CHAPTER 12 

 THE ACTION OF FUNGICIDES 



The never-ending warfare which man must wage against parasitic fungi 

 in order to protect his crops has been ably chronicled by Large (1940). 

 The saprophytic species which decay wood and other cellulosic materials 

 cause great economic loss, although these species perform a necessary 

 and indispensable role in maintaining the carbon cycle in nature. It is to 

 man's interest and profit that the deterioration of textiles and lumber be 

 prevented or delayed and that his crops be protected from pathogenic 

 fungi. This is done by the use of fungicides, which either kill or inhibit 

 the action of fungi. 



By definition, an agent which kills fungi is a fungicide. A fungistatic 

 agent merely causes inhibition. The same agent is commonly capable of 

 producing both actions. A discussion of the terms fungicidal and fungi- 

 static is given by McCallan and Wellman (1942). These authors point 

 out that the fungistatic activity of an agent is broader than its fungicidal 

 activity. 



Both physical and chemical agents may be fungicidal and fungistatic. 

 Of the physical agents, heat and ultraviolet radiation are probably most 

 commonly used, while many chemical compounds are ''toxic" to fungi. 

 Whether an agent is fungicidal or fungistatic is primarily a matter of 

 degree of intensity and duration of exposure. We may assume that these 

 agents, whether chemical or physical, act directly upon certain specific 

 enzymes or enzyme systems. If the action is less severe and may be 

 reversed, the result is fungistasis, while if it is irreversible, the action is 

 fungicidal. Since most of the agents employed by man are chemical 

 compounds, much of the following discussion will be limited to the 

 mechanism of action of these compounds. 



Chemical fungicides may be applied as eradicants or as protectants. A 

 protectant is applied to the plant or other material before the inoculum 

 arrives at the infection court and often functions only after the fungus 

 spore germinates. An eradieant kills the fungus already present on or in 

 the substrate material. 



The lethal action of a chemical depends upon both the concentration 

 of the active compound or ion and the time of exposure. Species of 

 fungi exhibit great variation in ability to resist the action of certain 

 fungicides. Many fungi are killed by exposure to a few parts per million 



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