1919.'] 



ROOT DISEASE OF CACAO IN TRINIDAD. 



195 



I 



organic matter. There is a farther probabihty that by neutraUzing soil 

 acidity lime produces conditions lerfs suitable to tii3 survival of 

 the fungus. 



If the view here taken of the function of lime is the correct one, the 

 main effect may be obtained by the use of slaked lime, which is the 

 only form that can be conveniently obtained in some islands. Quick- 

 lime, because of its additional sterilizing effect, is to be preferred when 

 equally available. 



Where sulphur has to be imported, its price will not euable it to 

 compete with lime, but there would appear to be possibilities in some 

 places of obtaining it cheaply from local sources. Sulphur has a more 

 definite fungicidal action than lime, and one which may be expected to 

 develop slowly and have considerable persistence. In this connexion 

 Fawcette (1915) reports an experiment in which a trench 4x4 mches 

 was filled with soil mixed with sulphur, using about 15 grams of sulphur 

 to each metre of trench. The fungus travelled through the soil to the 

 edge of this barrier, but did not pass to the plants on the other side. A. 

 part of the trench which received no sulphur was crossed by the fungus. 



The effect of sulphur on the soil reaction is to increase acidity, and 

 from this point of view it is desirable to accompany or follow its appli- 

 cation with an application of lime. 



Isolation of contacts.*— The diseased tree having been removed, it 

 is necessary to take steps to prevent the spread of the disease among 

 the trees around. In a regular field, with ordinary distances of planting, 

 the roots of the trees form a continuous system of contacts, interrupted 

 only by the deep drains. By the time a tree is so far diseased as to be 

 noticeable, the probabilities are great that the roots of one or more of 

 the neighbouring trees have also become infected. It is required to cut 

 the connexion between the infected roots and those of the surroundmg 

 healthy trees, and to break the continuity of the possibly infected 

 surface soil. To be reasonably sure of doing this, it is necessary to carry 

 an isolation trench outside the trees in contact with the one deceased. 

 Thus in an undrained field, if D in the first diagram (I) represents a 

 a diseased tree, C. the possible contacts, and H the healthy trees, the 

 isolation trench should take the course indicated by the unbroken line, 

 and will include nine trees. 



HHHHH HHH 



H 



H 



H 



H 



H 



H 



HHHHH 



I. 



II. 



* This section, with minor alterations, is reprinted from thu author's pamiiiilet 

 on diseases of lime trees. (No. 70, Imperial Department of Agricult<iie. ) 



k 



