CARTER: ILLINOIS TREES: THEIR DISEASES 



15 



cyanide, which, with water in the soil, forms hydrocyanic acid. 

 Severe injury by gas usually shows as rapid wilting, browning, 

 and death of leaves (Fig. 17), followed by withering of branches 

 and death of the affected trees. Bluish-black discoloration of 

 small roots on some trees can be detected after the bark is re- 

 moved. On London plane trees, the inner bark may appear water 

 soaked and pink, and long, narrow cankers may form in the 

 bark at the bases of the trunks. A gas leak sufficient to cause 

 severe injury to trees will kill grass, weeds, and shrubs in the 

 vicinity of the leak; from such a leak, a strong odor of gas 

 usually can be detected in the air. Most gas companies have ma- 

 chines that can detect even small quantities of gas in the soil. 



Fig. 16. — Injury to trees from applications of excessive dosages of fer- 

 tilizer shows as curling, yellowing, browning, and stunting of leaves. Ir- 

 regular areas of tissues along the margins and between the veins turn 

 brown and die. The living tissues between the veins become yellowish 

 green, and the leaves are stunted, as shown by the two leaves on the right, 

 in contrast to the uninjured leaf on the left. 



