32 Disease and Death of Trees — Generalitie 
Nevertheless, since different parts are interdependent 
for their nutrition, any interference in feeding arrangements 
will be more or less felt, at least in all growing parts which 
organically belong together. It is, therefore, usually, one 
side of the crown, in sympathy with the same side of the root 
system, which is diseased or dead, while the other side may 
remain alive and even vigorous. 
This fact, together with the ability of trees to replace lost 
parts, makes it possible to resuscitate and restore quite 
dilapidated specimens by surgery and proper treatment, 
and to prolong their life almost indefinitely. 
General Causes of Disease. There are two general causes 
of disease, namely, unfavorable environment and parasitic 
organisms (which, however, should really be considered a 
part of the environment). Under the influence of these 
two we may group for diagnosis and treatment four classes 
of troubles to which trees are exposed, namely: physiolog- 
ical diseases which are produced by generally unfavorable 
conditions of growth in atmosphere, climate, or season, and 
soil; mechanical injuries, which destroy living parts or else 
weaken the dead wood of the body; attacks of fungi and 
other plants; and attacks of insects. 
Usually several of these causes work together or are even 
interdependent, and it is not always easy to tell which is the 
primary, which the secondary, cause of the diseased or abnor- 
mal condition. This difficulty of diagnosis is especially 
experienced when there are no viszble signs of insect or fungus 
pests present and when the trouble seems not localized but 
exhibits itself in general debility or sickly appearance. 
Fungus attacks are usually secondary results of mechan- 
ical injuries, and insect pests become in most cases fatal 
only when other debilitating causes are at work simulta- 
neously. 
