[Illustration: 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.
[Illustration: 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.