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, 7.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, 1.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. 
430 
