32 



DISEASES OF DECIDUOUS FOREST TREES. 



of the disease. As stated before, the aspen, the beech, and the but- 

 ternut, in the order given, seem to be the most frequently diseased of 

 all the hosts upon which the fungus has been found. 



The age of the host tree is probably the most important factor 

 connected with the distribution of the disease. It has been indi- 

 cated that the principal mode of entrance of the fungus is through 

 dead branches. Trees which are not old enough to have many dead 

 branches or which are not old enough to shed their lower branches 



are free from the 

 disease. When 

 a branch dies and 

 is broken off its 

 wood is left ex- 

 posed to the at- 

 tacks of the fungus. 

 Where trees are 

 very much sup- 

 pressed the fungus 

 may be found on 

 trees only 1 or 2 

 inches in diameter ; 

 but it will be found 

 on examination 

 that such sup- 

 pressed trees are in 

 reality of a consid- 

 erable age. In gen- 

 eral it may be said 

 that none of the 

 common deciduous 

 forest trees are af- 

 fected with the 

 white heart-rot be- 

 fore they are 20 

 years of age, and 

 most of them will 

 not become affected until they are considerably more than 20 years 

 old. The aspen and the butternut become infected after they are 

 20 to 25 years of age. This is especially true of the aspen, on which 

 dead branches remain on the lower part of the tree trunk for years 

 after they are dead (fig. 4). A clean-stemmed forest of beech stand- 

 ards will show far less susceptibility when 50 to 60 years old than 

 a similar stand in which the dead branches stick out in large num- 

 bers from the lower portions of the trunk. 



149 



Fig 



4.— An aspen tree with many dead lateral branches; each of these 

 offers a good entrance for wound fungi. 



