Oak Wilt

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Oak Wilt is a harmful and debilitating disease that affects healthy oak trees when they become infected by a fungus called bretziella fagacearum. Fungal mats grow underneath the tree bark disease travels throughout the tree's vascular system and obstructs water movement, which leads to the decline and often death of the tree. Oak wilt is well established in southern and much of central Wisconsin. The northern part of the state is still largely unaffected, however, small areas of oak wilt have been detected.

A detailed image that gives an indication of what happens to the leaves of red and white oak trees infected with oak wilt. [1]



How do oak trees become affected?

Sap-beetles from infected trees move to other healthy trees.[2]

One way oak trees can become infected is through the connected root networks between healthy and unhealthy trees. Another way is from sap-feeding beetles, which feed on the fungal mats hiding beneath the bark. The fungus produces a sweet odor that attracts the sap-feeding beetles. The beetles then fly to healthy oaks and feed on the sap that drips from open wounds.[1]




What types of oak trees are affected?

Oak trees can be divided into two groups including red and white oak. Some species of red oak include black, northern red, and northern pin. Their leaves are characterized by pointed edges. White oak trees have leaves have rounded edges and include white, swamp white, and burr. The red oak group is most susceptible to oak wilt than white oak. [2]

The difference between red and white oak leaves are the edges.[3]


What are the signs and symptoms of oak wilt?

One of the most common symptoms of oak wilt is brown discoloration of leaves. This occurs starting at the tip of the leaf and progressing down through the center. Red oaks can experience an uncharacteristic dropping of leaves during seasons other than fall, when leaves traditionally fall off the tree. The disease in White Oaks progresses slower and signs may take longer to appear.[3]


There are two common methods to identify a tree with oak wilt that is not yet showing symptoms. One way is to look for the presence of sap beetles on the tree. Another is to look for fungal pressure pads located underneath the bark.[4]


How widespread is oak wilt in Wisconsin?

The image of Wisconsin shows the scope of the oak wilt problem in Wisconsin.[4]

Oak wilt is widespread in southern Wisconsin. About half of the northern counties have some degree of oak wilt infestation, however it is limited in scope. Affected areas in the north are generally mapped out in 6 mile blocks and are usually concentrated together in patterns as it continues to spread. The Door County Peninsula does not have any know infestations at this time.



How is oak wilt affecting specific regions

In 2010, trees exhibiting the characteristics of oak wilt were discovered on lakeshore properties near Eagle River and three miles west of the Chequamegon-Nicolet National Forest. The Wisconsin DNR has expressed more concern with this small outbreak because the trees in this region are less likely to survive oak wilt, unlike trees in the south where the trees are more adapt and disease is already widespread. The disease in this area was thought to have spread from transporting firewood, where disease carrying insects conceal themselves.[5]

The find is unfortunate for cabin owners and people who have enjoyed these trees for years, although the economic impact could be more worrisome. In a study done in the neighboring state of Minnesota, the cost of tree removal in Anoka County would be $143 million over 20 years if no management of the disease occurred.[6]

What can be done to stop the spread of oak wilt?

Prevention

Oak wilt is most likely to spread during the spring and summer months. It is during this period when sap-beetles are most active and can infect fresh wounds, which can be caused numerous ways including from storms. To help prevent new infections and spread, it is important to remove any cracked or broken branches during the cold winter months. Any open wounds should then be treated with a pruning sealant, which will keep the beetles from doing further damage. It is also recommended to call a professional who can better assess the situation and take the appropriate actions. The easiest method to prevent the spread of oak wilt is by taking precautions when transporting firewood. Trees that have died from oak wilt can be used for firewood, but because they can still harbor the fungal mats which harbor the disease, it is important not to transport the wood to new areas. Firewood whose bark falls off naturally is deemed to be safe and no longer a threat. [7]

Management

Another way for oak wilt to spread is underground through a connected roots system. The best method of control for this type of spread is to create a root graft barrier, which severs the connection between infected trees and healthy trees. Effective ways to do this is by using a cable plow, vibratory, or trencher. Finding the correct location to create the barrier is critical since it can be difficult to identify healthy oaks that are infected, but not yet showing symptoms.[8]

References

  1. "Oak Wilt." Wisconsin Department of Natural Resources. Accessed December 18, 2018. https://dnr.wi.gov/topic/foresthealth/oakwilt.html.
  2. "Oak Wilt." Wisconsin Department of Natural Resources. Accessed December 18, 2018. https://dnr.wi.gov/topic/foresthealth/oakwilt.html.
  3. "Oak Wilt." Heartwood Tree Company. Accessed December 18, 2018. https://www.heartwoodtreecompany.com/oak-wilt.
  4. "Oak Wilt." Heartwood Tree Company. Accessed December 18, 2018. https://www.heartwoodtreecompany.com/oak-wilt.
  5. Seely, Ron. "Oak Wilt Found in Trees in Wisconsin's 'cabin Country'." Waterloo Cedar Falls Courier. August 30, 2010. Accessed December 18, 2018. https://wcfcourier.com/news/local/oak-wilt-found-in-trees-in-wisconsin-s-cabin-country/article_ee79d118-b413-11df-9939-001cc4c002e0.html.
  6. Seely, Ron. "Oak Wilt Found in Trees in Wisconsin's 'cabin Country'." Waterloo Cedar Falls Courier. August 30, 2010. Accessed December 18, 2018. https://wcfcourier.com/news/local/oak-wilt-found-in-trees-in-wisconsin-s-cabin-country/article_ee79d118-b413-11df-9939-001cc4c002e0.html.
  7. "Oak Wilt." Heartwood Tree Company. Accessed December 18, 2018. https://www.heartwoodtreecompany.com/oak-wilt.
  8. "Oak Wilt." Wisconsin Department of Natural Resources. Accessed December 18, 2018. https://dnr.wi.gov/topic/foresthealth/oakwilt.html.

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