CH. XV] DISEASES OF PLANTS AND THEIR TREATMENT 135 



in general only by white planters in the tropics that measures 

 of this kind are taken with any precision or regularity, but it 

 so happens that it is upon their cultivations that disease stands 

 the greatest chance of getting out of control, for the natives 

 have usually either the mixed cultivations in which disease 

 does not so readily spread, or rice or grain fields, which are 

 annually left fallow or rotated with other crops for considerable 

 periods. Thanks to the efforts of many students, especially in 

 the United States, the methods employed in combating disease 

 are becoming more and more systematised. It would lead too 

 far to go into details here, but the chief methods in use may 

 be summed up under the following heads: 



(1) The destruction, in general by fire, of diseased trees, 

 plants, or parts of plants. In this very simple way the spores 

 which carry the infection of the disease are absolutely destroyed, 

 and if this method be steadily practised, the disease may be 

 prevented from spreading, and may even in time become almost 

 eradicated. One great difficulty in the way of such treatment 

 upon many estates, is the lack of any vacant areas upon which 

 the fires may be lighted. 



(2) The collection and killing of parasitic insects and their 

 eggs. This is obviously the corresponding remedy to the last- 

 mentioned. 



(3) Spraying. This in general means the wetting of the 

 diseased, or in preventive cases the healthy, areas of the 

 infected plants (or in some preventive cases, the healthy plants) 

 with a stream of some fluid in which, so long as it remains 

 upon the leaf, stem or fruit, the spores of the fungus cannot 

 germinate, or the eggs of the insect cannot live, or which makes 

 the plant distasteful to the fungus or the insect, or which kills 

 the insect or fungus upon the plant. There are almost innu- 

 merable forms of spraying compounds, but in general they 

 contain, if for attack upon fungi some salt of copper (as for 

 instance copper hydrate in the commonest spray of all, Bor- 

 deaux mixture), if for attack upon sucking insects an emulsion 

 of kerosene oil, and if for attack upon biting insects some 

 compound of arsenic. For attacks upon mites, sulphur is the 

 usual foundation of any insecticide. The forms of instrument 



