EXAMINATION OF DISEASED TKEES. 17 



Sunscald is common on thin-barked trees, particu- 

 larly after sudden exposure to strong sunlight by thin- 

 ning, snowbreak, etc. 



Heavy snowfall often causes serious damage by bend- 

 ing and breaking young trees or branches, or by favor- 

 ing the development of certain fungi (Herpotrichia 

 nigra). 



EXAMINATION OF DISEASED TREES. 



The nature of a disease can be determined only by 

 keeping in mind all the possible factors which make 

 for disease (see "Causes of disease," p. 12), and the 

 way they affect the functions of the normal tree (see 

 "Disease a disturbance of normal functions," p. 11). 

 By taking into consideration all possibilities and by 

 discarding everything that does not suit the particular 

 case we finally arrive at a comparatively small group 

 from which by careful examination we will as a rule 

 be able to choose the correct answer to our problem. 

 It should be remembered, however, that a disease is 

 not always encountered under its most typical aspect. 

 A tree attacked by a fungus may show signs of suffer- 

 ing, but the fungus itself may not yet be sufficiently 

 developed to form fruiting bodies (p. 25). In other 

 cases the fruiting bodies may be very small, very in- 

 conspicuous, or hidden on the roots or under the bark. 

 The fruiting bodies of certain fungi even vary con- 

 siderably with host and climate (Pis. IV and V). Ex- 

 amination of diseased trees, therefore, can not be too 

 careful. 



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