THE BOTANY OF THE APPLE TREE. 23 



DISEASES OF THE TRUNK. 



Rotten Heart. This is probably closely related to the root-rot. 

 The heart wood becomes brittle and punky, and may be easily snap- 

 ped off with a little force. The threads of some of the larger fungi 

 are found ramifying through the woody tissues. It may be that 

 several fungi (pore-fungi, and toadstools) are concerned in this dis- 

 ease. Certain it is that a small ear-shaped toadstool (Schizophyllum 

 commune) always appears on the trunk about the time that the disease 

 is well under way. Another umbrella-shaped toadstool (Agaricus 

 melleus) is known to be concerned in the heart-rot of various timber 

 trees,* and it is thought by many botanists to be the chief cause of the 

 present trouble. 



It is probable that the fungi of Rotten Heart enter through wounds 

 from careless and injurious pruning. It is possible that they may 

 enter through the wounds made by grafting. These points demand 

 careful inquiry. 



Black Heart. The cause of this common disease is obscure. It 

 may be due to too low a winter temperature, and again it may be the 

 effect of the earliest invasion of fungal filaments. Possibly it is the 

 result of some of the other general causes discussed above. It de- 

 mands further study. 



Sun Scald. While not due to parasitism, this is entered here in 

 order to refer the reader to the discussion above under " Unfavorable 

 Atmospheric Conditions." 



Blight. Twig-blight sometimes extends down upon the trunk of 

 the tree. 



See the following discussion of blight on twigs and branches: 



DISEASES OF THE BRANCHES AND TWIGS. 



Blight. The following account of the blight of the apple tree is 

 supplied by Mr. A. F. Woods, until recently assistant in the botanical 

 department of the State University, now assistant pathologist in the 

 Department of Agriculture, Washington, D. C.: 



THE "TWIG BLIGHT" OR "FIRE BLIGHT" OF THE APPLE. 



This disease is best characterized by the dying of the twigs from 

 the end down. In severe cases the smaller limbs are often attacked, 



* H. M. Ward's Timber and Some of Its Diseases, p. 155. 



