CH. XV] DISEASES OF PLANTS AND THEIR TREATMENT 135 



numerous, but in general they consist of small receptacles 

 which can be carried by one man, from a compressed-air 

 chamber in which a stream of air drives a fine jet of the 

 compound through a nozzle which breaks it up into a fine 

 spray. The large horse and steam sprayers employed in 

 California and elsewhere have not yet come into use in the 

 tropics. 



(4) Isolation of the diseased plants by digging trenches. 

 This is in general adopted for the purpose of checking the 

 spread of root-attacking fungi, and is usually very effective, 

 provided the trench be made outside of the outermost range as 

 yet covered by the fungus. Jungle stumps left in the ground 

 when clearing are the great hotbed for such root fungi. 



(5) Prohibition of the removal of plants, seeds, or parts of 

 plants, from districts already infected by disease, into districts 

 not already infected. This is more a precautionary measure, 

 and simply checks the transfer of infection into new districts. 



(6) Isolation of small areas of the cultivated crops by the 

 planting of " shelter belts " of trees through them, as is so 

 common in Ceylon, where the tea estates are divided into 

 "fields" by such belts. In this way, a pest is checked from 

 spreading to another field, when it has devastated the first 

 one attacked. 



The mention of method (5) above leads on naturally to the 

 subject of quarantine regulations against diseases, which are in 

 force in a few countries. These regulations may take various 

 forms, as for instance the prevention of the importation of 

 plants of a particular kind from a country in which plants of 

 that kind are known to be subject to a disease not as yet known 

 in the country in which the law is put into force. Thus the 

 importation of pepper plants into Ceylon is prohibited, so far 

 as South India is concerned, there being a very bad disease 

 widely spread among pepper plants in that country; so also 

 the importation of cacao from the Dutch East Indies is prohi- 

 bited, on account of the disease known as " krulloten-ziekte." 



A very common phase of the quarantine regulations is the 

 compulsory " fumigation " at the port of entry of the plants 

 or fruits or seeds. The skins of oranges or other fruits very 



