Overview

Several well-known diseases and pathogens continue to threaten the trees in our community. Look for signs of the disease in your oaks and elms.

For invasive pathogens such as Emerald Ash Borer (EAB), please click here.

Dutch Elm Disease

Dutch elm disease is a widespread problem throughout the city each year. Tell-tale signs of the disease include "flagging," where an entire branch of leaves will turn yellow and brown, creating a "warning flag" in the tree canopy.

Dutch elm disease can kill an elm very quickly during the summer and poses a threat to neighboring elms as well. According to city ordinance, the property owner must remove infected trees within 20 days of notification by the city. The disease can be controlled through proper sanitation measures and chemical treatments.

Elms are excellent shade trees. There are now disease-resistant varieties available at local garden centers.

Bur Oak Blight

Bur oak blight is often mistaken for oak wilt. This fungal disease only affects bur oaks, causing their leaves to form wedge-shaped dead areas, brown leaf veins, and dead leaves that persist through the winter.

Bur oak blight is not fatal to bur oaks. Still, repeated infections over several years can weaken the tree leaving it vulnerable to fatal attacks by two-lined chestnut borer or armillaria root rot.

Oak Wilt

Oak wilt is caused by a non-native fungus, Bretziella fagacearum, whose tiny spores can move through the vascular tissue of our native oaks. Like humans, the trees undergo an immune response when they have an infection. Oaks restrict the movement of the fungal spores by closing off vascular pathways. With these pathways closed, water and other nutrients can't reach targeted areas, thus causing wilting. 

How It's Spread: Overland and Underground

•  Underground oak wilt infections spread through root grafting, where diseased trees can pass on the fungus to healthy trees through connecting root systems.
• Sap beetles are attracted to the sweet scent of the fungal mats produced by oak wilt. These beetles can then transfer the disease to healthy trees.

Symptoms:

• Browning/wilting of leaves starts at the crowns of the trees and progresses downward. This causes leaf loss during the spring and summer months.
• Sweet-smelling spore mats under the bark. In Red and Pin oaks, these spore mats can be exposed as they cause the bark to split.
• Dark brown or black discoloration can be seen when the bark is peeled back; it can also be observed in the cross-section of an infected branch. 

Symptoms of oak wilt are found in all types of oak trees. However, trees in the red oak group are particularly susceptible, leading to them experiencing more rapid and aggressive symptoms.
It's also important to note that although these are all symptoms of Oak Wilt, you can't have an accurate diagnosis without testing (University of Minnesota Plant Disease Clinic). Many lookalikes with similar symptoms to oak wilt exist, such as bur oak blight, two-lined chestnut borer, and anthracnose.

Management Strategies:

• The number one way to prevent the spread is to not move infected wood from areas where oak wilt is present. Oak wilt fungus can survive in the wood and be spread up to one year after the tree is taken down or has died.
• Pruning of oaks during oak wilt season (April-July) should only be done if necessary. When pruning is needed, immediately cover wounds with shellac to minimize sap beetles’ attraction to wounded oaks.
  • Prune only during the “safe” months (see the pruning table below).
• Timely removal and proper disposal of diseased oaks limit insect spread.
• Chemical fungicide injections can help prevent oak wilt symptoms for up to two years before needing to be injected again. This treatment is only recommended for white oaks.
• In some cases, a vibratory plow with a 60” blade is used to break up root grafts.

Two-Lined Chestnut Borer

The two-lined chestnut borer is a native, slender, and dark-colored beetle. It can be identified easily by looking for the distinct yellow stripes running down their wing covers. These beetles attack all species of oak trees in Minnesota but typically attack those already stressed or declining. It is not the beetle that directly threatens Minnesota oaks, but rather their larvae. Their larvae grow up to an inch long, are pale white, and have flattened bodies. Adult beetles lay their eggs in branch or bark crevices of oak trees, usually in the late summer months. When these eggs hatch, the larvae burrow into the tree’s phloem layer to feed. They usually feed all fall, live through winter in pupal chambers, pupate in the spring, and emerge in early summer. 

Symptoms:

• Wilting of leaves and dieback starting at the crown of trees. The dieback will continue downward towards lower branches and into the main stem.
• Frass, a mixture of sawdust and larvae feces, can be seen packed in s-shaped galleries when the bark is removed.
• On trunks and branches, there will be a 1/8-inch D-shaped exit hole where the pupated beetles emerge from the tree.

Management Strategies:

• To mitigate stress, keep existing trees healthy by adding organic mulch, frequent watering, and protecting roots. This can minimize damage caused by the two-lined chestnut borer.
• Avoid fertilizing already stressed trees, as this will increase the tree’s demand for water.
• There are various chemical treatments in the form of pesticides.
  • Systemic Pesticides – applied around the soil of the tree
  • Canopy Sprays – applied on the trunk and branches where eggs are laid

Pesticides are ineffective when more than 40 to 50% of the canopy has been killed. Get an opinion from an arborist on whether your tree would benefit from treatment.

Similarities Between Oak Wilt and Two-Lined Chestnut Borer:

• Both diseases primarily affect oak trees.
• Trees infected/infested with either disease will experience wilting, starting at the crown of the tree and working its way downward.
• Chemical treatments are available for both diseases when conditions are sufficient. However, the oak wilt will need fungicide, and the two-lined chestnut borer will need insecticide.
• Like oak wilt, the two-lined chestnut borer can be transported via firewood.

A two-lined chestnut borer infestation and an oak wilt infection can mimic each other at first glance. Therefore, knowing what to look for when trying to differentiate the two is essential.

Differences Between Oak Wilt and The Two-Lined Chestnut Borer:

Some differences in symptoms include:

• The D-shaped exit holes and S-shaped galleries found in two-lined chestnut borer-infested trees.
• Discoloration and fungal mats on oak wilt diseased trees.
• The two-lined chestnut borer mainly attacks stressed trees, mitigating damage within healthy forests and leading to healthy new growth.
• Oak wilt targets all oaks, healthy or not. This can lead to a great amount of damage within a healthy forest.
• Two-lined chestnut borer is spread by adult beetles laying their eggs in crevices of oak branches and barks. This can occur from spring through fall.
• Oak wilt is spread primarily by connecting root systems and can also be spread by native sap-sucking beetles. The spread through beetles occurs mostly in the late spring and early summer months.

Resources

The city provides a free diagnosis of tree diseases. If you suspect your elm or oak may be infected or have other trees in your yard that are unhealthy, contact the Environmental Resource Specialist.