20 DISEASES OF TREES 



If investigation shows that neither animal nor vegetable 

 organisms are the primary cause of the disease, then the latter 

 must be due to some influence in the inorganic environment. If 

 it is suspected that unfavourable properties of the soil are 

 responsible for the disease, the unhealthy tree should be removed, 

 and if possible, at the spot where it stood a hole should be dug 

 to the depth reached by the lowest roots. During the opera- 

 tion attention is to be directed to the consistency of the layers 

 of soil, and to the quantity of water which they contain, and 

 especially to their greater or less accessibility to atmospheric 

 air. In the forest a change in the amount of mineral food, so 

 great as to induce disease in a previously healthy tree or planta- 

 tion, would occur only under circumstances that would at once 

 attract the attention of the skilled observer. For instance, top- 

 drought may be a consequence of the removal of litter or the 

 laying bare of the soil ; sickness or death may be occasioned 

 by the presence of injurious substances derived from factories, 

 or owing to flooding with sea-water, &c. Chemical investigation 

 will very seldom be necessary. More frequently the cause is 

 attributable to atmospheric influences, such as variations of 

 temperature, moisture, or precipitations, or to lightning, noxious 

 gases, &c. If it can be determined when the disease first 

 appeared, the task will often be more quickly mastered by col- 

 lecting information, and by ascertaining the external conditions, 

 than by investigating the diseased plant, although the latter 

 course will, in many cases, lead to the desired end. 



As a rule, diseases produced by animals and plants are 

 characterised by their occurring first of all on single plants or 

 parts of plants, and then gradually extending from these 

 centres ; whereas diseases which are due to the influence of soil 

 or atmosphere generally appear regularly and simultaneously 

 over large areas, because such influences are seldom bounded 

 in the forest by narrow limits or confined to the neighbourhood 

 of single plants. 



Mistakes are most likely to occur when a disease is preceded 

 by an abnormal predisposition, because this alone, and not the 

 disease rendered possible by it, is apt to be kept in view. It 

 frequently happens that we meet with different diseases on the 

 same tree, each of which is at work independently, and when 



