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 over a 1- to 6-month period 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. A few live oaks in oak wilt centers escape infection or may be resistant to the fungus and apparently remain unaffected by the disease.

Red oaks seldom survive oak wilt and often die within 3 to 4 weeks following the initial appearance of symptoms. During summer months, diseased red oaks often can be spotted from a distance because of their bright autumn-like coloration in contrast to the surrounding greenery.

Foliar Symptoms - Leaves on diseased live oaks often develop chlorotic (yellow) veins that eventually turn necrotic (brown), a symptom called veinal necrosis. Defoliation may be rapid, and dead leaves with brown veins often can be found under the tree for months after defoliation. Leaves may exhibit other patterns of chlorosis and necrosis, such as interveinal chlorosis, marginal scorch, or tip burn, but these symptoms are less reliable than veinal necrosis for diagnosing oak wilt in live oaks.

Veinal Necrosis - area around leaf vein turns brown rest of leaf is still green, found on the tree or on the ground. Veinal Necrosis is the most diagnostic.

Vein Banding – the leaf vein is a darker green then the rest of the leaf. This is symptom is seen before the veinal necrosis. These leaves are found on the tree.

Foliar symptoms of oak wilt on red oaks are less distinct. In early spring, young leaves simply wilt, turning pale green and brown. Mature leaves develop dark green water soaking symptoms or turn pale green or bronze, starting at the leaf margins and progressing inward.

Red Oak Symptoms - Leaves turn pale green then brown usually remaining attached for a period of time. This can begin on one branch and quickly engulf the entire tree. Trees generally die within 4-6 weeks

Fungal Mats - Fungal mats are reliable indicators for diagnosis of oak wilt. 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.

Crack in bark where fungal mat is formed underneath. Fresh fungal mat with oak wilt spores.Fungal mats all over tree.

Laboratory Diagnosis - Oak wilt diagnoses may be confirmed by isolating the fungus from diseased tissues in the laboratory. Samples can be submitted to: Texas Plant Disease Diagnostic Laboratory, 1500 Research Parkway, Suite A130, Texas A&M University Research Park, College Station, TX 77845. A county extension agent, Texas Forest Service forester, or trained arborist should be consulted for proper collection and submission of samples.