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 

 extinction as a timber tree. Investigations have determined that the Euro- 



Fio. 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 



