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 iillings 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 



