ACTION OF FUNGICIDES 249 



these "solvents" the solutions were blue in color, which indicates that 

 the copper was oxidized to the cupric state. Marten and Leach investi- 

 gated the toxicity of the copper-glycine compound to Pythnim debaryanum. 

 It was noted that an excess of glycine protected the fungus from the 

 action of the copper. Some 200 times as much copper was required to 

 inhibit growth when glycine was present in the medium as when it was 

 absent. Thus, it seems that whether a given amount of copper is toxic 

 or not depends upon the nature and amount of certain constituents in 

 the medium or substrate. 



One may ask, By what mechanism does the copper ion cause fungistasis, 

 or how does the copper kill? The common explanation of the toxic 

 action of the heavj'^ metals (copper, mercury, and silver) is based upon the 

 property of these ions of precipitating or denaturing proteins. Enzymes 

 are proteins, and it would be expected that the heavy metals would 

 inactivate these catalysts. However, not all enzymes are equally inac- 

 tivated by low concentrations of heavy-metal ions. The enzymes which 

 require free sulfhydryl groups for activity appear to be especially suscepti- 

 ble to inactivation by ions of heavy metals. It is probable that copper 

 causes fungistasis by combining with the sulfhydryl groups of certain 

 enzymes. At this stage, the action of copper is reversible. Goldsworthy 

 and Green (1936) found that spores of Sderotinia fructicola which had 

 been treated with insufficient copper to kill made normal growth when 

 sown on copper-free medium. As long as an inhibition is reversible, the 

 process is one of fungistasis. Death of the spore results when irreversible 

 changes occur. 



There is reason to believe that the injurious effect of copper fungicides 

 upon the host plant is due to the same mechanism that operates in fungus 

 spores. Foster (1947) attributed the sensitivity to copper of certain 

 seeds to their content of sulfhydryl enzymes. 



mercury' 



While a number of inorganic salts of mercury have been used as anti- 

 septics, only two have had wide application as fungicides. Mercuric 

 chloride (corrosive sublimate, bichloride of mercuiy, HgCl2) is a soluble, 

 highly poisonous compound. It is commonly used for surface steriliza- 

 tion in a concentration of 1/1,000. Mercuric chloride is occasionally 

 used as a special-purpose fungicide. 



Mercurous chloride (calomel, HgCl or Hg2Cl2) is essentially insoluble 

 in water, sufficiently so to be used in medicine. Calomel slowly decom- 

 poses into mercury and mercuric chloride. This decomposition is accel- 

 erated by sunlight, which may account for the successful use of calomel 

 to control dollar spot, brown patch, and other turf diseases. 



The organic mercury compounds have won wide acceptance in the 



