Studies on Fungicides — III 69 



Cumulative action 



The abstraction, by spores, of copper from large volumes of water flowing over 



copper protectants 



Hypothesis. The protective action in copper fungicides, especially 

 bordeaux mixture, may be dependent on a cumulative action of the copper 

 in equilibrium in solution. That is, bordeaux mixture may contain always 

 in solution a minute trace of copper infinitely beyond detection by ordi- 

 nary chemical tests or even spore tests. In the presence of spores or 

 other reactive bodies, this soluble copper is absorbed or otherwise re- 

 moved from solution. An equilibrium having existed between the in- 

 soluble bordeaux and the soluble copper, this is now disturbed. There- 

 fore, more copper will enter into solution to maintain the equilibrium. 

 The spores will in turn remove this copper, and so on until finally the 

 spores will have accumulated sufficient copper to ultimately destroy 

 themselves. This, of course, assumes that the source of soluble copper, 

 the bordeaux particles, is always present. 



Experiments. In a study of this hypothesis, some preliminary experi- 

 ments were performed. It was thought that if this hypothesis were true, 

 then large quantities of water slowly dripped over " insoluble " copper 

 protectants and then over the spores not in contact with the fungicides 

 would carry traces of soluble copper to the spores. Because of the con- 

 stantly changing equilibrium conditions about the fungicide, fresh copper 

 would continually enter into solution and the spores would abstract enough 

 copper to eventually become poisoned. If, however, the excretions of 

 spores are the only solvent agent, then the spores would not be killed. 

 So if the water were first dripped over spores capable of excreting the 

 solvent agent, and then the water plus this agent were passed over the 

 fungicide, sufficient copper should be brought into solution to kill the 

 test spores. 



An apparatus was accordingly devised and set up. Tests were made 

 with spores of Sclerotinia americana and bordeaux mixture 4-4-50, Woburn 

 bordeaux, and copper-lime dust. Since the experiment was, in general, 

 unsatisfactory, a description of the apparatus is considered unnecessary. 

 The chief difficulties encountered were the poor germination of the spores 

 in the control apparatus, the rapid disorganization of the Woburn bor- 

 deaux, precipitating black copper oxide — • which occurred also, though 

 to a less extent, in the bordeaux mixture and the copper-lime — - and the 

 precipitation of calcium compounds from the bordeaux mixture and the 

 copper-lime. 



The results of three typical experiments are given in table 18. 



