CHAPTER IX 
GENERAL REFLECTIONS AND CONSIDERATIONS 
ON DISEASE TREATMENT—Continued 
Sprayins and Dusting 
THE value of spraying has long been recognized by the 
potato and fruit growers, the more progressive of whom 
regard it as an essential cultural operation—almost as a 
form of insurance. The glasshouse owner, however, has 
not yet learned to regard it in the same light, and many 
of those who have attempted to spray against fungus 
diseases have been disappointed by the results, mainly 
because they have not understood the rationale and 
function of the process. 
Spraying consists of the application of fungicidal and 
- insecticidal fluids to the surface of a plant as a protection 
against pests causing diseases. 
Some fungi and most insects live on the surface of 
the leaves and stems and may be destroyed directly by 
contact when a suitable spray is applied. On the other 
‘hand, the majority of parasitic fungi and bacteria spread 
deeply into the plant tissues, and therefore cannot be 
touched by spray fluids, which are external. In such 
cases the application of a fungicidal spray destroys the - 
fruiting parts of the fungus, which are produced on the 
outside of the plant, but does not prevent the further 
development of the deeply buried parts, which in course 
of time are able to grow out into the air and produce new 
fruiting bodies. 
The true function of a fungicidal spray is protective, 
not curative; and realization of this fact will lead the 
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