DISEASES CAUSED BY WOUND FUNGI. 33 



The chances for the healing of branch wounds are generally very 

 much better for a tree with thick sapwood than for one with thin 

 sapwood. From an examination of trees affected with the white 

 heart-rot the following average ages have been determined as indi- 

 cating the periods beyond which infection is liable to result: Butter- 

 nut, about 15 to 20 years: black walnut, about 20 years; aspen, 20 

 to 25 years: yellow birch, 20 to 25 years: beech, 25 to 35 years; 

 silver maple, about 35 years. 



The rate of growth of the host tree affects chances for infection 

 only so far as it influences the rate with which such trees are able to 

 heal wounds. Vigorously growing trees of a certain species will as a 

 rule show less tendency toward infection than slow-growing trees of 

 the same species. Anything which tends toward the healing of the 

 wounds formed in a tree as it grows older will reduce its chances of 

 becoming diseased. 



The formation on trees of wounds other than those caused by the 

 natural dying and breaking away of branches on the lower part of 

 the trunk has a material influence on the chances for infection. 

 Short-lived trees, like the aspen, in which with increasing age there 

 is a rapidly increasing tendency for all branches to be broken off by 

 windstorms, show a greater tendency toward diseased conditions as 

 the trees reach maturity and afterwards than is the case with trees 

 such as the beech and oak, which are long lived, where the tendency 

 toward the breaking of the branches, due to ice storms and wind- 

 storms, is smaller. 



Summing up the factors which control the entrance of the false- 

 tinder fungus, one finds that the chief factors, given in the order of 

 their importance, are: First, presence of wounds on the tree, this 

 Involving the natural rate of healing of lateral branches, especially 

 on the lower parts of the trunk; second, age of the tree; and third, 

 greater or less tendency on the part of trees to maintain a close crown, 

 reducing the chances for the breaking off of large branches by wind- 

 storms and ice storms. 



ULTIMATE FATE OF DISEASED TREES. 



A tree affected with the white heart-rot may continue to live for 

 many years, even if badly diseased. This is particularly true of long- 

 lived trees, like the white oak and the beech, and especially if the 

 amount of wood destroyed in the trunk is so small as to reduce the 

 strength of the trunk but slightly. YThere the disease encroaches 

 upon the sapwood, as in the case of the aspen, trees may be killed 

 by the disease; and this actually does take place in many instances. 

 The chief destructive results, however, should be considered from the 

 point of view of the wood destroyed by this fungus. 



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