Elm phloem necrosis is a disease of elm, caused by a virus or virus-like
organism. The disease has occurred in the United States for many years,
probably as early as 1882. The pathogen is transmitted from infected to
healthy trees by the adult white-banded elm leafhopper, _Scaphoideus
luteolus_, which feeds on the leaf veins. It is now present throughout
most of the central, eastern, and southern portions of the United
States. The disease is known to occur on American and winged elm, but
all native elms are probably susceptible to attack by the pathogen.
[Illustration: Foliage symptoms of elm phloem necrosis.]
The earliest symptoms of the disease appear in the top of the crown, at
the outer tips of the branches. Here the elm leaves suddenly wilt, turn
yellow, the margins curl upward and the leaves die. Leaf-fall causes the
crown to appear sparse. In large trees, the foliage symptoms may
initially appear on one branch or only a portion of the crown. However,
the symptoms during the advanced stage of the disease are exhibited
throughout the crown. The most reliable symptom appears as a yellow to
butterscotch discoloration on the inner bark surface or phloem. This
symptom initially appears under the bark of large roots, later spreading
to the base of the main stem and finally to the larger branches. Phloem
and cambial discoloration is often found in advance of the foliage
symptoms. The moderately discolored phlomen has a slight odor of
wintergreen. Thus far, all trees known to be infected with the pathogen
have died. Acutely infected trees, while initially appearing healthy,
may wilt and die in three to four weeks.
[Illustration: Stem sample showing discoloration of sapwood.]
No effective controls are known. The “Christine Buisman” elm, which is
highly resistant to Dutch elm disease, has demonstrated resistance to
elm phloem necrosis.