General Causes of Disease 33 
Shade trees in city streets are especially subject to many 
adverse conditions, which make the attack of fungi and 
insects much more dangerous than it would be in the virgin 
forest. Injuries from gnawing horses, abrasions from 
passing vehicles and other agencies give entrance to fungi, 
reduce the feeding area and the vigor of the tree; pavements 
and macadamized roads reduce the water-supply and the 
supply of air to the roots; excavations for buildings, sewers, 
water-, gas-, and steam-pipes destroy parts of the root system; 
gradings do the same, and earth fillings change the water 
and air supply; gas leakage at the root and other obnoxious 
gases at the foliage interfere with the health and vigor; 
inattention to the wounds and broken limbs invite fungus 
attacks; and electric wires introduce the danger of electric 
discharges. All these untoward conditions of the environ- 
ment together with the mechanical injuries, if not fatal in 
themselves, tend to reduce the vigor of the tree and make 
it more susceptible to the attacks of parasites. 
The first concern of the tree doctor, therefore, is to secure 
a favorable environment, — sanitary surroundings for his 
charges, which will keep them in vigorous condition and 
enable them to fight off their enemies in the plant and animal 
world. ‘The conditions in the atmosphere are only very 
partially controllable; drouthy and wet seasons and the ills of 
frost and lightning have to be endured without much, if any, 
chance of counteracting their unfavorable influences. But 
the medium in which the roots perform their function, the 
soil, is much more readily influenced, and here, therefore, 
lies the main activity for securing favorable conditions. As 
we shall see more fully later, the regulation of the water- 
supply to the roots and a sufficiency of air or oxygen for 
their respiration, and, to a minor degree, a satisfactory 
amount of available mineral material are the means of 
