Importance. -- Oak wilt is one of the most destructive diseases of oaks in the Eastern United States. Red and live oaks (in Texas) are more severely affected than white oaks. Susceptible trees may be killed within a few months after infcction. The disease generally progresses more rapidly in the homogeneous stands of semievergreen live oaks in the shallow soils of central Texas than in the mixed hardwood stands of deciduous oaks in deeper soils of northern and eastern States.

Identifying the Fungus. -- The fungus is recognized by the presence of grayish fungal mats beneath cracks in the bark of infected red oaks (figure 63a). Fungal mats do not form on white or live oaks. The imperfect stage (Chalara quercina) may be identified microscopically in the laboratory.

Identifying the Injury. -- In early stages of the disease, symptoms range from leaf bronzing, marginal and veinal leaf necrosis, and twig dieback. Defoliation, branch dieback, and eventual death of the tree (figure 63b-e) are later symptoms.

Biology. -- The fungus spreads short distances by root transmission through root grafts and common root systems (live oaks) shared between infected and healthy trees. Sapwood beetles (Nitidulidae) are confirmed vectors in northern States and are responsible for long-distance transmission by carrying spores from fungal mats to wounds in healthy trees. Vector transmission is not yet well understood in Texas.

Control. -- Controls used in northern and eastern States include the use of silvicides to kill infected red oaks to reduce fungal mat formation and insect transmission, and the avoidance of tree wounding during periods of insect activity. Trenching (figure 63f) to sever root grafts helps control root tranmission of the fungus from infected to uninfected roots. Oak firewood should be covered with plastic and used within a year after cutting to reduce vector activity. The triazole fungicide (propiconazole) is useful in preventing infections in healthy, threatened trees with high economic value.