444 



CHEMICAL AGENTS 



Figure 2. The inhibition of 

 Aspergillus niger by p-hydroxy- 

 benzoic acid in an agar medium. 

 Drawn from data of Vincent 

 (426). 



-0.5 +0.5 



Log millimolar concentration 



p-hydroxybenzoic acid 



This is related to toxicant concentration by the equation: 



e6 



y = ax" (1) 



where y stands for degree of inhibition, x for concentration of the fun- 

 gicide, and a and b are constants. A plot of the logarithm of the degree 

 of inhibition against the logarithm of the concentration yields curves 

 of the type shown in Figure 2. Not all curves are as clearly linear as 

 this one (426). 



A variety of special methods are essentially similar to the assay of 

 Bateman just described (26, 102, 108, 109, 114, 151, 203, 271, 283, 

 296, 297, 317, 318, 359). Criteria used include the least concentration 

 preventing all growth, the colony diameter at a fixed time, or the dry 

 weight in liquid culture. A rather different approach is found in the 

 use of filter paper discs impregnated with the toxicant (218, 415); here, 

 of course, diffusion becomes an important factor. 



These methods all have particular and limited value; thus, the medi- 

 cal mycologist has a practical interest in the inhibition of established 

 infections rather than in prevention of spore germination. Fungi 

 which do not sporulate in culture may have to be studied as mycelium. 

 For theoretical work on mode of action, however, the presence of nu- 

 trients and, in agar media, of undetermined contaminating substances, 

 severely restricts their value. It is almost always found that nutrient 

 materials reduce the apparent toxicity of fungi toxic agents (214, 352, 

 404). In a qualitative way, it is usually true that the agar plate method 

 and the spore germination method agree, but they may give divergent 

 results for particular compounds (174). As a rule, fungitoxic ma- 

 terials are more active against spore germination and spore respiration 



