ACTION OF FUNGIDICES 



247 



The next advance in copper fungicides was not until 1885, whei 

 Millardet published the formula for making the famous fungicide 

 Bordeaux mixture. Millardet recommended that 8 kg. of copper sulfate 

 pentahydrate (bluestone) be dissolved in 100 liters of water. This solu- 

 tion was then mixed with 15 kg. of quicklime slaked in 30 liters of water. 

 The chemistry of Bordeaux mixture is more complicated than was 

 assumed at first. Instead of cupric hydroxide, a series of basic sulfates 

 are formed, the composition being dependent upon the ratio of coppei 

 sulfate and calcium hydroxide used (Frear, 1948). Bordeaux mixture is 

 a copper compound or compounds of low solubility. According to 

 Goldsworthy and Green (1936), Bordeaux mixture in equilibrium with 

 water yields a solution containing about 4 p. p.m. of copper. However, 

 McCallan and Wilcoxon (1936) found that well-washed 4-4-50 Bordeaux 

 mixture was soluble only to the extent of furnishing 1 p. p.m. of copper. 

 After this material was thoroughly dried, as in a spray film, the solubihty 

 in terms of copper decreased to 0.2 to 0.3 p. p.m. McCallan and Wilcoxon 

 have reported a comparison between the amounts of Bordeaux mixture 

 and copper sulfate required to inhibit the germination of 90 per cent of 

 the spores of a few species. These data are given in Table 43. 



Table 43. The Relative Efficiency of Bordeaux Mixture and Copper Sulfate 



IN Inhibiting Spore Germination 

 (McCallan and Wilcoxon, Contribs. Boyce Thompsori Inst. 6, 1936.) 



If Bordeaux mixture or other copper spray or dust of low solubility 

 furnishes less than 1 p. p.m. of copper to the solutions with which it is in 

 equilibrium, it is obvious that the concentration of copper is too low for 

 any great amount of toxicity. We must also take into account the rate 

 ^ of solubility of the ''insoluble" copper compounds, for if the rate of 

 solution is slow, the maximum concentration may not be attained in 

 time to prevent infection. The only hypothesis which would account 

 for the lethal action of copper compounds of such low solubility would be 

 that of cumulative action. A germinating spore in a saturated solution 

 of the copper compound in equilibrium with the solid copper compound 

 would remove copper from the solution. This process would cause more 



