151 



lations have suffered more than an aesthetic loss. The removal of dead and dying 

 elms has cost up to an estimated $100 million per year nationwide (Mazzone and 

 Peacock 1985). The disease has also virtually eliminated American elm as a timber 

 species (Burkman et oL 1993). The five other species of native elms, e.g., red or slip- 

 pery elm (Ulmus rubra), are also vulnerable to infestation, but appear to have more 

 resistance. Since arriving in America, the disease has evolved more aggressive strains 

 which now are adding to the threat in Europe as well (USDA Forest Service 1991b). 



Fungal infection usually occurs in wounds made by insect vectors and may be lethal 

 in a small tree within a single year. Dual efforts have been made to control the dis- 

 ease, targeting the fungus and its primary vector, the introduced European elm bark 

 beede {Scolyhis multishiatus) . The removal of dead and dying elms and any dead wood 

 left on the ground is essential to an effective long-term conu-ol program. This ap- 

 proach is feasible only in urban plantings, due to the expense of removal of the dead 

 materials. Unfortunately, the disease also can be spread by root grafts between 

 healthy and infected trees, which was particularly devasuting to urban street plant- 

 ings of American elms. Efforts to prevent spread of the disease by root grafts are usu- 

 ally frustrated by transmission of the disease by beedes (Swank and Smith, personal 

 communication). Pcsucides and fungicides have been used only in urban settings and 

 have had variable success in preventing the disease or arresung existing infesution. 



i\al\kTai Tange oj Ammuin etm 



Hybridization between American elm and disease-resistant Asian elms has been pre- 

 vented by differences in chromosome numbers (Dcrmen and May 1966). Intraspecific 

 and interspecific hybridizations have been made among selections of resistant Asian 

 species and among American species (Townsend and Santamour 1993). Over 20 pure 



