406 GENETICS IN RELATION TO AGRICULTURE 
parasitica. The parasite is a native of eastern Asia where it is parasitic 
upon native species of chestnut, to which it appears to do relatively 
little harm. In other words these species are highly resistant to the 
parasite. However, when the fungus was introduced into America, pre- 
sumably in nursery stock some 25 years ago, it found in our native species, 
Castanea americana, a very susceptible host (Fig. 165). The parasite has 
already caused the destruction of the American species throughout the 
northern Appalachian region and is strongly threatening its complete 
extinctionasatimbertree. Investigations have determined that the Euro- 
Fie. 165.—An advanced stage of the chestnut bark disease, caused by Endothia 
parasitica, a virulent pathogenic fungus from China. (From the Journal of Heredity.) 
pean chestnut is also susceptible to the attacks of this fungus, so that the 
future existence of this species is also jeopardized. The American chest- 
nut is one of our most valuable forest trees and its destruction will entail 
an enormous loss. A very promising Chinese species is known pro- 
visionally as C. mollissima. While it is scarcely a timber tree as compared 
with our native species, yet it may thrive in our climate. As the nuts 
are of good quality and the tree has shown marked resistance to experi- 
mental inoculations on plants already established in this country it is 
hoped that it will prove to be a successful substitute for the vanishing 
American species. Even the culture of the American’ species for com- 
mercial nut production in western North America will be constantly 
threatened, Hence it is fortunate that breeding experiments with the 
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