466 CHEMICAL AGENTS 



effective toxicant (256, 311, 312). Very little information is available on 

 heavy metals other than these three; chromate complexes have been 

 studied recently (146, 439). 



Horsfall (174) summarizes the action of metallic cations, measured 

 by external concentration, against fungi in the order: Ag > Hg > Cu 

 > Cd > Cr > Ni > Pb > Co > Zn > Fe > Ca. For any one organ- 

 ism the order may differ slightly (23, 256, 300). Roughly the same 

 order of effectiveness is found in experiments on animals (373). 



Because of the reactivity of the heavy metals, growth experiments in 

 complete media are of dubious value; the spore germination technique 

 is to be preferred. Proteins and peptones are especially active in de- 

 creasing fungitoxicity (153, 214, 458), but other materials present in 

 culture media have the same effect (247). 



Present information allows us to consider the three most toxic heavy 

 metals — silver, mercury, and copper — together, i.e., to assume provi- 

 sionally that all have the same general mode of action; silver may, how- 

 ever, differ. We can also, but perhaps with less justification, consider 

 that the final toxic effect of heavy metal compounds is to be ascribed 

 to the metal itself. However, some problems are raised by this second 

 assumption. Different inorganic salts of copper, for example, are not 

 always of equal toxicity (28, 279); when copper is supplied as cupric 

 ammonium sulfate, it is more firmly bound by spores than copper sup- 

 plied as the sulfate (34). These differences cannot be explained as yet 

 in detail, but we may guess that the form in which copper is supplied 

 affects its uptake by the spore. Provision of silver as the iodide sharply 

 reduces both uptake and toxicity as compared to those of other silver 

 halides (288). 



The toxicity of coordination complexes of copper deserves special 

 attention. For years the effectiveness of Bordeaux mixture as a plant 

 protectant was a mystery, inasmuch as the copper is present on the leaf 

 in an insoluble complex. A number of investigations, reviewed by 

 Martin (278) and by McCallan (244), established firmly the proposi- 

 tion that materials from the spore solubilize the copper. The most 

 important of these substances are the amino and hydroxy acids, com- 

 pounds which form soluble chelate complexes with copper. Mono- 

 methylamine liberation by spores of Tilletia caries also results in solu- 

 bilization of copper (327), and seeds of higher plants liberate copper 

 from Bordeaux mixture (429). 



This hypothesis requires that soluble copper complexes be toxic. 

 Some data show that, under specified conditions, the glycine and malate 

 action may not be so great (130). Concentration factors are critical: 

 complexes are as fungistatic as copper sulfate itself (257); fungicidal 



