What is Dutch Elm Disease (DED)?
DED is a destructive wilt disease caused two closely related fungi, Ophiostoma ulmi and Ophiostoma novo-ulmi, which were introduced to the U.S. in the early 1930s. DED was first identified in Evanston in 1955. DED only affects American elm trees.
This fungal disease can be transmitted through two major vectors: proximity to another infected American elm tree (root graft, infected wood material, etc.) and elm bark beetles. The disease can be transmitted through root grafts that are prevalent in cramped urban and suburban parkways. The native elm bark beetle and the European elm bark beetle are responsible for transmitting DED.
The disease is most easily detected during early summer when the leaves on an upper branch curl and turn gray-green or yellow and finally brown. This condition is known as flagging. Brown streaks in the wood beneath the bark of affected branches are further evidence.
If a tree is less than five percent infected, it may be saved by pruning out the diseased branch promptly after seeing the first flag. If a tree shows more severe symptoms, it must be removed quickly so that the beetles and root grafts do not transmit the disease further. DED is an aggressive disease which can progress through the tree rapidly, killing trees within a few weeks to a few years.
Care and Maintenance
There are some things a homeowner can do to prevent the loss of American elm trees:
Preventative fungicide injections can be used to protect trees from infection by beetle feeding. Fungicide injections are not effective in preventing infection through root grafts, so it is important that all trees in an area be treated and root grafts severed before removal of an infected tree. Fungicides available for American elm inoculation include Propiconazole (Alamo™) and Thiabendazole (Arbotect™). Fungicide injections can only be done by a Certified Arborist and, depending on the chosen fungicide, must be repeated on a 1- to 3-year cycle. The City does treat selected American elms on the parkways and in City parks; these trees have been marked by a painted green dot on the lower trunk.
Good Tree Care
Dry soils cause the death of small roots and reduce a tree’s capacity to absorb water even after the soil is remoistened. The resulting drought stress increases a tree’s susceptibility to certain diseases and insects. Keep your trees and shrubs adequately watered.
Probe or dig in the soil to tell how much moisture is in the soil. A metal rod, such as the end of a root feeder may be convenient. Very dry soil will resist penetration of the rod and indicate the need for watering.
The top 12 to 18 inches of soil should be kept evenly moist around trees during periods of drought at least as far as the branches spread (drip line). Without adequate rainfall, established trees may have to be watered as often as once every 10-14 days.
Links to more information on Dutch elm Disease
https://www.na.fs.fed.us/spfo/pubs/pest_al/ded/elm.pdf (USDA’s Pest Alert)
http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/dutch-elm-disease/ (University of Minnesota Extension)