448 



CHEMICAL AGENTS 



100 



Uranium, ppm 



10,000 



Figure 4. Dosage response 

 curves for the action of uranium 

 acetate on spores of Alternaria 

 solani. Curve 1, fungistatic 

 action; curve 2, fungicidal action 

 at 20 hours; curve 3, fungicidal 

 action at 10 minutes. Redrawn 

 from McCallan and Wellman 

 (250), by permission of the 

 Boyce Thompson Institute for 

 Plant Research, Inc. 



relatively high concentrations of copper — at lower concentrations there 

 is no perceptible damage for a considerable period of time, after which 

 the exponential form of curve appears. Similarly, the shape of the 

 time-inhibition curve for the action of phenol is affected by the con- 

 centration of the toxicant (391). 



Killing is also a function of temperature; the fungicidal action of a 

 number of relatively mild materials is sharply and non-linearly in- 

 creased as the temperature is raised, with an especially sharp break as 

 the temperature approaches its lethal value for spores in the absence 

 of toxicant (183). 



Although a strongly fungicidal agent is fungistatic, the reverse is 

 not necessarily true. In particular, several compounds of high fungi- 

 static activity — potassium dichromate, uranyl acetate, imidazolines, 

 and 8-hydroxyquinoline are examples — have only very weak fungicidal 

 activity (250, 437). In general, dosage-response curves for fungicidal 

 action are much flatter than those for fungistatic, i.e., the range in 

 resistance of the population is greater (Figure 4). 



2. TOLERANCE AND RESISTANCE 



Species of fungi differ widely in their sensitivity to toxicants. This 

 has been especially noticed since the introduction of antibiotics and 

 organic fungicides; differences in sensitivity of 50- to 100-fold are com- 

 mon (203, 270, 349, 440). However, strains tolerant to more general 

 poisons are also encountered in studies on mercury (124, 232, 423), 

 fluoride (227, 423), arsenate (303), and creosote (65). Copper tolerance 

 is often associated with tolerance to hydrogen ion or other cations (195, 

 387, 397). This suggests that the mechanism of copper tolerance is a 



