Foliar symptoms, patterns of tree mortality, and the presence of fungal mats can be used as indicators of oak wilt. However, laboratory isolation of the fungus is recommended to confirm the diagnosis. A trained expert should be consulted when in doubt.
Patterns of Tree Mortality
Most live oaks defoliate and die within 3 to 6 months following initial appearance of symptoms. Some live oaks take longer to die, and a few untreated trees may survive many years in various stages of decline. Occasionally, a few live oaks in an oak wilt center may escape infection and remain unaffected by the disease.
White oaks rarely die from oak wilt. Most only exhibit some canopy loss and generally the disease will not spread to adjacent trees. However, Lacey oaks and white shin oaks sometimes form root connections similar to live oaks, offering a pathway for the disease to spread to adjacent trees.
Red oaks never survive oak wilt and often die within 4 to 6 weeks following the initial appearance of symptoms. During summer months, diseased red oaks can often be spotted from a distance because of their bright, autumn-like coloration in contrast to the surrounding greenery. This symptom is called flagging.
Foliar (Leaf) Symptoms on Live OaksLeaves on diseased live oaks often develop chlorotic (yellow) veins that eventually turn necrotic (brown), a symptom called veinal necrosis. This is the most commonly seen foliar symptom on live oaks that are infected with oak wilt. Another symptom that leaves can exhibit is vein banding, where the leaf vein is a darker green than the rest of the leaf. Tip burn or margin burn, which turns the edges of the leaf brown, can also be seen on leaves. Defoliation may be rapid, and dead leaves with brown veins often can be found under the tree for months after defoliation.
Foliar Symptoms on Red OaksFoliar symptoms of oak wilt on red oaks are less distinct. In early spring, young leaves simply wilt, turning pale green and brown, usually remaining attached for a period of time. Mature leaves develop dark green water soaking symptoms or turn pale green or bronze, starting at the leaf margins and progressing inward. This can begin on one branch and quickly engulf the entire tree. Red oaks generally die within 4 to 6 weeks.
Presence of Fungal MatsFungal mats are reliable indicators for diagnosis of oak wilt. In Texas, these specialized spore-producing structures most often form in the spring on red oaks that developed advanced symptoms of oak wilt the previous late summer or fall. Red oak infections in late spring and summer usually do not give rise to fungal mats due to high temperatures and low soil moisture conditions. Fungal mats can be found by looking for inconspicuous narrow cracks in the bark of dying red oaks leading to hollow areas between the bark and wood. They often have a distinctive odor similar to fermenting fruit. Fungal mats can be exposed for inspection by chopping away the loose bark. Only infected red oaks can form fungal mats. Mats do not occur on white or live oaks.
Laboratory DiagnosisOak wilt diagnoses may be confirmed by isolating the fungus from diseased tissues in the laboratory. The Texas Plant Disease Diagnostic Lab accepts and tests samples. Detailed instructions and mailing information can be found on this form, which must be completed and included with the samples. A county extension agent, Texas A&M Forest Service forester, or trained arborist should be consulted for proper collection and submission of samples.