 Disease & Insects Oak Wilt
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The fungus Ceratocystis fagacearum causes oak wilt. The fungus invades water-conducting vessels and induces
the formation of balloon like projections called tyloses, which plug the vessels. As water movement within
the tree slows, the leaves wilt and drop off the tree.
Oak species are divided into two groups. Trees in the red oak group (black, northern pin and other oak trees
with pointed leaf edges) and white oak group (white, swamp white, bur and other oak trees with rounded leaf
edges) are susceptible to oak wilt infection. However, oaks in the red oak group always are killed by the
fungus whereas oaks in the white oak group tolerate the infection better and often survive.
Red Oak Group
Trees in the red oak group usually drop their leaves within a three-week period either in late June or
throughout July and August. Some lose a portion of their leaves in September, then rapidly lose all
their leaves just after they bud in the spring.
White Oak Group
Trees in the white oak group drop their leaves from either one or several branches for several years
in a row. These trees do not always die and have a greater chance of surviving infection than red oaks.
Underground
Most oak wilt moves from diseased trees to healthy trees through interconnected roots (root grafts).
Root grafts normally form between oaks of the same species; grafts between red and white oaks are rare.
It is more likely that root grafts will form between red oaks than white oaks.
Overland
Some oak wilt moves overland via sap-feeding beetles. In the spring, fungal mats (small masses of
Ceratocystis fagacearum) develop under the bark of some trees that died from oak wilt the year before.
These mats force the bark to crack open. The fungus produces a sweet odor that attracts sap-feeding beetles,
which then pick up fungal spores while feeding on the mats. The beetles then fly to healthy oaks to feed on
sap flowing from fresh wounds, thus infecting healthy trees. Overland spread can also occur when firewood
or logs from infected trees harboring fungal mats are moved.
Collect three twigs (about 1/2-inch in diameter and 4-inch in length) from three different branches with
wilting leaves. Samples must still have live tissue. Scratch the sample branch with your fingernail. If the
wood under the bark is a light color (white to green), the sample is fresh. If the wood is brown or dark, it
is too old to sample. Wrap the sample in waxed paper and keep it cool until you mail it. Mail the sample,
along with $15 to:
Plant Disease Diagnostics Clinic
Department of Plant Pathology
University of Wisconsin-Madison
1630 Linden Dr.
Madison, WI 53706-1598
Wounding, timing, susceptibility
Oak trees are most susceptible to overland spread in the springtime, from bud swelling until two to three
weeks past full leaf development.
April 15-July 1. During this highly susceptible period, do not prune, cut or injure oaks! If an oak is
wounded during this time, cover the wound immediately with tree wound paint. Tree wound paint does not help
wound closure process, but does act as a barrier to the fungus. Limit the use of wound paint ONLY to the
time period described above.
Observations and unpublished research shows that overland infection can occur after July 1, yet these
midsummer and early-fall infections are not common. To take a cautious approach, do not prune or wound
oaks from April 15-October 1.
Management through disruption of common root systems
Spread of oak wilt through grafted root systems is the most common way oak trees become infected. If healthy
oaks of the same species are near an infected tree, removing the infected tree will not control the spread.
In fact, the movement of the fungus through the root grafts may hasten after the prompt removal of infected
trees. The best control measure is to install root graft barriers, which disrupt the common root systems
between healthy and diseased trees. The most successful methods to install root graft barriers involve
physically severing roots with a vibratory or cable plow or trencher. Barriers must be created in the correct
location for success. Often, adjacent oaks that appear healthy may already be infected, but have yet to
show symptoms. A forest pest specialist, forester or consultant trained in oak wilt management should work
with you to find a barrier location.
Management through use of injected fungicides
Fungicide applications--macro infusions of Alamo™ fungicide into the tree trunk has been shown to offer
up to three years of protection against oak wilt in red oaks and can be used therapeutically in the white
oak group.
After installing root graft barriers, diseased wood may be removed and used for firewood or other wood products.
Trees that have died from oak wilt can harbor fungal mats, so if this wood is moved, the fungal mats are moved
as well and the disease may spread to areas currently unaffected. Any trees that have died from oak wilt and have
bark that is tightly attached to the wood could harbor fungal mats. This wood must receive special treatment.
Once that bark is loose and falls off the wood, the mats are no longer viable and no special treatment is necessary;
movement of the wood is no longer a concern.
Firewood
Two methods of wood treatment are effective in preventing overland spread via firewood:
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Debarking (removing bark from the wood) the wood will prevent the fungus mats from forming. Debarking must be
conducted before fungal mats form, thus it should occur in the late summer, fall or winter following tree death.
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Cutting, splitting, stacking and covering wood with a 4 ml (or thicker) plastic will also prevent overland
spread. All sharp edges or stubs should be cut to eliminate the possibility of puncturing the plastic. The
entire pile must be sealed all around. Seal the bottom by covering it with dirt and logs or other heavy
objects. If the wood is not burned over the winter following tree death, leave the tarp on through the next
growing season (October 1) or until the bark is loose.
Other wood products
Wood from infected trees may be sold to a sawmill or chipping facility - preferably one which is several miles
away from the nearest red oak. Advise the purchaser that the infected trees with tightly attached bark must be
utilized over the coming winter.
The oak wilt fungal mat does not survive well when it is dried out, exposed to other adverse conditions, or put
in competition with other wood decay fungi. Thus, wood chips from infected trees are highly unlikely to serve
as a source of disease innoculum or spores, and can be used for landscaping.
The research on this topic is minimal, yet experience shows that the likelihood of spreading oak wilt via saws is
very low. At this time, it does not appear necessary to disinfect your saw.
If you allow the disease to progress, it will spread to healthy oaks that are grafted through roots to the diseased
trees. In stands where oak is common and root grafting prevalent, an ever-widening pocket of dead oaks will form.
In forests where oak is mixed with other species and is a minor component, spread will be slower and may actually
stop from a lack of root grafting. New pockets may also form via overland spread by sap-feeding beetles
(see description of "over land spread")
Dead oak trees can serve as excellent den trees for wildlife. Oaks do not decay as quickly as aspen, birch and red
maple, thus will provide shelter for wildlife for many years. Also, as oaks die, the site often becomes brushy for
about ten years. The brushy area will attract warblers, grosbeaks, cuckoos, cardinals, grouse, rabbits, deer and
shrews. Brown creepers may nest under the sloughing bark on dead trees. Dead trees will also furnish insects for
birds, and larger specimens may provide perches for raptors.
Revised: April 17, 2006
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