Studies on Fungicides — III 



45 



dilution, the ionization of this salt is presumably complete (Randall, 1916). 

 Hence, at the dilutions used in these tests — that is, 0.1 to 10 milligrams 

 of copper per liter of copper-sulfate solution — the copper sulfate is from 

 96 to practically 100 per cent ionized, or nearly all in the ionized state. 



In studying the toxicity of copper-sulfate solutions toward spores, at 

 least four factors must be considered. These are: the age of the spores; 

 the number of spores; the concentration of copper in solution; and the 

 volume of solution. 



In table 1 is recorded the germination of spores of different ages in 

 varying dilutions of copper-sulfate solutions. These figures are based 

 on the results from a number of tests. The tests are in close agreement. 

 The concentration of the spore suspensions was from approximately 30 

 to 32 spores per low-power field of the microscope. In all future references 

 to concentration of the spore suspension in this paper, the number of 

 spores is designated for a low-power field. The low-power field of the 

 microscope employed is 1575yu in diameter. The area of the field is thus 

 approximately 1/30 of that of the entire drop, since the drops are from 

 8 to 10 millimeters in diameter. Also, since the drops are 1 20 cubic centi- 

 meter in volume, the volume of solution in a low-power field would be 

 approximately 1/600 cubic centimeter. The temperature was from 21° to 

 25° C. 



TABLE 1. Percentage Germination of Sclerotinia Americana Conidia in Dilute 

 copper-sulfate solutions: relations of copper-sulfate concentration avd 

 Spore Age 



(Spore-suspension concentration. 30 to 32) 



Age of spores, in days 



Milligrams of copper per liter of solution 



2.0 | 



Per cent and type of germination 



5 

 6 



7 

 8 

 in 

 1-' 

 17 



The results, which arc shown graphically in figure 1, immediately 

 demonstrate the extreme sensitivity of Sclerotinia americana conidia to 

 copper. As may be expect id, the spores become less viable and more 

 copper-sensitive with increasing age. For the younger spores, 5 to 10 

 days old, 0.1 milligram of copper per liter of solution is slightly stimu- 

 lating. Increasing the concentration above this results in a rapid decrease 

 in the percentage germination. Spores of the ages commonly used, and 



