250 PHYSIOLOGY OF THE FUNGI 



treating of seed to control the attack of fungi which cause damping-off 

 and of certain seed-borne pathogens. The organic mercurials are free 

 from many of the objections inherent in the inorganic compounds of 

 mercury. In general, they combine less avidly with proteins, are more 

 selective in their action, and are far less toxic to animal life. As used for 

 seed protection, they are commonly diluted with an inert carrier. Most 

 if not all such organic mercury compounds are sold under trade names, 

 but the active components are required by law to be stated on the label. 

 Among these organic mercury compounds are ethylmercury phosphate 

 (Semesan Jr. and New Improved Ceresan), ethylmercury chloride 

 (Ceresan) and hydroxymercurichlorophenol (Semesan). 



The organic mercury compounds used as sprays and for treating seeds 

 are in general related to mercuric chloride in the following way: 



C2H6— Hg— CI CI— Hg~Cl 



Ethylmercury chloride Mercuric chloride 



The ethyl group has replaced a chlorine atom in mercuric chloride. 

 The type formula for compounds like this may be written as R — Hg — X, 

 where R may be any alkyl or aryl (or other) group and X represents any 

 anion, I~, Cl~, 0H~, NOs", P04~. The anion greatly modifies the solu- 

 bility of the compound in water. In general, this type of organic mer- 

 cury compound is volatile, and this property may be assumed to aid in 

 penetration. Other organic mercury fungicides are derivatives of alkjd 

 and aryl mercuric hydroxides. These compounds can react with organic 

 acids to form salts. The relation of these compounds to mercuric hydrox- 

 ide is shown below: 



CeHs— Hg— OH HO— Hg— OH 



Phenylmercury hydroxide Mercuric hydroxide 



Compounds of this type are used to protect cellulose and leather products. 



Parker-Rhodes (1942) investigated the toxicity of the following mer- 

 cury compounds to Macrosporium sarcinaeforme and Botrytis allii: mer- 

 curic acetate, mercuric chloride, methylmercury nitrate, and tolylmer- 

 curic nitrate. All these compounds were toxic to M. sarcinaeforme, and 

 all except methylmercury nitrate were toxic to B. allii. Perhaps methyl- 

 mercury nitrate is not soluble enough in fat for the spores of this fungus 

 to absorb a toxic amount of the compound. Dillon- Weston and Booer 

 (1935) found that vapor of ethylmercury iodide was toxic to TiUetia spores 

 in the laboratory but afforded no control in the field. 



The soluble inorganic mercury salts are protein precipitants, and this 

 property may explain in part their mode of action when used in high 

 concentrations. These salts are frequently fungistatic or bacteriostatic, 

 since the very firmness of the union between the mercuric ion and the cell 

 membrane may form a barrier to further penetration. The first action 



