Disease & Pest Management

Bur Oak Blight

Bur oak blight (BOB) is a native plant disease caused by the fungus Tubakia iowensis. First identified in Iowa in the mid-200's, 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. Fungicide 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:

• Bur oak blight 

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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. 

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. In certain cases, 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.

Additional Information

• Dutch Elm Disease Identification, Biology, & Management

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Emerald Ash Borer

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

Additional Information

• Emerald Ash Borer

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Nectria Canker

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

Additional Information

• Nectria Canker

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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.

Forestry only prunes oak trees in the winter in order to help prevent the spread of this disease. 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 recommends that private property owners not prune oaks between April and October in order to help prevent infection from this fungus.

Additional Information

• Minnesota DNR Oak Wilt Guide

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