144 DISEASES OF CROP-PLANTS 



separate contacts from healthy trees, and secondary cross trenches 

 between D and C, to save the contacts if they do not prove to 

 have become infected. The length of isolation trench required 

 is reduced to one-sixth. Where close planting is adopted or the 

 whale-back beds which this system produces are objected to, the 

 trenches may be made to include two rows instead of one, the 

 protection obtained being then less complete. Like the preven- 

 tive measures already indicated, this system should be applied 

 first of all in the situations where the losses are greatest, and 

 extended as opportunity permits. 



It is not necessary, from the point of view of root disease, 

 that the trenches should conduct water : indeed, where loss of 

 surface soil by wash is feared more than the alternate danger 

 of water-logging, it may be advisable deliberately to check 

 their function in this respect. They may then be periodically 

 cleaned out, and the deposit returned to the soil. It gives a 

 clearer point of view if the system is regarded as one of per- 

 manent isolation trenches with a secondary function as drains, 

 rather than as one of drains in the first instance. 



Returning to consideration of the treatment of infested 

 spots, there are two commonly existing situations which need 

 to be dealt with. These are (i) the case where a large patch of 

 trees has already been eaten out of a field, and the disease is 

 spreading outward around its circumference, and (2) the case 

 where one or more trees have been attacked around a large forest 

 stump, infested or likely to become so, which together with its 

 heavy roots prevents the cutting of trenches over an area 

 which may include quite a large number of trees. In both cases 

 the procedure has to be modified to suit each individual set 

 of circumstances but still follows the simple principles set oat 

 above. Where roughly parallel main drains or water courses 

 exist on each side of the area, they should be joined across above 

 and below to establish an outward limit, even though it may be 

 a wide one, to the spread of the infection. Then working inward 

 from this, successive trees or rows of trees which appear healthy 

 may be separated off wherever it is possible to dig a trench, and 

 the disease thus confined to the narrowest limits. 



In the situations sometimes met with, especially in cacao 

 plantations, where the trees are growing amongst a confusion 

 of fallen rocks, the possible measures are limited to early removal 

 of so much of an infested tree as can be got at, and the liming 

 of the soil about adjacent trees, with a view to preventing spore 

 infections of the material which accumulates in the enclosed 

 pockets of soil. 



It has been commonly recommended that a trench be carried 

 in a circle around diseased trees and their contacts. This 

 method has practical disadvantages. The extent of the existing 

 infection can never be ascertained by inspection, and a wide 

 circle, while enclosing many healthy trees, may prove too narrow 



