22 NEBRASKA STATE HORTICULTURAL SOCIETY, 



i 



bright winter's sunshine upon the trunk, to be as suddenly frozen 

 again at night, the result is usually fatal to the cells of the cambium 

 cells and those of the young wood and bark. 



III. MECHANICAL INJURIES. Aside from wounds by natural 

 causes, as the breaking of branches and leaves, injuries by hail, wind, 

 and lightning, many wounds are made by man himself, as in pruning, 

 pinching back, grafting, budding, etc. All these wounds, whether 

 made purposely or not, afford ready means for the access of bacteria 

 and fungi, resulting in more or less extended decay of the tissues. 

 Even when a wound is covered by the growth of living tissues over 

 it, decay usually continues, extending deeper and deeper until it be- 

 comes widespread. There can be no question that every wound is a 

 serious menace to the life of the tree. 



Under this head must be placed all the injuries by insects. These 

 affect all parts of the tree, the roots, trunk, branches, twigs, flowers, 

 and fruit, In many cases the injury is one of simple cutting, as in 

 the removal of parts of the leaves, the boring of the stems, etc., but 

 if the loss of leaves is excessive the tree may perish through starva- 

 tion, and the burrows in the stems may afford means for the entrance 

 of bacteria and fungi. Sucking insects by the withdrawal of water 

 and other food matters in so far decrease the nutrition of the plant. 

 Some insects, in addition, inject a poisonous or irritating fluid into the 

 wood, killing the tissues or setting up abnormal growths, resulting in 

 the production of galls and other malformations. 



IV. PARASITISM. One of the most fruitful sources of diseased 

 conditions in the apple tree is the presence of parasitic vegetable or- 

 ganisms in its tisues. More than one hundred fungi have been enu- 

 merated as growing upon the various parts of the tree, including the 

 flowers and fruit, a great majority of which are parasitic. The dis- 

 eases which they cause may be best discussed under the following 

 heads : 



DISEASES OF ROOTS. 



Root-rot This is a serious disease in which the roots undergo a 

 kind of dry-rot without any apparent cause. It is probably due to the 

 presence of a fungus, since in other trees with a similar rotting of the 

 roots the trouble has been traced to some of the larger fungi.* 



*See H. M. Ward's Timber and Some of Its Diseases, p. 142. 



