Studies on Fungicides — III 55 



seems improbable that atmospheric factors act, at least over a period of 

 24 hours, through the medium of distilled water to promote the liberation 

 of copper. The possibilities of " meteoric waters " and longer periods 

 of standing have not, of course, been precluded here. However, it is 

 very evident that in those cases in which the fungus spores were placed 

 directly on the insoluble protectant, the latter was decidedly toxic; that 

 is, in some manner the mere presence of the insoluble protectant is sufficient 

 to inhibit the germination of the spores. This may be explained by 

 assuming that the spores excrete a solvent capable of dissolving some of 

 the copper. 



The experiments with C. P. copper carbonate and C. P. copper hydrox- 

 ide resulted contrarily to expectations. These copper compounds are 

 ordinarily considered insoluble in water (Holland and Gilligan, 1928). 

 Apparently this is not strictly true, for it is very evident that the filtrates 

 contain substances in solution which are toxic to the spores. Presumably 

 (his toxic material is soluble copper. The filtrates from the copper car- 

 bonate gave no copper test when treated with potassium ferrocyanide; 

 the copper-hydroxide filtrates, however, gave a fairly clear test. Thus 

 the amount of copper present in the former is probably less than 10 milli- 

 grams per liter, while in the latter it is probably more nearly 20 milli- 

 grams per liter. This unsuspected fact of soluble toxic substances pre- 

 cludes the possible use of these copper compounds in subsequent studies 

 of this kind. 



The solubility of copper protectants in distilled and rain water, under forced 

 aeration 



Hypothesis. It is possible that in nature the copper protectants are 

 more exposed to the action of carbon dioxide and other atmospheric 

 agencies, than they are with plants merely standing in the laboratory. 

 Also, rain waters perhaps possess a greater solvent action than does dis- 

 tilled water. 



Experiments. A series of experiments were accordingly performed, 

 in which a prolonged and enforced action of the air and of rain water was 

 tested. Bordeaux mixture (4-4-50) and copper-lime dust were used. 

 The test fungi were conidia of Sclerotinia americana and of Macrosporium 

 sarcinaeforme. The latter fungus was used only in one test. It was 

 found, however, to be approximately as copper-sensitive as Sclerotinia 

 (imcricana. 



The fungicides wen; placed in specimen jars of 175 cubic centime) ers 

 capacity. The bordeaux mixture was sprayed on the inner side of the 

 jars, and also on both sides of glass slides. Four of these slides were 

 placed in each jar, this serving to increase Ihe surface exposed. After 

 thorough drying, about 50 cubic centimeters of distilled water was added 



