STUDIES ON FUNGICIDES 

 I. CONCEPTS AND TERMINOLOGY 

 H. H. Whetzel and S. E. A. McCallan 



There is now under way or projected, in the Department of Plant Pa- 

 thology at Cornell University, a series of investigations directed toward the 

 solution of fundamental problems in fungicidal efficiency. It is proposed 

 to publish the results of these investigations as units in a continuing series 

 under the general title Studies on Fungicides. As introductory to the 

 series, this first paper sets forth some concepts and terminology which 

 appear basic and indispensable to a rational discussion of hypotheses and 

 results to be presented in succeeding papers. 



The term fungicide is commonly employed to designate any chemical 

 element or compound which is toxic to fungi. It is a more restricted term 

 than germicide, and is coordinate with bactericide, insecticide, nematocide, 

 and the like. While commonly restricted in its application to chemical 

 elements or compounds, it may perhaps be extended legitimately to include 

 such physical agents as light, heat, or other forms of radiant energy. In 

 this paper it is employed in the more commonly restricted sense. Only 

 chemical fungicides are dealt with. 



Since the toxic constituents of fungicides can act on the fungi only by 

 diffusing into the protoplasm, it is evident that they must eventually 

 become soluble. Many of our commonly used fungicides are immediately 

 soluble in water. It is generally recognized, however, that certain of our 

 standard fungicides, notably bordeaux mixture and sulfur dust, are not suffi- 

 ciently soluble in pure water to exercise their fungicidal properties. A 

 number of hypotheses have been advanced to account for the evident 

 solution of the toxic constituents of these fungicides after they have been 

 applied to plants. Certain constituents of the atmosphere, excretions 

 from the plant itself, and excretions from the germinating spores, each 

 have been held by investigators to be the dissolving agent. 



Therefore, on the basis of their state in water, fungicides may be divided 

 into two classes: (1) those soluble in water, that is, those in which the 

 toxic principle acts directly and immediately upon the living protoplasm 

 of the fungus; and (2) those essentially insoluble in water, that is, those 

 in which the toxic principle can act effectively only on being liberated by 

 some other agent than water. While all chemical elements or compounds 

 are probably soluble in water to at least a very slight degree, the term 

 insoluble is here used in the sense that the toxic principle is of itself not suffi- 

 ciently soluble in water to inhibit or terminate infection by the pathogene. 



