46 



S. E. A. McCallan 



«5 



^ > 



O Control 



• 0/ mp. copper per l/Ccr 



A 



▲ 



0.2 



to 



/3 



/4 



16 



Age of spores in days 



Figure 1. germination op conidia of sclerotinia Americana in dilute oopper- 

 sulfate solutions: relations of copper concentration and age of spores 



sown at a suspension concentration of 30 to 32, are therefore definitely 

 sensitive to as small a quantity of copper as 0.2 milligram per liter of 

 solution. The potassium-ferrocyanide test commonly used for copper will 

 show fairly plainly at 20 milligrams of copper per liter, and with uncer- 

 tainty at 10 milligrams per liter. The Sclerotinia conidia are therefore, 

 under these conditions, one hundred times more sensitive to copper than 

 the potassium-ferrocyanide test. 



The second step was to determine the effect of the number of spores on 

 germination in the dilute copper solutions. The volume of solution and 

 the spore age were held constant. Spore suspensions were obtained, of 

 such concentrations that when they were added to the copper-sulfate 

 solutions the resulting concentrations were as nearly as possible 4, 8, 16, 

 and 32 spores per field. The variation from these numbers was small, 

 being not more than 2 or 3 for the dilute concentrations, and 6 or 7 for the 

 most concentrated. In all, five tests in duplicate were made. The ages 

 of the spores were 7 and 8 days. Copper-sulfate solutions of from 0.1 

 to 10 milligrams of copper per liter of solution were employed. The 

 results of these tests are recorded in table 2. 



