trees can die within a few weeks of infection. Species in the white
oak group are less susceptible, including white oak (Q. alba), bur
oak (Q. macrocarpa), and swamp white oak (Q. bicolor). When trees in
this group are infected, they may decline a period of two or three
years, rather than dying quickly.
the symptoms go into remission and the tree survives. It is
possible that the same characteristic that makes white oak useful
for whiskey barrels and wine casks helps protect them from oak wilt.
When trees in the white oak group are wounded or infected, small
plugs called tyloses block off the sapwood and wall off the damage.
These tyloses make the wood impervious to water and and appear to
inhibit the oak wilt fungus from moving throughout the tree.
Red Oak leaves
symptoms vary according to species and region. 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 or tan. This 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 branch ends begin
to fall soon after symptoms become noticeable, and often drop while
they are still green.
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. It is not always
possible to isolate the oak wilt fungus from samples in the
laboratory; trees that are already dead yield no results at all, so
oak wilt cannot be determined after the fact. Diagnosis is primarily
based on symptoms.
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.
FUNGUS: Ceratocystis fagacearum
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. Two
species of insects are responsible for long distance transmission
of oak wilt, oak bark beetles and sap beetles.
The Role of 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. Prune oak trees during winter
dormancy (November through mid-April) to minimize infection via this
route. Oaks are most susceptible in spring and early summer as new
growth is occurring.
Northern Red Oak tree
Oak Wilt spreads through
that oak wilt is spread is through naturally occurring root
grafts. This is the most common method of transmission in a
localized outbreak. Trees of related species growing in proximity to
one another – within about 50 feet - often have their roots grow
together as if they had been grafted. Vascular diseases such as oak
wilt are easily transmitted from infected trees to healthy ones
through shared vascular tissue in these grafts.
grafts must be broken to protect uninfected trees. This can be
done by digging a trench between infected and healthy trees, or by
killing the grafts chemically. Root grafts should be broken before
removing an infected tree. If infected trees are removed prior to
breaking those root grafts, there is a risk that infected sap can
“backwash” into the healthy tree. Healthy trees that are root
grafted to infected ones can be protected by breaking root grafts
prior to removals of the infected trees, then injecting the healthy
tree with propiconazole (Alamo).
trees should be removed as soon as possible. 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
debarking 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.
Making the Proper
It is wise to
contract with a certified arborist for an accurate diagnosis of oak
wilt and the safe removal of dead trees. If less than 30 percent of
the crown is affected, injections of Alamo (propiconazole) can put
the disease into temporary remission. Any root grafts between the
infected tree and neighboring oaks should be broken, because Alamo
does not kill the fungus present in the tree's roots. An arborist
can also advise you about protecting uninfected oaks with injections
of Alamo every other year.