Dutch Elm Disease

Dutch Elm Disease


Graceful, elegant American elm trees – photo © Mike Rollinger on Flickr noncommercial use permitted with attribution / no derivative works

The American Elm (Ulmus americana) is found thoughout eastern North America in mixed hard wood forests and was once planted widely in urban areas. It was the beautiful, iconic street tree in many Amiercan towns until most trees were killed by Dutch elm disease. The seeds and buds of native elms are a food souce for some birds, squirrels and chipmunks  and  yellow bellied sapsuckers drill holes for sap.   Native elms support 213 species of butterfly and moth larvae as well as other insects.   Elms are preferred nesting trees for Batimore orioles, the warbling vireo and the red shouldered hawk.

Dutch elm disease is present in the Richmond area.

Photo credit header: Joseph O’Brien, USDA Forest Service, Bugwood.org

Dutch elm disease (DED) is caused by a fungus that is thought to have originated in Asia.  It was spread to Europe, then from Holland to the United States, probably in the 1920s by transportation of wood products. A more virulent form of the fungus developed in the 1960s and killed many elms across the eastern and mid-western United States. According to Wikepedia, we shared this deadly fungus with Britain where it “proved both highly contagious and lethal to all of the European native elms; more than 25 million trees died in the UK alone.”

A few American elms survive either because they have some resistance to DED or because they are isolated from other elms.  The Siberian elm (Ulmus pumila), a species resistant to DED, was planted across the country during the first half of the 20th century to replace elms along highways urban areas. The Siberian elm has naturalized and is now considered a noxious weed or invasive species in 41 states. The Siberian elm is listed as occasionally invasive by the Virginia Department of Conservation and Recreation.

All native elms are susceptible to DED.  The non-native elms (Siberian elm and Chinese or lacebark elm) have variable resistance to the disease.

For identification of the American elm view a fact sheet 

Identification, spread and control

A characteristic symptom of Dutch elm disease is “flagging”.  This is wilting and yellowing of the leaves on one or more branches, usually in the upper portion of the tree.  Flagging is usually first seen in late spring / early summer.   (See the picture in the header of this page.)  Flagging and branch death can spread aggressively resulting in the death of the tree.   Also typical of Dutch elm disease is streaking in the sapwood.   The only way to identify DED with certainty is to culture and identify the fungus.

Consult an arborist promptly if you suspect Dutch elm disease.

The spread of DED is connected directly with the life cycle of the elm bark beetles.  The beetles tunnel under the bark to lay their eggs.  When the eggs hatch, the larvae feed on the tree bark and sapwood.  Mature adults then emerge to feed on other elms.  The fungus is spread from tree to tree when the adult beetles move to other trees.  The beetles, and therefore the disease, can be rapidly spread when people move wood (for example, firewood) from one location to another.    The fungus can also spread to adjacent trees through grafted roots.

Insecticide spray for the elm bark beetle is generally not considered an appropriate control measure in part because of the risks involved in spraying. This was done in the 1940s, 50s, and 60s using DDT until the chemical was banned.

Sanitation can be useful if the community is vigilant about removal of infected wood.  This removes the source of the infection and reduces the breeding sites for the beetles. Pruning may be an option for some newly infected trees.   Wood from tree removal or pruning should not be transported; it must be destroyed or appropriately treated since the beetle larvae can complete their life cycle in the dead wood.  Wood is usually destroyed by chipping.  Infected elm wood should not be kept as firewood unless is appropriately handled.

Grafted roots should be severed before an infected elm is removed.  When trees grow in close proximity, roots can cross and fuse together; the fungus can spread from tree to tree through the root.  Roots between the tree to be removed and healthy tree next to it should be severed using a trenching tool or other device.  It may be advisable to sever the roots between the healthy tree and the next healthy tree as well.

Pruning can save some trees if the infection is caught in the first year and if infection is limited to a small portion of the crown. The infected branch must be pruned at a branch fork that is at least five to ten feet below the discoloration of the sapwood which indicates the presence of the fungus.  Therefore, any tree with infection in any of its’ major branches is not a candidate for pruning.  Fungicide treatments in conjunction with pruning will improve survival.

Fungicide injections are effective in the treatment of DED but the tree must be treated every three years.  Due to the expense of repeated treatments, this is usually limited to trees of historic value.

Replanting DED resistant cultivars is an effective way to limit the spread of DED.  Most resistant cultivars can be identified by their patriotic names such as Liberty, Independence, and Valley Forge.

Visit this site for more information including details about the lifecycle of the elm bark beetle and management of DED.

http://www.na.fs.fed.us/spfo/pubs/howtos/ht_ded/ht_ded.htm

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