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The City of Saint Paul remains committed in its efforts to manage tree losses to pests and diseases such as emerald ash borer,  bur oak blight, Dutch elm disease, oak wilt, and Nectria canker. Learn about the identification, treatment, and management of these diseases below.

Bur Oak Blight

Bur oak blight (BOB) is a native plant disease caused by the fungus Tubakia iowensis. First identified in Iowa, BOB is a leaf disease that is beginning to affect more bur oaks in Saint Paul. More frequent precipitation over the last few years has increased the severity of the fungus which causes purple-brown lesions on the leaves.

Symptoms of this disease include late season leaf death starting in the bottom of the crown and moving upwards. If a tree is seriously infected one year, it tends to be severely infected the next year. Bur oaks infected with BOB generally decline and may die after several years as secondary pests such as two-lined chestnut borer invade the stressed trees. The combination of BOB and other stressors will eventually overwhelm the tree and cause mortality.

Compared to oak wilt in red oak trees, BOB does not spread rapidly. Healthy trees can be found adjacent to BOB-infected trees. Fungicides injections to control the fungus have shown to be effective when applied in low doses in the spring.

At this time, Forestry is not performing fungicide treatments on bur oaks infected with BOB. Forestry staff will continue to monitor the disease and assess trees with BOB.

Additional Information:

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Dutch Elm Disease

Dutch elm disease (DED) has killed hundreds of thousands of elms in Saint Paul since the late 1960s. Most of these elms were large trees lining the boulevards that created a tunnel-like effect down city streets. Management of the disease has allowed some of these iconic elm trees to remain scattered throughout the city. Continued management of this disease is necessary to mitigate the effects of this disease. 

Public Property Management

Forestry uses a sanitation approach, which is the prompt removal of infested elm trees, to prevent the spread of DED. Forestry locates diseased elm trees as soon as possible so that they can be removed before further spread occurs. Tree inspectors licensed by the Minnesota Department of Natural Resources perform a tree survey of Saint Paul every summer to locate and mark diseased elm trees for removal. Inspectors use identification characteristics of DED to determine whether or not an elm tree is diseased. In questionable situations, a sample may be taken and brought to the University of Minnesota's Plant Health Disease Clinic to determine whether or not the tree is diseased.

Private Property Management

City ordinance gives Forestry staff the right to actively pursue, identify, and require the removal of diseased elm trees and wood piles within Saint Paul on private property. Tree inspectors licensed by the Minnesota Department of Natural Resources locate and mark diseased elm trees and wood piles on private property for removal.

  • Forestry will mark infested elm trees or wood piles with a painted red ring.
  • An orange slip is left at the residence that informs the property owner of the diseased elm tree or wood pile and states that an official letter with removal specifications will be arriving in the mail.
  • Once the letter is sent, the property owner will have 20 business days to remove the diseased elm tree or elm wood and properly dispose of the diseased wood including debarking the stump to ground level.
  • If the work is not done in the time allotted, Forestry will have a hired contractor remove the tree and assess related costs to the property owner.

Fungicidal Treatment of Elms

Forestry does not use fungicidal treatments to help prevent DED in public elm trees. However, property owners may work with a licensed tree care company and apply for a free permit if they wish to treat boulevard elm trees.

Forestry also encourages property owners to treat their highly valued private property elms. A permit is not required when treating privately owned elm trees. However, Forestry recommends that property owners use a company with an ISA Certified Arborist® on staff.

Additional Information

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Emerald Ash Borer

Emerald Ash Borer (EAB) is a destructive incest that attacks and kills all species of ash trees (Fraxinus genus) native to North America. EAD is native to Asia and has no natural enemies in North America. The adults as small iridescent green beetles that live outside of trees during the summer months. The larvae are grub or worm-like and live underneath the bark of ash trees. Trees are kill by the tunneling of the larvae under the tree's bark. 

Learn more about Emerald Ash Borer on our EAB webpage. 

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Nectria Canker

Nectria canker is a fungal infection which can cause tree mortality, especially in honey locusts. Forestry manages this disease by pruning honey locusts during winter, as the disease is dormant during this time and the chance of infection is greatly reduced. Pruning honey locust during winter has proven to be an effective management strategy for this disease.

Additional Information

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Oak Wilt

Oak wilt is a fungal infection (Ceratocystis fagacearum), which causes oak mortality primarily in the red oak family. Oak wilt is spread by many species of sap beetles, but is most often transmitted via root grafting.

Public Property Management

Forestry only prunes oak trees in the winter in order to help prevent the spread of this disease.

Private Property Management

Tree inspectors identify and mark diseased oaks during the late summer or early fall. On private property, property owners have until the end of the calendar year to remove diseased trees.

Management of oak wilt is primarily focused in the Highwood/Battle Creek area where there are higher concentrations of red oaks and the disease is more prevalent. Forestry staff are available to advise property owners how to best manage and contain oak wilt. Forestry recommends that private property owners not prune oaks between April and October in order to help prevent infection from this fungus.

Additional Information

How to Tell the Difference between Oak Wilt and Twolined Chestnut Borer

Minnesota DNR Oak Wilt Guide

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