RESPONSE AND ITS MEASUREMENT 443 



under these conditions, although other explanations are possible (20). 

 Increase in oxygen consumption is also caused by poisons, e.g., 2,4- 

 dinitrophenol, which interfere with the utilization of respiratory en- 

 ergy (380). 



We may conclude that in general the activity of fungicides against 

 respiration does not parallel their effects on growth, and infer that 

 effective fungicides usually act on systems much more sensitive than 

 is the gross oxygen uptake. We may speculate that a compound which 

 acts on respiration and growth at approximately the same concentra- 

 tion — mercuric chloride is an example (247) — is a general enzyme 

 poison. It should also be clear that, although a respiratory system 

 may be inhibited by a fungistatic agent, this fact alone is not sufficient 

 evidence that the inhibition observed is the primary locus of toxicity. 



Sclerotinia laxa grown in contact with partially inhibitory concen- 

 trations of several fungicides is affected physiologically in its formation 

 of respiratory enzymes, some being increased and others decreased in 

 amount by specific compounds (55). 



Mutagenic Effects of Chemical Agents. Cultural variants of unde- 

 termined genetic status are induced by high levels of zinc (89) and 

 nitrite (Chapter 8). Mutation in fungi, verified by genetic studies, 

 is caused by a wide variety of chemicals, e.g., nitrogen and sulfur mus- 

 tards and related compounds (16, 286a, 400, 409), peroxides (87, 188), 

 epoxides (208), diazomethane (189), and caffeine (115). Some of these 

 are sulfhydryl reagents, others possibly generate free radicals, but no 

 single mode of action can be proposed at present. 



Fungistatic Action of Chemical Agents. Both practically and theo- 

 retically, the most important effect of toxicants on fungi is the inhibi- 

 tion of growth. 



This fungistatic action is the most frequently measured and should 

 not be confused with killing, fungicidal action sensu stricto. We may 

 distinguish two classes of fungistatic assay: (1) determination of my- 

 celial growth on a natural or artificial medium, and (2) determination 

 of spore germination. In both methods, the test fungus and the toxi- 

 cant are in contact throughout the period of incubation. 



The early development of agar plate methods is described by Bate- 

 man (22), who established a satisfactory quantitative treatment of the 

 data obtained with Fomes annosus. This treatment has since been 

 extended to other fungi (330, 426). After a more or less protracted 

 lag period, which is itself an index of toxicity, the rate of growth across 

 the agar surface is linear with time. Comparison with a control inocu- 

 lation on toxicant-free agar yields the basic datum, degree of inhibition. 



