CHAPTER XIX 

 INSECTICIDES AND FUNGICIDES (continued) 



THE FUNGICIDAL AND SCORCHING ACTION OF COPPER 



(Reports, XI, 93 ; XIV; App.) 



AN excellent resume of the work done prior to 1896 on the 

 fungicidal action of copper will be found in Bulletin 10 of the 

 Bureau of Vegetable Pathology, U.S. Department of Agriculture, 

 by W. T. Swingle ; though it must be borne in mind that the 

 chemical nature of Bordeaux mixture had not at that time been 

 elucidated, and, consequently, the statements made respecting 

 it are often wide of the mark and misleading. Six years 

 later J. F. Clark (Botanical Gazette, 33, p. 26) published an 

 important paper dealing with various fundamental aspects of 

 fungicidal action. Unfortunately there are uncertainties attach- 

 ing to his statements, owing to their not appearing always to 

 tally with the figures which he quotes, and to the loose, and often 

 unintelligible manner in which his ideas on chemical questions, 

 and his names for chemical compounds, are expressed (XI, 93). 

 In various series of experiments with 15 different fungi he deter- 

 mined the dose of copper which produced death after 24 hours' im- 

 mersion, as well as that which produced the smallest recognisable 

 effect on the fungus. Though the lethal strength varied from 0-005 

 to 0-00025 per cent, of copper a twentyfold variation in the 

 majority of cases it approximated to 0-0015 per cent. ; and this 

 lethal dose was the same in whatever form the copper was supplied, 

 namely, as sulphate, chloride, nitrate, acetate or formate. When 

 the fungus was supplied with nourishment in the form of molasses, 

 its resistant power was enormously increased, even up to eighty- 

 fold. It is probable, however, that this was not really a question 

 of nourishment, but was due to the alteration in the viscosity 

 of the solution, which would affect the rate at which the copper 

 penetrated the cells, since a similar increase was obtained on 

 the addition of such inert substances as the sulphates and 

 chlorides of potassium or sodium, and any other explanation of 

 the effect of these salts based on chemical grounds, such as was 

 given by Clark, is certainly untenable. 



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