Studies on Fungicides — III 53 



hydroxide solution. It is perhaps possible that the greater toxicity of 

 the calcium-hydroxide solution is due to its alkalinity. No determination 

 has been made of the possible toxicity of the sulfate radical or ion, either 

 in these tests or in those with the copper sulfate. 



Calcium-carbonate dust displays no toxic action toward the spores of 

 Sclerotinia americana. 



THE LIBERATION OF THE COPPER FROM THE PROTECTANT 



In studying the manner in which the copper is rendered soluble, various 

 types of experiments have been employed. These experiments have, in 

 turn, been directed toward proving or disproving certain of the more 

 important current hypotheses. In the presentation of each study, the 

 hypothesis is first stated, and this is followed by a statement of the experi- 

 ments and experimental data, and finally by a discussion of the results. 



Special emphasis has been laid on the hypothesis of spore excretions. 

 The first two of these studies have dealt with the action of atmospheric 

 agencies. The next three relate to the role of spore excretions, the third 

 of these being concerned especially with the sphere of activity of the 

 spore. Finally, brief studies have been made of the cumulative-action 

 hypothesis. 



Atmospheric agencies 



The solubility of copper protectants in distilled water 



Hypothesis. Copper protectants in distilled water may be brought 

 into solution when normally exposed to the action of the air. A copper 

 protectant is applied to a glass surface, and in one case distilled water is 

 allowed to remain standing on it for varying lengths of time. After 

 exposure thus to the air, spore-germination tests are made in the filtrate. 

 In the second case, a suspension of spores is placed directly on the fungi- 

 cide and the germination is recorded. If atmospheric factors alone are 

 operative in the solution of the copper, then in the first case there should 

 be little or no germination, and the toxicity of the filtrate should increase 

 with the time of standing; while in the second case, because of the short 

 exposure to the air the germination should be good. However, if an 

 action of the spores is the main factor, then in the first case none of 

 the filtrates, regardless of the time of standing, should be toxic; but in 

 the second case, because of the presence of the copper compound there 

 should be little or no germination. 



Experiments. The copper protectants employed in these experiments 

 were bordeaux mixture (4-4-50), Pickering or no-excess-lime bordeaux 

 (2.5-2-50), copper-lime dust, C. P. copper carbonate, and copper hydrox- 

 ide. The freshly made bordeaux mixtures were tested for free copper 

 by the use of potassium ferrocyanide, and none was found. Sclerotinia 

 americana conidia were used in the germination tests. 



