drops, in order 

 that the ratio of drops of solution to one drop of spore suspension may be 

 readily seen. The concentration of spore suspension prior to the dilution 

 was constant and was approximately 16. The age of the spores was 7 

 days. The percentage of germination observed in these tests is recorded 

 in the third column of table 3. 



TABLE 3. Percentage Germination of Sclerotinia Americana Conidia in Dilute 



CoPPER-SuLFATE SOLUTIONS: FACTOR OF SoLUT T ON VOLUME 



(Concentration of solution. 0.1 milligram of copper per liter; spore-suspension concentration. 



16; spore age, 7 days' 



A study of these figures shows the importance of the volume factor. A 

 small volume of very dilute copper-sulfate solution may be entirely non- 

 toxic toward a given number of spores, but a large volume of this same 

 concentration is decidedly toxic to the spores. This confirms the observa- 

 tions noted before, that toxicity is governed by the number of spores and 



