DISEASES OF THE CHESTNtJT AND OTHER TREES 79 



The Parasitism of the Fungus, 



Diaporthe parasitica is a virulent parasite. I have not yet, in 

 making over six hundred inoculations, found any strain of the 

 fungus that is not an active parasite. At the same time it is not 

 a highly organized parasite, i. e. adapted exclusively to a parasitic 

 mode of life, like the rusts for example: but grows readily as a 

 saprophyte upon a considerable variety of culture media, and in 

 nature upon the dead bark and wood of the trees that it has killed. 

 By virtue of its moisture-loving character, it often occurs on stumps 

 and at the base of trees, especially on the north side, and on roots 

 which are partly out of the ground. This capacity for sapro- 

 phytic life is shared by many of our most destructive parasitic 

 fungi : a character which, as it permits growth on artificial culture 

 media, has been of great service to plant pathologists. 



The Relation of the Disease to the Condition of the Tree. 



I have not been able to secure any definite evidence, experimental 

 or otherwise, to show that a tree with vitality reduced from any 

 cause is more susceptible to infection, or that the disease spreads 

 more rapidly in such a tree, than in a perfectly healthy and well- 

 nourished tree of either seedling or coppice growth, provided that 

 such reduced vitality does not result in or is not accompanied by 

 bark injuries through which spores can gain entrance. The 

 dominant fact in the spread of the disease appears to be the parasi- 

 tism of the fungus. Once inoculated into any variety of American 

 or European chestnut, it soon girdles it, regardless of its condition, 

 age, or previous history. 



Winter injury may have caused the death of chestnut trees 

 locally, especially of young sprout growth, but over the whole 

 range of the chestnut, or over the present range of the bark disease, 

 it is insignificant, so far as I can ascertain, as a cause of either 

 injury or death. In so far as cracks or dead areas of bark or other 

 wounds are produced through which the parasite can gain entrance, 

 winter injury may predispose to the bark disease; but not other- 



