Forestry
DUTCH ELM FAQ
1. What is Dutch Elm Disease (DED)?
DED is caused by a fungus called Ophiostoma ulmi, formerly Ceratocystis ulmi, that was introduced to the U.S. in the early 1930s.
The native elm bark beetle and the European elm bark beetle are responsible for transmitting DED. The disease can also be transmitted through root grafts that are prevalent in cramped urban and suburban parkways.
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 is 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 many flags or completely wilts and dies, it must be removed quickly so that the beetles and root grafts do not transmit the disease further.
2. Care and Maintenance
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 8-12 inches of soil should be kept evenly moist around trees during periods of drought at least as far as the branches spread (dripline). Without adequate rainfall, established trees may have to be watered as often as 10-14 days.
3. Parkway Trees
Each year, the Parks/Forestry Division plants approximately 700 new trees on parkways citywide. Although this area is City-owned, residents are required to maintain the turf and/or plantings. The City's parkway tree population currently stands at around 28,500 trees.
The City of Evanston’s Forestry Division maintains parkway tree diversity by making sure no one tree species makes up more than 10 percent of the overall population to help avoid a single pest from destroying a significant portion of the tree population.
A prime example of this is DED, where a single fungal disease has eliminated a large percentage of the tree population due to the overplanting of American Elms.
PARKS/FORESTRY & REC 
Forestry
Dutch Elm FAQ