DIEBACK AND GUM DISEASE 



583 



DISEASES NOT TRACEABLE TO 

 PARASITIC GROWTHS 



There are a number of prominent troubles which are not trace- 

 able to parasitic invasion of any kind, and yet may be in some cases 

 promoted by bacterial growth invited by preceding conditions. 



Sour Sap. There is a fermentation of the sap, quite noticeable 

 by its odor, which may be found in all parts of the tree, from the 

 root to the topmost twigs ; sometimes in one part and not in another. 

 Sour sap in the root is generally due to standing water in the soil, 

 and the remedy is drainage. Trees thus affected make an effort to 

 grow and then the young growth shrivels. Severe cutting back of 

 the top to reduce evaporation until the roots can restore their feed- 

 ing fibers is the only treatment of the tree, and its success depends 

 upon the extent of the root injury. Sour sap may also be caused in 

 the branches by the occurrence of frost after the sap flow has 

 actually started. Cutting back the diseased parts, as soon as dis- 

 covered, to sound wood, is the proper treatment. 



Die-Back. Dying back of twigs or branches may occur without 

 parasitic invasion through root-weakness or partial failure. It may 



Tuberculosis of the olive. 



