CHAPTER XVI 

 FUNGICIDES, THEIR PREPARATION AND APPLICATION 



MOST fungous diseases of fruits and fruit-trees are more or 

 less profitably controlled by the application of external pro- 

 tective substances known collectively as fungicides. The 

 essentials of a. good fungicide are : it must have ability to protect 

 the host from the attacks of the fungi ; it must be non-injurious 

 to the host; it must be adhesive, i.e. not readily washed off 

 by rains ; and must be relatively inexpensive and easy of 

 application. 



A fungicide usually acts to protect a host by destroying or 

 inhibiting, through some toxic property, the growth of the 

 fungous spores which fall on the exposed surfaces after the 

 fungicide has been applied. Fungicides do not penetrate 

 the host and kill the fungus after it has entered and established 

 itself, hence the futility of applications after the host is infected. 

 Fungicides prevent, they do not cure. In the case of a few 

 fruit diseases, the fungicide acts as a disinfectant rather than 

 as a protector, i.e. it destroys the spores of the pathogene 

 which are already present when applied. The control of peach 

 leaf-curl or of the surface-growing powdery-mildews are cases 

 of disinfection rather than protection. Pathogenic fungi vary 

 considerably in their sensitiveness to fungicidal poisons, both 

 as to kinds and strengths, hence the necessity for using dif- 

 ferent kinds and strengths of fungicides for different fungous 

 diseases. Sulfur fungicides, for example, so effective against 

 apple-scab are of no value for the control of bitter-rot, against 

 which copper fungicides must be used. 



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