CARTER: ILLINOIS TREES: THEIR DISEASES 17 



water out through the aeration holes and away from the root 

 area of the tree. After the soil has returned to a normal air-water 

 relationship, the tree should be fed. It may need to be fed an- 

 nually for several years. 



Injury to trees from sulfur dioxide occurs in areas where 

 industrial plants, in utilizing sulfurous materials, liberate sulfur 

 dioxide into the air. Injury is most apt to occur in clear weather 

 when the humidity is 80 per cent or above, when the air move- 

 ment is less than 5 miles per hour, and when the concentration 

 of sulfur dioxide in the air is 3 parts per million or greater. 

 Symptoms of injury decrease with increase in distance from the 

 source of trouble. 



Evergreens are more susceptible to this type of injury than 

 deciduous trees, old or mature leaves are more easily injured 

 than young or immature leaves, and weakened trees more read- 

 ily burned than healthy, vigorous trees. 



Sulfur dioxide produces two types or degrees of injury: 

 acute and chronic. Acute injury may look worse than it is since 

 the leaves of an affected tree show sharply defined discolored or 

 bleached areas along the margins or between the veins. Acute 

 injury caused by a single burn may retard growth, but usually 

 an affected tree recovers from it. Repeated burning causes pre- 

 mature defoliation, which may be followed by reduction in the 

 amounts of annual growth or by sudden death of the tree. Chronic 

 injury develops more slowly and is less severe; leaves may be 

 discolored but usually they remain on the tree, which is retarded 

 in growth. Sometimes chronic injury causes slight reduction in 

 growth but does not cause visible injury to the leaves. 



Stem Diseases 



Stem diseases, in branches or trunks, may develop as canker, 

 dieback, gall, witches'-broom, or woodrot. 



Canker. — Canker diseases produce localized dead areas in the 

 bark of twigs, branches, and trunks of affected trees. These 

 areas are oval to elongate in shape and vary from 1 or 2 inches 

 to 1 or more feet in length. Cankers may enlarge until they 

 girdle an affected stem ; that part of the stem beyond the girdle 

 dies. Cankers may develop as conspicuous sunken areas, as dis- 

 colored areas of bark that are not depressed, or as diseased areas 

 so inconspicuous that they cannot be detected by examination of 

 the surface of the bark. The diseased bark of conspicuous sunken 

 areas becomes fissured and cracks away from the surrounding 



