150 



strains of the fungus (Diller and Clapper 1965, 1969; Thor 1976). Currently, a 

 promising development involves the genetic engineering of the virus that induces 

 hypovirulence (Choi and Nuss 1992). Regardless of the approach, the end result will 

 be a tree that is resistant, not immune, to the chestnut blight. Resistant trees may be 

 able to survive in present-day forest situations, but could be more susceptible to se 

 ondary attack from other pests. 



Natural range oj Amtnuxn chutnul in itu UniUd States 



A chestnut in North American forests will have to contend with several exotic pesL 

 aside from chestnut blighL There is good evidence that an exotic fungus, Phytoph- 

 thara annamomi had infested southern populations of American chestnut and Alle- 

 gheny chinkapin {Castanea pumiUa) as early as 1824 (Crandall ct oL 1945). The fungus 

 attacks the roots and causes mortality or decline. Infestations by the chesmut gall 

 wasp (Dryocosmus kunphilus) viere first reported in 1974 (Payne et oL 1975). Initially, 

 this insect infested Castanea species in Georgia and gradually spread to adjacent 

 sutes. Chestnut gall wasp larvae feed upon bud and flower tissue forming a charac- 

 teristic gall. Mortality can occur with severe infestations. 



Dutch Elm Disease 



Dutch elm disease is the most devastating shade tree disease in the U.S. (Kamosky 

 1979). The American elm was once the primary ornamental tree in eastern and mid- 

 western cities. The species was noted for fast growth and a vase-shaped form that 

 made it an ideal choice for shading houses and streets. From the eastern forests, 

 American elm provided lumber for boat building, cooperage, fiuniture, and agricul- 

 tural implements. After 1930, however, use of American elm in urban landscapes and 

 as a forest product dramatically decreased due to an exotic fungal disease. Dutch elm 

 disease was first recorded in Cleveland, Ohio in 1930, and rapidly spread throughout 

 eastern North America from three different infestation centers (May 1930; cf. Stipes 

 and Campana 1981). The disease, caused by an introduced fungus, Ophiostoma (- 

 Ceratocyslu) uimi, was brought into the country on different shipments of unpeeled 

 raw veneer logs from Europe (USDA Forest Service 1991b). By 1977, the disease hat* 

 spread to most of the contiguous 48 sutes. In the Northeast U.S. alone, 75 percent ot 

 the elms had died by 1979 (USDA Forest Service 1991b). Cities with large elm popu- 



