Insects and Diseases of Trees in the South

by Anonymous

DUTCH ELM DISEASE

Dying tree infected with Dutch elm disease.

Dutch elm disease, caused by Ceratocystis ulmi, is the most devastating disease of elm trees in the United States. This disease has been recorded in most states east of the Mississippi and as far as Idaho in the Northwest and Texas in the Southwest. All of the native elm species are susceptible, while many of the ornamental Asiatic species are highly resistant.

Trees suffering from Dutch elm disease may show a variety of symptoms. Leaves become yellow, wilt, and turn brown. Premature defoliation and death of branches usually occurs, causing the crown to appear thin and sparse. Internally, a brown discoloration appears in the outer sapwood. C. ulmi is transmitted from diseased to healthy elms by elm bark beetles, mainly the small European elm bark beetle and the native elm bark beetle. These beetles make characteristic galleries under the bark of dead and dying elms. Adult beetles pick up the sticky fungus spores from under the bark and then feed on the young tender elm twigs of healthy trees, inadvertently inoculating the healthy tree with the fungus. The fungus may also spread from diseased elms to adjacent healthy elms through root grafts.

Discoloration and streaking symptoms in the sapwood.

Controls to combat this disease generally involve sanitary measures aimed at the beetles. Dead and dying elms should be burned. This eliminates the elm wood which normally serves as a breeding place for elm bark beetles and thus reduces the beetle population. In areas where most native elms are infected, other tree species or resistant elm species, such as the Chinese or Siberian elm, should be planted rather them native elms.