Dutch elm disease (DED) and elm phloem necrosis are the two most serious diseases of elm in the United States (Figs. 1 and 2). Most native species of elm are susceptible to both diseases. Dutch elm disease is caused by a fungus, Ceratocystis ulmi (Buisman) C. Moreau, and is transmitted by two species of elm bark beetles-the smaller European elm bark beetle, Scolytus multistriatus (Marsham) and the native elm bark beetle, Hylurgopinus rufipes (Eichoff). Elm phloem necrosis is caused by a mycoplasma-like organism and is transmitted by the whitebanded elm leafhopper Scaphoideus luteolus (Van Duzee) (Fig. 3). Both diseases also can be transmitted through root grafts between adjacent trees.