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Oak
wilt, caused by the fungus Ceratocystis fagacearum, is found
throughout Illinois. The disease continues to kill oaks in the state
every year in residential areas, parks, farm woodlots, and forests.
In mixed stands of white and red oaks, red oaks may die out leaving
a pure stand of white oaks.
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The
oak wilt fungus invades the water conducting vessels of the
sapwood through fresh wounds or by root grafts formed between
diseased and healthy trees. In a few days, balloon-like tyloses
and gums begin to plug the water conducting tissue, blocking
the flow of water and nutrients from the roots to the foliage.
As the supply of water becomes restricted, leaves wilt and
die (Figure 1). No complete control or cure for oak wilt exists.
However, proper care plus mechanical and chemical control
measures can keep the disease from spreading to healthy trees
nearby.
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Figure 1. Symptoms of Oak Wilt on Red Oak
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Figure 2. Oak Groups: Red, Black, and Pin Oak Have
Pointed Tips; White and Bur Oak Have Curved Lobes Without
Pointed Tips
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Symptoms
RED-BLACK OAK GROUP (leaf lobes pointed): includes black,
black jack, pin, red, scarlet, and shingle oak
(Figure 2). The leaves in the top of an infected tree and
the tips of the lateral branches discolor and wilt in late
spring and early summer. The wilt symptoms progress downward
and inward until all the foliage is affected. The leaves curl
slightly and turn a dull pale-green, bronze, or tan starting
at the margins. Defoliation may occur any time after the symptoms
appear. By late summer, an infected tree is often bare of
leaves.
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Mature
leaves usually remain stiff during the different stages of
wilt and for some time after the tree dies. Immature leaves
curl, droop, turn dark brown to black, and remain on the branches.
A brown or black discoloration usually develops in the current-season
sapwood of wilting branches (Figure 3). The discoloration
may appear as longitudinal streaks. In cross-section, a brown
ring or circle of dark-colored spots is evident. Oak wilt
is the primary suspect when clusters of red and black oaks
start dying. They often wilt completely within 4 to 6 weeks
after the first symptoms appear or during a single growing
season. Once infected, oaks in the red and black group do
not recover.
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Figure 3. Branches of Red Oak with Bark Removed Showing
Healthy Sapwood (top) and Streaked Wood (middle
and bottom) Typical of Infection by the Oak Wilt Fungus
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Occasionally,
large branches of trees infected late in the summer survive the
winter but die the following spring after producing a few scattered
leaves. Oak wilt is the primary suspect when localized areas of
dead and dying red and black oaks showing the described symptoms
continue to increase.
WHITE-BUR
OAK GROUP (leaves with rounded lobes): includes bur, swamp, white,
and chinquapin oak (Figure 2). The leaves on affected branches
usually become light brown or straw-colored from the leaf tip toward
the base. The leaves curl and remain attached to the branches. Infection
generally occurs in scattered branches of the crown. The trees may
die in one year, but usually die slowly over a period of several
years or more. After two or more years of progressive die-back,
infected white oaks have sparse crowns and eventually die from oak
wilt or secondary causes. Bur oaks are intermediate in susceptibility
and may be killed as quickly as red and black oaks or as slowly
as white oaks. Sapwood discoloration, similar to that in red and
black oaks, has occasionally been observed in wilting bur oak trees.
White oaks have been known to recover after one season of infection
when new wood is laid down over an infected annual ring effectively
“walling off” the fungus and allowing tree recovery. This
is usually only a temporary recovery.
Oak
wilt can be confused with other problems such as anthracnose, construction
damage (including soil compaction), changes in the soil grade or
water table, lightning damage, nutritional disorders, insect and
animal injuries, chemical damage and root decay. (Anthracnose commonly
affects the leaves on the lower branches of white oaks; oak wilt
usually affects the upper part of the tree first).
Laboratory
Culturing
Some communities use trained survey people to locate and identify
diseased trees by visual symptoms. Positive identification of oak
wilt is possible by laboratory culturing (especially from late May
to mid-July) and recovering the oak wilt fungus from the tree. For
laboratory confirmation, send two to four branch sections from live,
freshly wilting branches. Branches that are dead and dried out (where
the bark can not be easily peeled away) should not be submitted.
The sections should be about thumb thickness and 6 to 8 inches long.
The fresh branch sections should be sealed in a plastic bag attached
to a completed Plant Clinic specimen data form. These forms are
available at University of Illinois Extension offices, and online
at the University
of Illinois Plant Clinic. Do not add moisture to the branch
sections, but do protect them from excessive heat and from drying
out. Some southern states recently reported increased success in
culturing the fungus when samples were kept cool during shipping.
The University of Illinois Plant Clinic has experienced similar
results. We highly recommend mailing samples early in the week and
shipping them on ice. For example, you might include a small, plastic
soda bottle, partially filled with frozen water along with the sample.
The
fresh branch sections should be mailed to the Plant Clinic, 1401
W. St. Mary's Road, Urbana, IL 61802. There is a charge per sample
for culturing – call ahead (217-333-0519) for current prices.
Include a check, payable to the University of Illinois, with the
oak-branch sections. The results of the laboratory diagnosis will
be mailed as soon as they are known. The process usually takes one
to two weeks of lab time.
Hosts
The known hosts of the oak wilt fungus include 36 species of oaks
(Quercus species) as well as the closely related American
chestnut (Castanea dentata), Chinese chestnut (C. mollissima),
Spanish chestnut (C. sativa), Allegheny chinquapin (C.
pumila), bush chinquapin (Castanopsis sempervirens),
and tanbark-oak (Lithocarpus densiflorus). All species and
varieties of oak tested to date have been found susceptible to the
oak wilt fungus.
Spread
Oak wilt moves from diseased to healthy trees in two ways –
through root grafts formed between trees and through fresh wounds
via sap-feeding insects.
Several
species of sap-feeding beetles (Nitidulidae family).
Beetles spread the oak wilt organism by carrying spores to fresh
wounds on healthy oaks. The spores are produced on fungus mats formed
beneath the bark of wilt-killed trees of the red oak group and as
the beetles crawl over and feed on the fungus mats, the microscopic
spores stick to their bodies. The insects then fly to healthy oak
trees and feed on the sap oozing from fresh wounds. Squirrels are
potential carriers of the wilt fungus, too, but their importance
has not been established.
Root
grafts. Where trees are growing close to one another, their
root systems often become intertwined. Thus, the roots of one red
oak may graft to the roots of a nearby red oak. If one of these
trees becomes infected, the root grafts serve as natural “pipelines”
for the oak wilt fungus to spread, below ground, to the healthy
tree. Root grafts are most common between oaks of the same species.
For example, it would be rare to find root grafting between a white
and a red oak.
Management
Control measures are designed to keep the disease from spreading
by preventing unnecessary wounds, severing root grafts either mechanically
or chemically, and removing and destroying diseased trees early.
In some instances, fungicides may be used where high value trees
are in danger and when all other appropriate control measures have
been implemented.
- Avoid unnecessary wounds. Wound infections are most likely
to occur during the spring and early summer between the time the
buds begin to swell and full leaf development. If possible, do
not start construction on sites with oak trees during the critical
period of spring and early summer. Generally it is suggested that
oaks be pruned in the fall after a hard freeze. A very cautious
approach, for areas known to have Oak Wilt, would be to prohibit
cutting from bud swell to October. If you must prune during this
period, consider treating the wounds with a wound dressing. Where
oak wilt is not a threat, the use of tree wound paints and dressings
is no longer recommended.
- Control root grafts. Four factors influence the likelihood
of root grafting between two trees: 1) trunk diameter of both
trees; 2) distance between trees; 3) soil type and drainage; and
4) tree species. For example, research shows that large (12"diameter)
trees growing in sandy soil will likely form grafts if the distance
between them is less than 93 feet. However, the same size trees
growing in heavier, loamy sand may form root grafts if the distance
between trees is less than 74 feet. In other words, root s are
likely to spread farther in sandy, rather than heavy soil. While
there is no similar published research based on heavier (high
clay content) soils common to much of Illinois, we can make some
generalities.
Oaks growing in heavy clay soils that are within 30 to 50 feet
of diseased trees are likely root grafted and should be considered
as "infected suspects". This means that they may already
be infected but are not yet showing symptoms. As previously mentioned,
root grafts are most common between oaks of the same species,
so it is not necessary to place barriers between a member of the
red/black oak group and a member of the white/bur oak group.
All possible root grafts between healthy and suspect or infected
trees should be severed mechanically or chemically before the
diseased trees are removed. These two methods are described
below - choose the best method for the particular situation. You
should work with a experienced forest pest specialist, forester,
or consultant trained in oak wilt management to determine the
location of barriers. In addition, before placing any type of
barrier, it is important that you call JULIE (800-892-0123) or,
in the Chicago area, DIGGER (312-744-7000). These folks will arrange
to have your underground wires, pipes, etc. located and marked
within 48 (business) hours after your request. While it is important
that you sever root grafts as soon as possible after an oak wilt
diagnosis, do it right and don't be careless.
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Figure 4. Typical Oak Wilt "Pocket",
Indicating Where Barriers Should Be
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Where
there is a mixture of diseased, suspect, and healthy oaks,
you should make two barriers – one between the healthy
and suspect trees and another between the suspect and
diseased trees (Figure 4). If you plan to mechanically
disrupt the roots, be on the cautious side and make the
barriers in the above order. The goal is to sever all
roots to a depth of 3-5 feet in a line midway between
the two trees. Experience and limited research indicates
t hat deeper (4-5 feet) barriers are more effective t
han shallow barriers. When sidewalks, driveways, curbs,
or other obstacles prevent establishing a completed barrier,
extend the barrier along the obstruction and sever all
root connections.
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In residential areas, infected trees commonly involve more than
one homeowner. For best results, all homeowners in the neighborhood
must: 1) be informed; 2) understand the necessity of establishing
barriers regardless of property boundary lines; and, 3) carry
out a continuing neighborhood-control program.
- Cut the roots mechanically. While neither method
is foolproof, it has been shown that mechanical barriers are
much more effective than chemical barriers. Any trenching
machine or vibratory plow (sold by Ditch Witch or Vermeer
or other companies) that will cut or break the roots to a
depth of 3-5 feet (see above discussion) can be used. Many
practitioners prefer to use a vibratory plow because it slices
through the soil and does not require backfilling with soil.
Root-cutting equipment is commonly available from a local
forester, commercial arborist, utility company, or irrigation
installation company. However, it may be difficult to find
a vibratory plow with a shank longer than 3 feet (5 foot shanks
can be custom-built). The trenching technique is not suitable
near sidewalks, driveways, buried pipes, power lines, or telephone
cables. In those situations, chemical treatment might be necessary.
- Employ a chemical treatment. Metam sodium (sold as
Metam Sodium and other brand names) will kill segments of
grafted roots. Fumigants are restricted-use pesticides so
they must be applied by a licensed pesticide applicator who
has been trained in their use. Typically the fumigant is placed
into 1-2 inch-diameter holes drilled 18-24 inches deep and
spaced 4-6 inches apart. The fumigant diffuses into the soil
to various degrees and kills all roots (including grass and
other plants along a strip about 18 inches wide), blocking
the spread of the oak wilt fungus from diseased to healthy
trees. Because they are inherently dangerous, difficult
to apply properly, costly, and less effective than mechanical
barriers, fumigants should only be used as a last resort.
- Remove dead trees. Diseased and dead oaks should be
removed and burned as soon as possible after mechanical root cutting
or about 2 weeks after chemical treatment for root grafts, unless
removal would wound surrounding trees. If that is a possibility,
remove the diseased or dead oaks in late fall or winter. Since
fungal mats are not known to develop within members of the white
oak group, there is less urgency in removing dead or dying trees
that belong to this group. However, remember that all dead shade
trees present a landscape hazard and should be removed as soon
as possible after death.
If diseased white oaks are especially valuable, remove the wilting
and dead branches carefully at the trunk, treat the wounds promptly,
and water during periods of drought. Pruning tools should be disinfected
before trimming another tree or noninfected tissue. To disinfect
tools: (1) remove wood fragments; (2) soak for several minutes
in a disinfectant such as 70 percent rubbing alcohol or liquid
household bleach (diluted 1 to 5 with water), and then (3) rinse
the tools in clean water. If wilt symptoms continue to advance
beyond the pruned parts, the tree will probably die of oak wilt
or secondary complications. In such cases, the white oak should
be removed.
The use of diseased oaks for firewood is not recommended in residential
areas since the firewood is frequently not burned before the following
spring when insects may carry the wilt fungus out of the infected
wood. If diseased wood is to be used, it should be processed as
soon as possible and burned before spring. Firewood should be
cut to the proper length, split, stacked off of the ground in
a single tier, and protected from moisture in order to hasten
drying. Firewood that has been debarked or stored in a dry place
is not a source of infection. Diseased oak timber can be harvested
for lumber if it is sawed before the following spring.
- Control infections that jump barriers. Occasionally,
oak trees beyond the barriers become infected in spite of the
severed roots. Additional barriers should then be put down using
the same procedure to surround newly found diseased trees and
any neighboring oaks that are suspect.
- Create a barrier of poisoned oaks. In timbered areas
where it is not feasible to sever root grafts, use fungicides,
or properly dispose of infected oaks, foresters may contain pockets
of diseased oaks by using herbicides to deliberately kill diseased
and nearby "suspect" and healthy oaks. This poison barrier
should be set up as soon as possible after the disease is observed.
For example, creating one or two barriers of poisoned red oaks
around the single infected red oak or around a pocket of wilt-infected
red oaks (Figure 4) will prevent the fungus from spreading through
root grafts. The diseased trees should also be poisoned. Both
the tops and the roots of the poisoned oaks must be killed to
stop transmission by root grafts. Consult the Brush Control chapter
of the Illinois Agricultural Pest Management Handbook (updated
annually) for current herbicide recommendations. The poison barrier
system of oak wilt management should only be performed by a appropriately
licensed and trained forester or arborist, and should be considered
only for oak trees having value as timber and not for high-valued
trees having ornamental value.
- Use systemic fungicides for high value trees. Researchers
and practitioners continue to investigate the use of systemic
fungicides in an attempt to provide safe, long-lasting protection
against oak wilt. While there are several injectable fungicides
labeled for the control of oak wilt, many researchers and practitioners
agree that Alamo is currently the most effective. While Alamo
can be applied using the newer "micro-injection capsules",
most practitioners prefer the traditional "macro-injection"
technique. The drawback to using any of the current fungicides
is cost (typically $300 or more per tree) and the potential need
for re-treatment in one to two years. Thus, fungicides are suggested
only where high value trees are in danger and when all other appropriate
control measures, listed above, are used. Alamo is labeled for
use as preventative and therapeutic injections. Therapeutic injections
have the best chance of working if the tree shows less than 10-20%
crown loss due to oak wilt. However, researchers and practitioners
tend to agree that it is a waste of money to inject members of
the red oak group that show any symptoms of Oak Wilt. Tree injections
should only be made by trained arborists or others trained in
injection techniques and diagnosis of Oak Wilt. Consult the Illinois
Commercial Landscape and Turfgrass Pest Management Handbook (updated
annually) for current chemical recommendations.
For
further information on Oak Wilt or other tree diseases, contact
Nancy R. Pataky, Extension Specialist and Director of the Plant
Disease Clinic, Department of Crop Sciences, University of Illinois,
Urbana-Champaign. We acknowledge with thanks the expertise of Bruce
E. Paulsrud, Extension Specialist and Pesticide Applicator Educator,
in the revision of this publication.
University
of Illinois Extension provides equal opportunities in programs and
employment.
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