How can I determine if my oak tree has oak wilt?
Oak wilt is a serious disease that can infect many oak species. It is caused by the fungus Ceratocystis fagacearum. Red and pin oaks are very susceptible to the oak wilt fungus and can die within 4 to 6 weeks. White and bur oaks are susceptible, but the symptoms develop slowly. Trees can be infected by the fungus through root grafts or by sap-feeding beetles that carry spores to newly wounded trees. When a tree is infected, it tries to protect itself by producing a gummy material called tyloses which clog the water conducting vessels. Water is prevented from moving to the canopy and leaves begin to wilt. Leaves of infected oaks wilt, turn brown at the edges and fall off. The outermost ring of sapwood turns brown and appears as streaks when the bark is peeled or as a ring when the branch is cut in cross-section.
Diagnosing oak wilt can be difficult. Other factors, such as construction injury, drought stress, and insect borers, can produce symptoms that resemble those of oak wilt. The best way to determine if a tree has oak wilt is to submit a plant sample to a laboratory for diagnosis. A sample submitted for diagnosis should consist of several branches that are at least 1 inch in diameter and 6 to 12 inches long. If possible, collect the material from several different branches that are exhibiting symptoms. The sampled branches should have freshly wilted leaves but must be alive as the fungus cannot be isolated from dead wood. Samples should be kept cool and dry before and during shipment. It is also helpful to include a few partially discolored leaves. In Iowa, the ISU Plant and Insect Diagnostic Clinic can test for this disease. See their oak wilt article. There is a fee for this service. Additional information on submitting plant samples can be found at the clinic's website at clinic.ipm.iastate.edu