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



converted by the action of the carbon dioxide of the air into other com- 

 pounds and free sulfur, that its protective function is exercised (Haywood, 

 1907). The elemental sulfur precipitated from the lime-sulfur chiefly 

 functions as the protectant. 



It has long been known, though not generally appreciated, that certain 

 essentially water-insoluble arsenical compounds — for example, arsenate 

 of lead and arsenate of lime — exhibit marked protective action against 

 the apple-scab pathogene, Venturia inaequalis (Butler and Doran, 1928). 

 The writers and T. C. Loh (Whetzel, McCallan, and Loh, 1929) have 

 recently reported that calcium arsenate exhibits marked specificity against 

 Alternaria panax, a pathogene but slightly susceptible to the toxic action 

 of copper or sulfur protectants. 



That insoluble fungicides may sometimes act as disinfestants, or even 

 as disinfectants, is not to be denied. The elimination of powdery mildews 

 by applications of sulfur dust to infected plants may be looked upon as a 

 case of disinfection by an insoluble fungicide. Such cases are, however, 

 few and exceptional. 



A good protectant is (a) practically insoluble in pure water, (b) highly 

 adhesive to the surface of the suscept organs to be protected, (c) active 

 only in the presence of some solvent, (d) essentially harmless to the 

 suscept, (e) miscible with insecticides if desired, the efficiency of neither 

 being impaired nor the suscept endangered, (f) easy of application, and 

 (g) economical. 



It is evident, then, that water-soluble fungicides act chiefly as disin- 

 festants or disinfectants, while water-insoluble fungicides function, with 

 few exceptions, as protectants. 



In the foregoing presentation, the writers offer what they hope may 

 appeal to plant pathologists as a much-needed clarification of the chaotic 

 state of the terminology of our current concepts of fungicides and fungi- 

 cidal phenomena. It is believed that we have now reached a point in the 

 development of the science of plant pathology where a revision in our 

 terminology, consistent with the present state of our concepts in this field, 

 will conduce to more nearly accurate and more logical thinking. It may 

 be argued that general usage justifies the application of terms in a sense 

 contrary to their strict meaning. This position can hardly be maintained, 

 however, in the face of the current confusion in our terminology of this 

 phase of plant pathology. Neither is such a position tenable on the ground 

 of want of words adequate to convey the specific concepts involved. Even 

 where such words are not already available, the language offers abundant 

 material for acceptable coinage. The concepts and terms presented in 

 this paper have long been employed by the senior author in his college 

 course on the principles of plant-disease control. 



