Studies on Fungicides — I 5 



those cases in which the destruction of the pathogene results in the cessa- 

 tion of infection (for example, in the treatment of oat seed with formalin 

 for control of loose smut). Here the seed is actually infected (Gage, 1927). 

 Where destruction or elimination of the pathogene from mere physical 

 association with the suscept or other harborers is involved, the terms 

 disinfest, disinfestant, and disinfestation are to be preferred (for example, 

 treating wheat seed for control of stinking smut, or drenching soil with a 

 solution of formaldehyde to eliminate phytopathogenic organisms). 



Water-soluble fungicides, with few exceptions, function as either disin- 

 festants or disinfectants. They are usually effective only in cases of infesta- 

 tion, though sometimes they may actually disinfect, that is, destroy the 

 pathogene after it has established pathogenic relations with the suscept. 

 The use of lime-sulfur or potassium sulfide to destroy the powdery-mildew 

 fungus already established on roses or other plants, is a good example of 

 disinfection by water-soluble fungicides. There appear to be few water- 

 soluble fungicides employed in plant-disease control which are capable of 

 penetrating the tissues of the plant to destroy an invading pathogene 

 without economic injury to the suscept. The action of formaldehyde in 

 the case of loose smut of oats (Gage, 1927), and that of borax or boric acid 

 in the disinfection of oranges infected by Penicillium (Fawcett and Lee, 

 1920:361), appear to be cases in point. The water-soluble fungicides most 

 commonly employed for purposes of disinfestation in plant-disease control 

 are formaldehyde, some mercury compounds, and certain copper com- 

 pounds. 



A good disinfestant is (a) toxic to the infesting pathogene, (b) practically 

 harmless to the infested organs of the suscept at effective concentrations, 

 (c) innocuous to the soil or other non-living harborers infested with the 

 pathogene, (d) easy of application, and (e) economical. 



Where used as a disinfectant, the fungicide must be (a) able to penetrate 

 to the pathogene if that is within the tissues of the suscept, (b) toxic to 

 the infecting pathogene at a concentration practically harmless to the 

 tissues of the suscept, (c) easy of application, and (d) economically profit- 

 able in control of the disease. 



Water-insoluble fungicides commonly function as protectants. They are 

 usually effective only when applied to the plant before it has become 

 inoculated with the spores of the pathogene. Bordeaux mixture is the 

 classical protectant. Its protective function was clearly perceived by its 

 discoverer, Millardet (Crandall, 1909:205). Next to bordeaux mixture, 

 sulfur in its elemental state, either in suspension (dry mix) or as dust, is 

 perhaps the most widely used protectant. Lime-sulfur, a water-soluble 

 fungicide, at first thought might be assumed to function directly as a pro- 

 tectant. A moment's reflection will at once discover the fallacy in this 

 assumption. It is chiefly after the lime-sulfur has dried and has been 



