GENERAL CONSIDERATIONS 85 



most slowly developed affections, but when spores are produced 

 quickly and in vast numbers, as they are in so many diseases, 

 the effects are not as a rule satisfactory unless the plants are 

 also sprayed. 



Spraying. 



The object of most spraying with fungicides is not, as seems 

 to be commonly thought, the cure of disease, but the prevention 

 of further infection. Fungoid and bacterial parasites, with the 

 exception of the powdery mildews and a few others of similar 

 habit, enter more or less deeply into the tissues and cannot be 

 reached by a spray which does not at the same time destroy the 

 affected part itself. The action of the copper sprays, of which 

 Bordeaux mixture is the type, is to prevent the successful 

 germination of spores lying on the surface ; this they do most 

 effectively at the time of germination, so that protection is 

 best afforded by an enduring film in which the toxic ingredient, 

 the copper, which is at the same time the substance injurious 

 to the plant itself, is rendered soluble by slow degrees. For 

 the destruction of developed mycelium where it can be reached, 

 as in the mildews, the sulphur sprays are found more effective. 



From the nature of the process spraying is most applicable 

 to affections of leaves, flowers, and fruit, especially when the 

 disease is of a seasonal and more or less epidemic nature. 



Permanent Crops. 



Considerations different in some respects from those applying 

 to arable crops arise in reference to permanent crops, in which 

 the same trees occupy the same ground for many years. The 

 protection given by rotation of crops is not available, and the 

 death of individual plants is much more serious, since they may 

 take years to replace. The adoption of more careful and expen- 

 sive treatment may be commercially sound, for not only the 

 immediate crop, but the future history of the trees has to 

 be considered. 



In the West Indies there are all stages of tree cultivation 

 from the simple afforestation of land with limes, coconuts, or 

 cacao — further attention being confined to cutlassing the under- 

 growth and removal of the fruit — to careful orchard methods in 

 which the cultivation of the soil, the use and sequence of the 

 different types of manure, and the pruning of the trees receive 

 their due consideration. In the former system, as in forestry 

 proper, individual treatment for disease is seldom regarded as 

 practicable ; dead branches may be removed at intervals, 

 dead trees are replaced by " supplies," and that is about all, 

 except where the persistent march of a root disease may compel 

 attention. In the more intensive system reached on some cacao 

 estates the recommendation of sanitary measures and of the 

 use of sprays is not considered unreasonable. No doubt as 



