Monday, January 31, 2011

Threats to American Elms

During the past month, we have discussed introducing cultivars of the American Elm to resist Dutch elm disease. It has been a devastating threat to elms in DC and around the country, but there are other pests and diseases that can harm elms as well. Many American Elm cultivars are selected and propagated for their DED resistance, but not yet for their resistance to other threats. Here is a comparison of some of the major threats to the American Elm:

Dutch Elm Disease
  • Fungal infection - Ophiostoma novo-ulmi and O. ulmi
  • First described in 1921 in Holland and spread to the United States by 1930. Reached Washington, DC mid-20th century.
  • Transmitted primarily by the native and European species of the elm bark beetle that feed on elms. Can also spread from tree to tree when roots cross and become grafted together.
  • American Elm street trees can have rates of death higher than 5% annually. DC lost 70% of its street elms between 1959 to 2002 when Casey Trees did its first city-wide tree inventory.
A withered DED-infected branch alongside healthy branches.
Leaves on affected individual branches shrivel and brown. Symptoms commonly are observed in early summer, but  can be seen at other times of the year. Other elms are susceptible to DED, but the American Elm is especially vulnerable, unless it is a cultivar that has significant resistance (but no immunity). The disease is spread by beetles that carry fungal spores from the diseased elms to healthy elms. The beetles movement and the threat of spreading from a DED-infected tree to neighboring trees means that an entire street of elms can be at risk if there is even one diseased tree in the area. 

The virulence of DED necessitates an intensive management strategy that requires infected trees to be identified and removed as quickly as possible. Insecticide or fungicide methods are also used, but may be expensive. Planting resistant cultivars is a good strategy, but even those trees may be susceptible to elm yellows. Often only one branch is initially infected. In such cases, an arborist can quickly undertake a sanitation pruning to remove a diseased limb. Without intensive management, DED is fatal for an American Elm. 

Phloem Necrosis (Elm Yellows)
  • Viral infection - Morsus ulmi
  • Trees usually die within a year of symptoms.
  • Transmitted by the whitebanded elm leafhopper, and also through root grafts.
Healthy elm (left), elm yellows (right).
Photo credit: Wayne A. Sinclair, Cornell University,
The entire crown of leaves turns yellow (not brown and shriveled as with DED) and drop prematurely, often at the end of summer, far ahead of the normal fall cycle. One of the distinguishing characteristics of elm yellows is a wintergreen odor emanating from the inner bark. 

Bacterial Leaf Scorch
  • Bacterial infection - Xylella fastidiosa
  • The disease clogs the xylem, impeding water transportation through the tree which prevents water from reaching the leaves.
  • Causes annual summer/fall symptoms of browning (scorched) edges of leaves, creating a "halo" effect.
  • Can be transmitted by a xylem-feeding leafhopper.
  • Trees decline over many years rather than dying immediately.
Verticillium Wilt
  • Fungal infection - Verticillium albo-atrum
  • Carried through the roots of elms, transmitted through the soil.
  • Dieback symptoms are similar to DED - leaves begin to wilt on individual infected branches.
Verticillium wilt can affect the growth of twigs and branches, and may result in discolored leaves. Branches may die over the winter. An infection can be managed by pruning wilted branches.

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