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Oak
Wilt in Apple Valley
In an effort to maintain the vitality of
the community's urban forest, the City of Apple Valley has a Shade Tree
Disease Control Program to help control oak wilt and Dutch elm disease
throughout our community.
The City's Natural Resources Coordinator
is responsible for the coordination of the Shade Tree Disease Control
Program. The Natural Resources Coordinator is responsible for identifying
and controlling the spread of oak wilt and Dutch elm disease on public
and private property. A crucial component of this process is the identification,
communication and coordination of shade tree disease control efforts with
the residents of Apple Valley.
What
is Oak Wilt?
Oak Wilt is caused by a fungus
called Ceratocystis fagacearum. The fungus travels through the water-conducting
vessels (xylem) of the tree which are located just beneath the bark. In
efforts to control the fungus within its own system, the tree produces
gums and tyloses to compartmentalize it. This effectively prevents water
from reaching the crown, and the leaves will wilt. In time, the entire
tree may die. Link
to U of MN Extension Service Web Site for more information >>>
All species of oaks are susceptible
to this disease. In Apple Valley, you will find predominantly two species
of red oaks. The red oak group, which is comprised of northern red and
northern pin oaks, is highly susceptible to oak wilt and does not recover
once infected. The initial symptoms of oak wilt are easily identified
in red oaks by the rapid wilting of the uppermost crown of the tree and
progressing downward. The tips and edges of individual leaves wilt leaving
the main vein of the leaf green. Fallen leaves are often still green at
the base or in the area of the leaf stem. Once the initial symptoms of
oak wilt appear, red oaks defoliate and die within a matter of weeks.
At this stage, the fungus has already spread throughout the tree and into
the root system thereby setting up the potential to infect adjacent red
oaks. Link
to the U of M Extension Service Site for information on Self Diagnosis
>>>
Figure 1: Oak Wilt Leaves
(Photo: University of Minnesota Extension Service)
In contrast, the white oak
group, comprised of bur and white oaks, are somewhat resistant to oak
wilt. If white oaks become infected with the disease, they generally wilt
very slowly, dying one branch at a time. White oaks do not wilt as uniformily
as red oaks. The wilting leaves may be more random and scattered throughout
the crown from year to year. White oaks can live with this disease for
a number of years before they die. However, a new fungicide called Alamo
is showing great promise for saving the white oak group.
How
Does Oak Wilt Spread?
The oak wilt fungus can
be spread in two ways. The overland infection is a direct transmission
of the fungus via a beetle vector, or the tree can become infected via
root graft transmission which results from an adjacent infected oak.
Overland
Infection
Healthy oaks can be infected
above ground by several species of sap beetles of the Family Nitidulidae.
These beetles are attracted to the fruiting body of the fungus called
a spore mat. These mats are produced under the bark next to the wood.
The spore mats produce a pressure pad which cracks the bark and allows
the beetle to access it.
Spore mats generally develop
between April and late June on red oaks that wilted during the previous
summer. The sticky fungus spores adhere to the beetles when they visit
the mats. During this same time period, these beetles are attracted
to healthy oaks with fresh wounds (less than 5 days old). Inadvertently,
the beetles deposit the fungus spores while they feed on fresh open
tree wounds. Remember,
the spring of the year is the only time new pockets of infection can
be caused by overland spread. Although this activity
by sap beetles accounts for less than 10 percent of the spread of oak
wilt, this is the only way new infection centers get started. Once inside
the tree, the fungus spreads through the xylem tissue of the tree.
Figure 2. Spore Mat
(Photo: Gregg Hove, City of Eagan)
The second way the oak
wilt fungus spreads is via root graft transmission. Oak trees of the
same species form common root grafts when growing close together. As
these roots intertwine, they encapsulate one another causing an underground
network of roots. These root grafts allow the trees to compete for available
water and nutrients. Once an infected oak dies, it no longer has water
pressure. This allows the next adjacent tree to pull available nutrients,
water, and the fungus into its system via water pressure, thereby causing
infection and spreading the disease. Root graft transmission accounts
for approximately 90 percent of the spread of oak wilt.
Managing
the Spread of Oak Wilt
Prevention
The overland spread of
Oak Wilt can be controlled simply by not wounding or pruning oaks during
the months of April, May, and June. An easier way to remember these
dates is from Tax Day (April 15) to the Fourth of July (July 4). Remember,
the spring of the year is the only time new pockets of infection can
be caused by overland spread.
If trees are accidentally
wounded or pruning is unavoidable, cover the wounds immediately (within
minutes) with a pruning paint available at your local garden center
or hardware store.
The best time for pruning
oaks is when they are dormant, generally December through March. In
warmer winter seasons, pruning should be curtailed sooner. Do
Not Prune Oak Trees During April, May, and June!
Root Graft Barriers
Root graft transmission
accounts for approximately 90 percent of the spread of oak wilt. In
Apple Valley soils, root graft barriers are approximately 86 percent
effective in controlling the underground spread of oak wilt. The root
graft barrier isolates the infected pocket of oak wilt by severing the
root graft connection to healthy trees. This, in essence, creates an
island of infection preventing the underground spread. Root graft cutting
involves using a machine called a vibratory plow which has a 5-foot
blade. Removal of the infected oak tree will quicken the spread of the
disease through the root system. It is important that the root graft
barrier be installed prior to the
removal of the infected oak tree.
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Figures 3,
4, and 5: Root graft cutting involves using a vibratory plow
with a 5-foot blade
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Although City Ordinance
does not regulate the installation of root graft barriers, City staff
may recommend the installation of a root graft barrier if it is deemed
appropriate.
Chemical
Treatment
Foresters now have a new
tool to help combat Oak Wilt. An experimental chemical called Alamo
is being used to suppress oak wilt in diseased white or bur oaks. The
preliminary results indicate that Alamo is effective in suppressing
oak wilt in the white oak group.
Although Alamo is not effective
on infected red and northern pin oaks, it is being used on these species
in order to help prevent the disease
from infecting healthy oaks that are within root graft distance of a
wilting oak.
Research is ongoing to
determine the effectiveness and limitations of this relatively new application
for Alamo. There are several private companies in the Twin Cities metropolitan
area that offer this service. They should be contacted for more detailed
information regarding the use of Alamo for oak wilt control.
What
You Can Do
As a property owner, you can help control
the spread of oak wilt by:
- Checking your neighborhood for wilting oak trees,
and
- Checking your woodpile if you removed a diseased
oak after October 1, and covering it with a 4-mil poly making a
tight seal to the ground if the wood is still wet.
- If you suspect a problem, contact the Natural
Resources Coordinator at (952) 953-2570.
City
Regulations
Under City Ordinance 633, Section 20-55,
Shade Tree Disease Regulation, only pre-sporulating infected oak trees
(i.e. red and pin oaks) are required to be removed and properly disposed.
Because bur and white oaks do not have the potential of producing a spore,
they are not required by City Ordinance to be removed. City
Ordinance does not regulate the installation of root graft barriers.
A provision of this Ordinance allows for
City staff to inspect all properties on an annual basis in order to combat
the spread of oak wilt.
For specific Ordinance information
or if you would like a copy of the Ordinance, please contact the City's
Natural Resources Coordinator
at 953-2570.
Diagnoses
and Identification
The University of Minnesota
has a self-guided website to help homeowners diagnose their own tree,
shrub, vegetable, and turf-related diseases. Link
to the U of M Extension Service site for information on plant disease
diagnostics >>>
If you think your tree is
infected or your tree is diagnosed with oak wilt, please contact the Natural
Resources Coordinator at (952) 953-2570.
If the Natural Resources Department or Tree
Inspector identifies oak wilt infected trees on your property, the trees
will be clearly mark and number the trees with yellow paint. A packet
will be sent which includes the following:
1. An official Notification Letter that explains:
a) Diseased tree removal procedures,
b) Assessment of the installation of a root graft barrier and/or the
use of Alamo,
2. A map showing the location of the infected trees
3. 2003
Oak Wilt and Dutch Elm Disease Cost Share Program Policies
4. A list of tree contractors licensed
to work in Apple Valley.
5. Oak Wilt in Minnesota brochure
Content updated on:
19-Apr-2004
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