122 BELL SYSTEM TECHNICAL JOURNAL 



the choice of test organisms, the ease of manipulation, the replication of 

 results, and the duplication by other investigators have been the paramount 

 objectives. 



A hypothetical test which would meet these requirements could be 

 performed with distilled water to which could be added increasing concen- 

 trations of the chemical to be tested. A known amount of fungus mycelium 

 or spores could be shaken with the toxic solution and left for several different 

 time intervals. The fungus filaments or spores could then be removed to a 

 nutrient agar, and the viability of the fungal filaments or percentage of 

 spores germinating could be readily determined. Comparative toxicity of 

 a large number of compounds could be quickly and easily ascertained. 

 The results, however, would be applicable only to a distilled water-poison 

 system, and the concentration of most toxic materials necessary to inhibit 

 growth would be very low. 



The addition of nutrients would necessitate larger amounts of the toxic 

 materials (Van den Berge,'^ 1935). Therefore the mineral solutions — 

 nutrient agar, soil extract agar, soil or wood substrate would in general 

 necessitate an increase in toxic material, the amount of increase depending 

 upon which substrate best meets the nutritional requirements of any partic- 

 ular fungus. Some toxicity values would also be afifected by chemical or 

 certain physical changes resulting from interaction between the toxic 

 material and the substrate. 



In the petri dish method, the fungi selected for studies of wood destruction 

 grow well on the nutrient substrate containing 1.5% malt extract and 2% 

 agar. Although such a mixture has been recommended as a standard 

 substrate,- it should be pointed out that the malt syrup is somewhat variable 

 in composition and constituents and that even the agar varies in the amounts 

 of various growth substances present, Robbins and Ma,'" 1941. The inter- 

 pretation of results obtained by the assay of a fungicide when dispersed 

 in an agar system should be restricted to that specific system and not 

 applied to a wood-fungicide system. 



When wood preservation studies are carried out, reliance cannot be placed 

 on the results of petri dish tests. The use of wood permits the testing of a 

 large variety of the more common preservatives and fungus-proofing agents, 

 many of which may react with the wood or are precipitated in the wood 

 upon loss of solvent. Organic preservatives which are relatively insoluble 

 in water are not readily tested by petri dish assay. 



Comparison of the wood-soil contact method with the wood-water method 

 when untreated wood blocks are used is shown in Fig. 2. The greater 

 uniformity in the amount and rapidity of decay and the better control of 

 moisture showed the soil technique to be superior. It is obvious that if 

 the amount of decay is variable and adversely affected by other factors, 



