262 PHYSIOLOGY OF THE FUNGI 



ing the diameter of the zone of inhibition. Disks of blotting paper to 

 which fungicides have been added may be used to determine their potency. 

 Thornberry (1950) has suggested the use of filter-paper disks for the 

 evaluation of fungicides and bactericides. Filter-paper disks appear 

 to be more suitable than blotting paper. In this method seven filter- 

 paper disks are uniformly spaced on a Petri dish, and 0.09 ml. of the 

 toxicant in aqueous solution is added per disk. The zone of inhibition 

 is a measure of the effectiveness of the fungicide. 



The glass-slide method appears to simulate more closely the conditions 

 under which the spores of plant pathogenic fungi germinate in nature. 

 The Committee on the Standardization of Fungicidal Tests of the Ameri- 

 can Phytopathological Society has considered this method important 

 enough to publish a detailed and documented summary (1943), to which 

 the student is referred for further information and references. This 

 committee recommended the use of spores of the following species for 

 this test: Alternaria solani, Glomerella cingulata, Macrosporium sarcinae- 

 forme, Sclerotinia fructicola, Penicillium expansum, and Rhizopus nigri- 

 cans. For accurate work, at least two of these test fungi should be used. 

 The effectiveness of a fungicide is determined by calculating the percent- 

 age of inhibition of spore germination. The methods of evaluating data 

 obtained in fungicide tests are discussed by Horsfall (1945). 



SUMMARY 



A fungicide is an agent capable of killing some fungi. Fungicides may 

 be either water-soluble or nearly insoluble. The action of fungicides 

 of the first class is immediate; that of the second class is delayed. Eradi- 

 cant fungicides are of the first class, w^hile protective fungicides are of 

 the second. Fungistasis is the complete or partial inhibition of one or 

 more life processes of a fungus. This inhibition is reversible. The same 

 chemical compound may cause fungistasis or may be a fungicide, depend- 

 ing upon the concentration and time of exposure. The same substance 

 may be a fungicide for one species, cause fungistasis of a second, and be 

 without effect upon a third. Fungistasis precedes fungicidal action. 



Before a fungicide can act upon a fungus, the toxicant must get into 

 the fungus cells, or at least reach the protoplasmic membrane. While 

 other factors undoubtedly enter into the mechanism of fungicidal action, 

 the principal point of attack appears to be enzyme systems. The heavy- 

 metal fungicides appear to act by inhibiting various sulfhydryl enzymes. 

 Fungus spores transform sulfur into hydrogen sulfide, which inhibits the 

 metalloenzymes. Organic fungicides, so far as is known, are also enzyme 

 inhibitors. 



In the past, fungicides containing copper, mercury, and sulfur have 

 been the most useful. Recently, organic fungicides have become impor- 



