Studies on Fungicides — II 9 



conflict. However, the author's experiments strongly support the third 

 hypothesis. The evidence for this is presented in the next paper of this 

 series. 



It is evident that if fungicidal action involves essentially only a relation- 

 ship of spore and protectant, the laboratory testing of fungicides, confined 

 also to this relationship, may be expected to be both reliable and prac- 

 ticable. The advantages of laboratory tests lie in the saving of time, 

 expense, and labor, and in the more definite control of environmental con- 

 ditions. A set of tests may be made in a month or two which under field 

 conditions would require several years. Thus, from many possibilities a 

 few probabilities may be selected for actual field testing. 



It must be emphasized that in the testing of fungicides the determination 

 of their toxicity is of primary importance. The toxicity having been 

 determined in the laboratory, the investigator is then in a position to 

 proceed to the field for tests of adherence and injury to the plant. Spray- 

 ing or dusting experiments on a large scale in the field must serve as the 

 final test in the evaluation of any fungicide. 



In the preparation and selection of fungicides, laboratory testing is 

 playing an increasingly important role. Not only in many phytopath- 

 ological research laboratories does this constitute a significant part of the 

 program, but also in commercial laboratories. 



The practicability, advantages, limitations, and uses of laboratory tests 

 of fungicides have been outlined. What, then, is necessary in a reliable 

 method of testing the toxicity of protective fungicides in vitro? First of 

 all, it must duplicate, in so far as possible, actual field conditions. Some 

 methods depart from this requirement to a large degree. Wallace, Blod- 

 gett, and Hesler (1911 : 167) state: 



It is true that many experiments have been conducted to determine the effect of certain 

 fungicidal solutions on spore germination. In most such cases, however, the spores were 

 placed directly in a solution or mixture of the fungicide. The results, therefore, must be 

 considered as of scientific interest rather than as serving to indicate what these same sub- 

 stances will do after being sprayed and dried on the plant. The latter is the condition in 

 which fungicides, with very few possible exceptions, must prevent infection. 



Other considerations call for a method that is accurate and quick, with 

 a simple technic, and capable of wide application. 



DEVELOPMENT OF THE METHOD 

 In 1910 Reddick and Wallace published a brief note in Science, entitled 

 On a Laboratory Method of Determining the Fungicidal Value of a Spray 

 Mixture or Solution. This was the first published record of this method, 

 the fundamental principle of which was developed by Dr. Reddick in 

 connection with some class work during the winter of 1908 (Wallace, 

 Blodgett, and Hesler, 1911). A few months later, Wallace, Blodgett, and 

 Hesler (1911) devoted considerable space to a discussion of the method. 



