Dutch Elm and Oak Wilt
Dutch elm disease and oak wilt are infectious tree diseases controlled
annually under the provisions of
City Ordinance 1055. A
designated tree inspector hired by the City of Edina and certified by the
State of Minnesota has the lawful right to enter upon private property for
the purpose of inspecting for and controlling the spread of Dutch elm
disease and oak wilt.
Although the biological make-up of these two diseases varies, the removal of
infected trees is a necessary and proven method of minimizing the spread of
both. While removal alone will not stop the spread of disease, without it, the
instances of tree loss throughout the City would be greatly magnified.
 Dutch Elm Disease
Dutch elm disease is a fungal disease spread by the elm bark beetle. Once an infected elm tree is identified, it is marked, (usually with a
green dot or ring on the trunk), and the property owner is notified. Property
owners are responsible for removing diseased trees at their own expense within three weeks
of notification. This time period is essential because the entire tree will quickly
be colonized by more elm bark beetles, which mature
quickly and are able to further spread the disease to other elms in the
area.
Spread
Dutch elm disease may also spread through connected roots from one elm
to another. In this instance, it may be advisable to mechanically sever
the roots as soon as possible before the infected tree is
removed.
Chemical Treatment
As stated earlier, removal of infected elms protects the
remaining healthy elms. Individual elms can also be protected with
fungicide injections. Many years of performing such a procedure has shown a high rate
of success. The expense of this procedure, which typically needs to be
repeated every three years, may limit its feasibility.
Identification
During the summer months, the City Forester routinely inspects the
community for Dutch elm disease. Residents are also encouraged to watch
their own elms. If Dutch elm disease is detected early in an infection
stage, it is possible at times to prune it out. Typical symptoms to look
for are yellowing of leaves -- usually in the upper crown -- followed by leaf
browning, curling and falling to the ground. If residents suspect Dutch
elm disease, they may call the City Forester for an inspection.
Oak Wilt
Unlike Dutch elm disease, oak wilt is thought to be a
native disease. It affects oak trees primarily in the Midwest. The
greatest concentration in Minnesota is in the seven-county metropolitan
area.
 Although oak wilt affects both the red oak group and
white oak group, red oaks are most susceptible to this disease. White oaks are moderately susceptible
and northern white
oaks are the least susceptible. To distinguish between the two groups, note that
red oak
leaves have pointed leaf margins and lobes while white oaks have rounded
margins and lobes.
Spread
Oak Wilt is spread in two ways. Most new infections
are spread from diseased oaks into neighboring oaks through grafted roots.
This can happen when surrounding oaks are within 50 to 60 feet of each
other. Where feasible, the common control method in these cases is to
mechanically break the root grafts using a vibratory plow. This must be done
before the diseased tree is removed.
The other way oak wilt can be spread is through insect
vectors that transmit the disease from oak wilt fungal spore mats to fresh
wounds or cuts on healthy oaks. Oaks are most susceptible to new
infections from April through June. Therefore, all pruning
operations on oaks should be suspended during this period.
Most, but not all, red oaks killed the prior year can produce
infectious spore mats under the bark during this critical period. These
trees may not be kept, with bark intact. Residents are urged to use
extreme caution when bringing in diseased oak wood to their property.
The probability of starting new infection centers on their
property or throughout the neighborhood is high.
Chemical Treatment
In high value white oaks, systemic injection with propiconazole by qualified arborists may protect nearby healthy oaks
adjacent to infected oaks. It may also be a curative to white oaks
exhibiting early symptoms of oak wilt.
Identification
Oak wilt is not the only disease affecting oaks in
this area. Therefore, proper disease identification is necessary before
devising any control recommendations. In Edina, the Kunde Company --
Oak Wilt control specialists -- are contracted to the State of Minnesota.
They have the right to enter upon private property to conduct necessary
inspections.
If residents observe unusual symptoms of their oaks,
they may call the Park and Recreation Department to arrange for an
inspection.
Residents who wish to receive further information on
this subject may contact the City Forester Tom Horwath at 952-826-0308.
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