Oak Wilt Update From The Clinic

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Oak wilt is a disease caused by the fungal organism Bretziella fagacearum. The disease can spread both underground and above ground. The disease is spread underground to healthy oaks through the root system of infected oak trees. Additionally, sap-sucking beetles can be attracted to the fungal mats that form under the bark of infected, dead oaks and can spread the disease above ground as they move to uninfected oaks.


The family of red oaks are more susceptible to this disease than trees in the white oak group; red oak trees infected with the fungus rapidly spread the disease throughout the vascular tissue and die within weeks of developing symptoms; whereas white oaks develop the disease at a much slower rate and tend to show symptoms more or less a single branch at a time. White oaks can sometimes recover from this disease, where it is normally fatal to oaks in the red oak group.


In the spring, as the oak wilt fungus becomes active and progresses, physical symptoms of the disease in red oaks tend to begin higher up within the canopy and include a sudden leaf discoloration progressing from the leaf margin into an almost half-leaf scorch, and intermittently the leaf veins may remain green.  The leaves develop leaf wilt and begin dropping from the tree. Branch symptoms will often have a stem discoloration under the bark and/or within the branch tissue.


How do you know whether your tree has oak wilt? Often, oak wilt symptoms can look like other tree diseases or abiotic issues so getting a positive diagnosis from the Clinic is essential. To test for the presence of the disease, the homeowner or arborist ought to send a branch sample showing symptoms of streaking under the bark or discoloration within the branch tissue.


Oak Wilt Cross Section
Photo Credit: Joy Pierzynski, OSU Dept of Plant Pathology 


Oak Wilt Streaking Under Bark
Photo Credit: Joy Pierzynski, OSU Dept of Plant Pathology 


Cut at least an 8-to-12-inch piece of the discolored branch and package up to send to the Clinic for testing. The cost for testing is $20.


Samples should be collected and send immediately to the lab for testing. Samples can be delivered by the homeowner or green industry professional or mailed using overnight or priority shipping. Samples should arrive to the clinic early or mid-week. Avoid weekends and holidays as the sample would be sitting and not be received at its freshest.


If the test for Bretziella fagacearum is positive, the diseased tree ought to be removed from the landscape as it poses a disease threat to healthy oak trees growing nearby. Since the fungal disease can be spread through the roots of one oak to another, an arborist ought to be called in to trench and break oak root grafts between the diseased oak and nearby healthy oaks and remove the tree..  This will decrease the spread of the disease.


For additional information, check out the OSU FactSheet on Oak Wilt at: https://ohioline.osu.edu/factsheet/plpath-tree-02