2G0 PHYSIOLOGY OF THE FUNGI 



In addition, Nabam on decomposition yields an unidentified toxic gaseous 

 compound, which is neither hydrogen sulfide nor sulfur dioxide (Rich 

 and Horsfall, 1950). 



Specific organic reagents for metals. The essential nature of certain 

 micro elements for fungus growth and the role of these elements in 

 enzymes were discussed in Chaps. 4 and 5. The chemistry of these 

 specific organic reagents is treated by Yoe and Sarver (1941). These 

 reagents form insoluble or slightly ionized compounds with metals. 

 Zentmeyer (1944) tested various organic analytical reagents and found 

 8-hydroxyquinoline (Oxine) and ammonium nitrosophenylhydroxylamine 

 (Cupferron) to be fungistatic. 8-Hydroxyciuinoline inhibited the growth 

 of Fusarium oxysporum var. lycopersici, Ceratostomella ulmi, and a species 

 of Penicillium. The effectiveness of 8-hydroxyquinoline in forming 

 chelate salts increases as the pH values increase. Below pH 3.5 complex 

 formation does not take place with zinc, copper, iron, and manganese. 

 Zinc ion reacts with 8-hydroxyquinoline as shown below: 





+ H+ 



I I r 



OH O— Zn 



8-Hydroxyquinoline Zinc complex of 8-hydroxyquinoline 



The fungistatic effect of 8-hydroxyquinoline on Fusarium oxysporum 

 var. lycopersici and Ceratostomella ulmi was overcome by increasing the 

 zinc content of the medium. In the presence of 8-hydroxyquinoline 

 there was competition between this compound and one or more enzyme 

 systems for the zinc present in the medium. Whether or not an organic 

 compound such as 8-hydroxyquinoline will act as a fungistatic agent 

 depends upon the concentration of the reagent, the amount of fungus 

 mycelium, and the concentration of the metallic ion for which the two 

 sj^stems compete. One would expect that such fungicides, in common 

 with all others, would be more effective when the mass of the fungus is 

 small. 



Other organic fungicides. Many other types of organic compounds 

 are fungicides, and an intensive search for new ones is in progress. Brief 

 mention of some of these developments is made below. Geiger (1948) 

 reports various unsaturated ketones to be active against Aspergillus 

 niger, Trichoderma koningii, Cryptococcus neoformans, and Trichophyton 

 mentagrophytes. The mode of action resembles that of the naphtho- 

 quinones in that sulfhydryl enzymes, including succinic acid dehydro- 

 genase, triose phosphate dehydrogenase, and urease, are inhibited. The 

 fungistatic activity of ethylenic and acetylenic compounds has been 



