ROOT DISEASES 



143 



and from this point of view it is desirable to follow its use 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 connection between the 

 infested roots and those of the surrounding healthy trees, and to 

 break the continuity of the possibly infested 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 diseased. Thus, 

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



H H H H H 



I. 



II. 



Neither the roots of the trees nor the course of the disease 

 follow regular lines, and it is quite improbable that all the eight 

 contacts are infected, but there is no practical means of knowing 

 which remain free. In order to save those which are still un- 

 infected, it is advisable to cut up the enclosed area chess-board 

 fashion as indicated by the dotted lines. If these secondary 

 trenches are omitted it is likely that the contacts will all go off 

 sooner or later in the manner already described. 



In situations where there is a risk of soil infections the trees 

 still fiurther out (H H in the diagrams), should have some 

 special attention, especially if D is an advanced case when dis- 

 covered. 



In situations liable to root disease there is much to be said 

 for the general adoption of the system represented in the second 

 diagram (II.), where a permanent drain (denoted by a double 

 line) exists between every row and the next. All that is required 

 for isolation is the cutting of cross trenches between C and H, to 



