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Dutch Elm Disease
Elm with flagging
in canopy

Dutch elm disease (DED) is caused by a fungus, Ophiostoma novo-ulmi. The elm bark beetle is identified as the vector of the fungus. The beetles breed under the bark, transmitting the fungus into the tree. It can also spread via root grafting, but this is less common. When wilting occurs in an isolated part of the canopy, also called flagging, this is a sure sign the tree has been infected. After flagging is spotted the elm can rapidly decline, eventually leading to the demise of the tree in a very short time period (weeks to months). 

In questionable situations, a sample will be taken to confirm that the tree has DED. This is done by taking five twigs from the assumed infected area, which are then brought to the USDA testing facility. In a diseased tree, there will be a brown staining under the bark. If a tree is not diseased, the wood under the bark will be clear of staining. 

The City of Saint Paul uses a sanitation approach to prevent the spread of DED. It is important to locate diseased trees as soon as possible so that they can be removed before further spread occurs.  Controlling the spread of DED relies on controlling the infected beetle population, which in turn relies on controlling the number of dead and diseased trees.

City DED Program

Tree inspectors, who are trained and licensed by the state of Minnesota, perform a survey of Saint Paul every summer to locate and mark diseased, dead and dangerous trees. Dutch elm disease is the most common illness among Saint Paul’s trees, which has recently experienced a resurgence. After devastating our elm population in the late 1970s, DED continues to affect elms in Saint Paul.

  • A tree that is marked for removal is marked with red paint in a circle around the tree at breast height, about 4.5 feet up from the base. A number will also be painted, which indicates the diameter of the tree.
  • If the tree inspector feels the disease can be trimmed out and the tree temporarily saved, a "T" is painted on the base of the tree.
  • If an "S" is painted on the tree, a sample will be taken to verify that there is disease in the tree as well as trying to determine how far the disease has progressed

Removal of trees on public property (boulevards, parks, right-of-ways, etc.) that are marked will be performed by Saint Paul Forestry crews. The affected adjacent residence will receive a yellow letter with detailed information and contacts for any questions.

Private DED Trees
Private Property DED marked tree

City ordinance, Chapter 175 (Disease Tree Inspection), gives Forestry staff the right to actively pursue and identify dangerous/nuisance trees within Saint Paul on private property. 

When a tree is marked on private property, an orange slip is left at the residence, informing that a letter will be coming soon. Once the letter arrives, the owner will have one month to remove and properly dispose of the infected wood. The stump must also be debarked to ground level (elm firewood must also be debarked or disposed of). 

When such removals are done contractually, work must be completed 
by a company that is licensed by the City of Saint Paul to do tree work. 

The City of Saint Paul does not offer a subsidy to offset the cost of this 
work. If the work is not done in the time allotted, the City of Saint Paul 
will complete the work and assess the costs to the property owner.

Treatment of Elms
Example of fungicidal injections

Private companies do provide treatment services for Dutch elm disease.  This is done through injections of a fungicide at the base of the tree.  These treatments are primarily preventative or therapeutic in nature, and therefore only highly-valued and specimen trees are good candidates for treatment.  

An elm that is in the early stages of infection can be saved with treatment and trimming out of the infected area.  Generally the rule of 
thumb is less than 5 percent wilting in the canopy to be 
successful. Depending on the size of the tree, an injection 
can cost between $300 and $600. Injections must be done 
every three years. 

The City of Saint Paul does not inject public trees or assume any 
costs, but will work with private property owners to stop removals 
and issue a free permit if they wish to try a fungicide injection. 
The permitted company then assumes the cost of treatment, 
necessary pruning, and disposal of infected wood. 

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