Q. My neighbor
has lost several red oaks to oak wilt. I have three red oaks that
really make my backyard. I would hate to lose them. What should I do
to protect my seemingly healthy trees?
A. It is wise
to contract with a certified arborist for an accurate diagnosis of
oak wilt and the safe removal of dead trees. An arborist can also
recommend fungicide injections that can protect healthy trees
growing near infected ones. Fungicides such as Alamo (propiconazole)
injected every two years have been shown to protect uninfected trees
growing near those killed by oak wilt. Even trees that have begun to
show symptoms can be successfully treated with Alamo if less than 30
percent of the crown is affected. Since Alamo does not kill the
fungus present in an infected tree's roots, such trees should be
treated every year. These products are not available to home
gardeners. Check the Yellow Pages to find a certified arborist in
your area.
Trees of related species growing in proximity to one another often
have their roots grow together as though they had been grafted.
Vascular diseases such as oak wilt are easily transmitted from
infected trees to healthy ones through shared tissue in these
grafts. This is the most common method of transmission in such a
localized outbreak. If your neighbor's trees are within 50 feet of
your trees, it is likely their roots have grafted together. Those
root grafts must be severed BEFORE your neighbor's trees are
removed. This can be done by digging a three-foot deep trench
between infected and healthy trees, or by killing the grafts
chemically with a soil fumigant. If infected trees are removed
before the root grafts are broken, fungus-laden sap from the
infected trees can "backwash" into your trees' vascular systems.
Red Oak leaves
Infected trees should be removed as soon as possible once the root
grafts have been broken. There is no chemical control for oak wilt
once symptoms are apparent in more than 30 percent of the crown.
Prompt removal of infected oaks is important to protect those trees
not yet infected. Destroy the wood immediately, including the stump,
by burning, burying or removing the bark so that it is not
attractive to the insects responsible for the spread of oak wilt. Do
not stack the wood for firewood, or transport logs with intact bark,
since insects in the infected wood can leave and carry the fungal
spores to healthy trees.
Perhaps the most important way to prevent healthy trees from being
infected with oak wilt is to prune oaks only when they are dormant,
from late November to mid-April. If oaks are damaged in a summer
storm, some pathologists recommend cleaning up the damage and
painting the pruning cuts with wound dressing to avoid attracting
the bark and sap beetles that spread oak wilt fungi. (Pruning paint
is rarely recommended because it can interfere with the tree's
natural ability to heal pruning cuts).
Oak wilt occurs in the United States in an area delineated by
Minnesota, Texas, Pennsylvania, and South Carolina. It is found west
of the Susquehanna River in Pennsylvania. Although all oaks can be
infected, those in the red oak group are most susceptible. These
include northern red oak (Quercus rubra), pin oak (Quercus palustris),
black oak (Quercus velutina) and scarlet oak (Quercus coccinea). Chestnut
trees are also susceptible, including American (Castanea dentata),
Chinese (Castanea mollisima), European (Castanea sativa) and bush
chinquapin (Castanea pumila). Susceptible trees can die within a few weeks of infection.
Species in the white oak group are less susceptible. These include
white oak (Quercus alba), swamp white oak (Quercus bicolor), bur oak
(Quercus macrocarpa), and post oak (Quercus stellata). When trees in this group
are infected, they may decline over a two to three year period, or
the disease may go into remission and they recover.
White Oak (above) is less susceptible than Red Oak
Oak wilt symptoms vary according to oak species and geographical
area. Generally for red oak group trees in our area, symptoms start
near the top of the tree and progress downward. Leaves on infected
trees turn dull green, then bronze, and browning is frequently
evident at the leaf tips or margins. Sometimes the leaves droop and
curl lengthwise. Browning may also occur along the veins. Leaves at
the ends of branches begin to fall soon after symptoms become
noticeable, and often drop while they are still green. Twigs and
branches die, and you may be able to see brown streaks in the
sapwood of infected trees, but this symptom is not always apparent.
Leaf discoloration and defoliation continue throughout the crown of
the tree for several weeks until the tree is dead.
Symptoms in the white oak group are similar, but advance much more
slowly, and do not cause the sudden defoliation and death seen in
the red oak group. The symptoms are often confined to a single
branch, and can look like typical fall coloration.
Oak wilt is caused by the fungus Ceratocystis fagacearum. The
disease kills infected trees by clogging their vascular systems
until they are unable to transport water and nutrients throughout
the tree. It is transmitted from infected trees to healthy ones by
oak bark beetles and sap beetles. Oak bark beetles lay their eggs in
infected trees. The adults emerge from egg laying covered with
spores of the fungus and transmit the disease to healthy trees when
they feed. Sap beetles are attracted to the fungal mats produced by
the disease because of their fruity odor. They also become covered
with disease-causing spores and transmit oak wilt to healthy trees
by feeding. They are particularly attracted to fresh pruning wounds.
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