246 PHYSIOLOGY OF THE FUNGI 



of cupric ion, while a few species have been reported to grow in a saturated 

 solution of copper sulfate (Starkey and Waksman, 1943). There is no 

 useful universal fungicide. 



The intelligent choice of a fungicide depends upon a number of factors, 

 the major ones being the species of fungus to be controlled and the nature 

 of the material to be protected. The solubility of the fungicide is of great 

 importance. For most efficient preservation of wood or protection as a 

 spray, a fungicide must have a low solubility in order that the protection 

 may extend over a long period of time. For surface sterilization a highly 

 soluble fungicide is used. When a fungicide is to be used on a living 

 plant (or other organism), the relative sensitivity of the host and of the 

 fungus to the fungicide must be considered. Host sensitivity limits the 

 use of many potent fungicides. A useful fungicide must be more toxic 

 to the fungus than to the host. For example, copper fungicides are quite 

 toxic to cabbage, cucumber, and pea seed, while beet, eggplant, pepper, 

 and spinach seed are relatively tolerant to copper. 



Although there is an enormous accumulation of literature on fungicides, 

 their composition, application, limitations, and economic value (see 

 Frear, 1948, and Horsfall, 1945), relatively little has been published on 

 the mechanism of fungicidal action. This is a practical as well as an 

 academic question, for the intelligent use of known fungicides and the 

 search for new and better ones are based upon a knowledge of how they 

 act. 



In the past the most important inorganic fungicides have contained 

 compounds of copper, mercury, or sulfur. In the future, however, excel- 

 lent fungicides may be made from other toxic elements. For example, 

 cadmium is of potential interest, but the present supply is limited. In 

 controlling fungi and other pests, there is always the danger that they 

 will become tolerant, or fast, to a given toxicant. This means that the 

 more susceptible individuals are killed and that a greater amount of a 

 given fungicide is required to control the more tolerant population which 

 is then built up. It is desirable from several viewpoints to have satis- 

 factory reserve fungicides in the armory of the plant pathologist. 



COPPER 



The first copper salts to be used as fungicides were the sulfate and 

 acetate (Prevost, 1807). These salts are soluble, and even in low con- 

 centration they are too toxic for many uses. Since all the copper is 

 available at once, these salts are toxic to plants, especially to young parts. 

 These soluble salts have a further disadvantage when used as a spray, for 

 a heavy dew or rain will easily wash them off. However, these salts, 

 especially copper sulfate, were successfully used for treating seed grain to 

 destroy surface contaminants. This treatment was devised by Prevost 

 to control bunt. 



