8 S. E. A. McCallan 



STUDIES ON FUNGICIDES 

 II. TESTING PROTECTIVE FUNGICIDES IN THE LABORATORY 



S. E. A. McCallan 



The laboratory testing of fungicides to determine their toxic properties 

 has gained ground rapidly during recent years. The growing interest in 

 this matter is doubtless due, in part at least, to the ever-increasing number 

 of proprietary compounds on the market, a condition which makes field 

 testing all but prohibitive in time and labor. In view of these facts, a 

 short discussion at this time relative to the general scope of fungicidal 

 tests in vitro, and the presentation of a method developed in the Labora- 

 tory of Plant Pathology at Cornell University and long in use there, may 

 be serviceable to other workers. Such a discussion is presented in the 

 following pages, and will serve as an introduction to the technic employed 

 in certain studies on fungicidal action which constitute the third paper 

 of this series. 



Experience here has demonstrated, over a period of years, that the 

 toxicity of a protective fungicide can be most readily and effectively deter- 

 mined in the laboratory. When the difficulties involved in measuring the 

 other requirements of a good protectant (page 6) under the unnatural 

 conditions of the laboratory or even of the greenhouse are considered, it 

 is probable that testing under natural field conditions will give, for the 

 present at least, the most reliable results for such factors as adhesiveness, 

 toxicity to the plant, and the like. 



A study of the literature on the nature of fungicidal action shows that, 

 in general, one of three hypotheses usually has been advanced to explain 

 the liberation of the active fungicidal principle from the insoluble protect- 

 ant. These hypotheses are: (1) that this is accomplished by means of 

 atmospheric agencies, especially rain, dew, and the carbon dioxide in the 

 air; (2) that it is accomplished by the suscept, especially by the excretions 

 from the organs to be protected; and (3) that it is accomplished through a 

 solvent action exerted by the fungus spores themselves upon the protectant. 



The literature relative to the first hypothesis is much in conflict, and 

 experiments by the author, which are still in progress, show that, at least 

 for certain common protectants, the role played by atmospheric agencies 

 is almost negligible. Convincing evidence to substantiate the hypothesis 

 of suscept excretions is lacking in the literature. The data on the 

 hypothesis of spore excretions are very limited and are somewhat in 



Author's acknowledgments. The writer desires to acknowledge his indebtedness to all those workers 

 of the Laboratory of Plant Pathology at Cornell University, both past and present, who have been instru- 

 mental in the development of the method of testing fungicides here described. Especial thanks are due 

 to Professor H. H. Whetzel, of the Department of Plant Pathology, for the valuable advice rendered by 

 him in the progress of these studies. 



