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.
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.
the Injury. --
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.
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.
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.