Oak Tree Diseases

Oak Wilt Disease

Oak wilt is a very common and serious tree disease in Lakeville. It is a fungal pathogen that is spread is primarily between the interconnected (grafted) roots systems of “like-species” oak trees growing within 50-100 feet of each other. Trees share water and nutrients this way in a forest setting, but unfortunately oak trees can also share the oak wilt fungus. The less common mode of spread is called “overland spread” because picnic beetles harboring the oak wilt fungus on their bodies land on pruning, storm damage or construction wounds and introduce the disease into the tree. They cannot bore into the tree and need an opening to be created to contact the living tissue of the tree. Oak wilt affects red, white and bur oaks. 

When the fungus is introduced into the water conducting (vascular) system of a red oak tree, the leaves wilt, and drop from the branches. Often it looks like autumn in the middle of summer due to the rapid leaf drop. The disease can kill a red oak rapidly, sometimes within two weeks. The process is usually slower in white and bur oaks. The oak tree dies because during its attempt to block the flow of the fungus, it blocks its own ability to transport water. 

Control measures to limit the spread of oak wilt should always be done before an infected tree is removed. Keep in mind that oak wilt does not discern property lines and often the best solution is to work with neighbors to stop the progression of tree death. The best management strategies for controlling the spread of oak wilt are:

  • Root graft disruption using a vibratory plow 
  • Preventative fungicide injection treatments of red oaks within 50-100 feet of the diseased tree, completed every other year
  • Practice good sanitation and remove diseased red oaks capable of producing a spore mat before they produce a spore mat the spring following their death - no later than April 1st. Typically, red oaks are the only species recommended for removal once infested because they produce a spore mat under the bark. Picnic beetles are attracted to the spore mats and get the fungal pathogen on their body. The beetles are also attracted to freshly cut oak wood where they will introduce the fungus that is on their body from a previously visited spore mat.
  • White and bur oaks rarely produce a spore mat, so tree removal is not as effective in limiting the “overland” spread of the disease. They can be therapeutically treated if the disease is caught early enough and are often not preventatively treated.
  • Do not prune or wound oak trees/roots from April-June, the highest risk time for new infections to start. Aim to prune oaks November- early March during the time that carries “no” risk for oak wilt transmission.
  • Tip:  If you accidentally wound an oak tree during the high-risk season of April-June, cover the wound within 15 minutes with a shellac or water-based spray paint to avoid oak wilt risk. Be sure to put up erosion control/snow fences around oak trees in construction areas BEFORE construction begins if you want to avoid wounding by equipment.
  • Find up-to-date information on the status of oak wilt in Minnesota.
  • For more information on oak wilt management techniques, including strategies for private properties, download the City of Lakeville's Oak Wilt Management Guide (PDF).

Bur Oak Blight

Bur oak blight (BOB) is a fungal disease found only on bur oak trees. The disease is common in Lakeville and the entire Twin Cities area. It is fairly new to Minnesota, though, so if you have bur oaks, it is important to know the difference between BOB and oak wilt because the diseases are managed differently.

Although BOB doesn't kill infected trees outright, several years of severely infected leaves weaken a tree's defenses and allow secondary pests such as two lined chestnut borers or armillaria root rot to kill the tree. A tree that has been affected by construction equipment over the soil, or is growing in less than ideal conditions, such as next to a driveway can be more susceptible to the damaging effects of BOB because it compounds the stress the tree is experiencing.  Infections are most common from late July to early August but can vary from year-to-year depending on the weather conditions during the growing season. The signs and symptoms of bur oak blight include:

  • Wedge-shaped areas of yellow/browning on leaves
  • Leaves that are wilted or appear scorched
  • Discoloration starting in the lower canopy that progresses up into the canopy
  • Black, pimple-like dots at the leaf base
  • Veins on the underside of infected leaves turning from green to purplish-brown, while the veins on the upper leaf surface appear darker than normal
  • Dead leaves that remain attached throughout the winter (this is common on healthy red oak trees, but not on bur oaks unless they suffer from BOB)
  • Extensive branch dieback with recurring infestations
  • The tree may look good when it leafs out in spring, but then suffers the above symptoms by late summer each year, moving progressively into the upper canopy each year

Watering trees regularly can also help maintain their vitality and prevent secondary pests such as two lined chestnut borers. Provide your tree(s) with sufficient water when less than one inch of rain per week is received and consider mulching around the dripline of the tree to conserve soil moisture and reduce competition from grass.

The spring following moderate to severe summer infection of BOB, consider having a company that employs a certified arborist to inject bur oaks with a fungicide. Treatment timing is crucial and should take place early in the spring immediately after the leaves have fully expanded but before BOB symptoms develop. Unfortunately bur oaks have a big response to the treatment and may drop some leaves afterward, but it is working to knock back the fungus and is the only option to combat the disease. After the initial injection, treatment is typically done every other year, but research is ongoing to determine the most effective way to tackle the disease. Some trees with extensive die back in the upper canopy may already have two lined chestnut borer and may need at least one treatment for that as well to reduce the pest pressure on the oak while it recovers from BOB. 

To determine if your bur oak tree has BOB, send a sample to the University of Minnesota's Plant Disease Clinic for a lab test or hire a private certified arborist. If you are unsure whether the symptoms you observe are oak wilt or BOB, the City Forester can inspect your tree during the months of June through September. Call 952.985-2724 for assistance or visit these online resources: