46 MANUAL OF TREE DISEASES 



ing it into healthy plants. Often, however, an active patho- 

 gene shows characteristic parasitic tendencies, especially to the 

 trained eye, which make diagnosis more certain. A comparison 

 of the edge of a canker on chestnut caused by Endothia 

 parasitica with that caused by winter-injury, lightning or 

 mechanical agents will illustrate the general characteristics 

 between an actively enlarging canker caused by parasitic 

 fungus and an area of dead bark invaded by a saprophyte. 

 From the outside such cankers may appear very similar, 

 but on cutting into the margin of the cankered area the one 

 caused by the active parasite shows a more or less gradual 

 gradation between the color and organization of the tissues of 

 the healthy light colored bark and the disorganized, usually 

 brown-colored diseased bark. Usually also, the fine mats of 

 mycelium can be seen advancing into iminjured and healthy 

 bark-tissue. On the other hand, in the case of an area of bark 

 killed by some other agent than an active parasite, the margin 

 of the canker when cut into is usually definite and the distinc- 

 tion in color and other characters between the healthy and dead 

 bark-tissues readily proves that the lesion is not becoming 

 larger, and that any saprophytic fungus present in the dead 

 bark is not advancing into healthy tissue. 



In this chapter are discussed several of the most common 

 diseases and injiu-ies of trees. It should be remembered that 

 the general health and appearance of the bark and wood of the 

 tree depend naturally on the health and condition of the foliage 

 on the one hand and the condition of the roots on the other. 

 The stunted and dwarfed condition of trees, slow annual growth, 

 deformed crowns, stag-head, dead branches, irregular branching 

 and sucker development are all signs of abnormal conditions 

 either at the source of starch production in the leaves or in the 

 food supply and general soil conditions around the roots. The 

 most common causes of such general symptoms of abnormal 

 growth shown by the branches or trunk are: (1) smoke-, 



