24 



James G. Horsfall 



with the standard monohydrated copper-lime dust are included. Since 

 in all the tests the 6.8-per-cent metallic copper, diluted with lime, talc, 

 or bentonite, prevented germination entirely until a content of 0.5-per- 

 cent metallic copper was reached, the data in the table refer only to the 

 latter dilution. 



The fluctuations of the determinations around the mean of 53.3 indicates 

 that the 0.5-per-cent copper content is at the lower toxic limit. 3 Many 

 factors probably operate to vary the germination from day to day, but 

 the chief one is the variation in the amount of copper on the slides. These 

 tests indicate that a copper dust containing 1 per cent or more of metallic 

 copper should prove effective in field work. 



A study was made on the effect of diluents upon the efficiency of the 

 copper lime. In making these tests, one of three substances, lime, talc, 

 or bentonite, was generally used as a diluent for the copper-lime dust. 

 The results obtained from testing these alone for their fungicidal properties 

 are presented in table 9. 



The probable error of the differences reveals that the lime and the 

 talc appear slightly toxic, but the differences are so slight that they prob- 

 ably do not have any great significance. Bentonite appears not to be 

 toxic at all. 



Greenhouse experiments 



A preliminary experiment made in the greenhouse when plants were 

 inoculated after having been sulfured, presaged the results just set forth 

 even before any spores were ever germinated on slides in the presence of 

 fungicides. Plants of mammoth red and medium red clover in pots were 

 selected for inoculation and were divided into two lots, one of which 

 was sulfured with Bank's colloidal sulfur dust and the other was left 

 untreated as a check. The plants were placed in a large moist chamber 



3 One experiment with Macrospo-ium sarcinarforme demonstrated that a dust diluted to a point near 

 what appears to be the minimum of toxicity inhibits germination in varying degrees, depending upon the 

 amount deposited on each slide. Copper-lime dust diluted to 0.5 per cent with talc was applied in heavy 

 and light dosages, as is shown in the following table where the results of the germination tests appear. 

 Except for the check, which was prepared only in duplicate, each treatment was replicated four times each 

 in duplicate, making actually ten tests in all, so that the results should be significant. 



Effect of Copper Dosage on Conidial Germination of Macrosporium Sarcinaeforme 



A large number of tests with the same fungus and 1-per-cent copper-lime dust diluted in talc gave no 

 germination, regardless of the thickness of the application. Hence it appears that 0.5 per cent is at or near 

 the minimum point of toxicity. Variations in dosage may be overcome by replication of the experiments. 



