340 PLANTS AND MAN 



caused by the eating habits of insects, natural pruning of branches, 

 fire scars, injuries by grazing animals or careless logging opera- 

 tions. In the case of leaf diseases, entrance is usually by way of 

 the stomata of the leaves. Such diseases do their worst damage 

 on the evergreen coniferous trees, since these retain their leaves 

 for periods of three to seven years and are dependent upon them 

 during that time for photosynthesis. The persistent nature of 

 such leaves allows attacking organisms to work back through 

 them to the twigs and branches. In the case of deciduous trees, 

 leaf diseases are not of much consequence, since the leaves are 

 normally shed before invasion has progressed far enough to 

 interfere with food manufacture. The fungus rarely penetrates 

 beyond the leaf, so that even repeated attacks interfere but little 

 with the normal functions of deciduous trees. 



As examples of particularly interesting tree diseases are the 

 following diseases of seedlings, stems, and leaves. Seedling 

 DISEASES, which attack very young trees, are a constant threat in 

 the nursery growing of forest tree species. Outstanding in this 

 group is the damping-ofF disease, which is caused by several 

 species of fungi which live in the soil, ordinarily utilizing dead 

 plant remains as food. They are, however, also capable of becom- 

 ing parasitic upon very young seedling stems of all kinds of 

 forest trees, causing a break in the continuous water supply 

 between roots and leaves, and eventually resulting in death of 

 the plants. 



Stem diseases of forest trees are probably more numerous and 

 more harmful in natural forest growth than any other type of 

 disease. One of the outstanding diseases of this group is the chest- 

 nut bark disease, or chestnut blight, which was introduced from 

 Europe and Asia, and has practically exterminated this once 

 valuable and dominant eastern forest tree. Attack by this parasite 

 results in a killing of the cambium and inner bark, and ultimately 

 the tree itself. White pine blister rust causes the development of 

 a large spindle-shaped swelling or canker on the stem of its host 

 with the death of all parts beyond the canker, and soon the entire 

 tree, especially if it is a young specimen. It has caused widespread 

 damage to the eastern and western white pines, and now 

 threatens the California sugar pine, unless adequately controlled. 



