Dutch Elm Disease and Oak Wilt
These two infectious tree diseases are controlled annually under the
provisions of Minnesota Statue 18.023 and 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 complex of these two diseases varies, an
important aspect of control efforts necessitates the removal of infected
trees. These sanitation procedures are necessary and proven methods of
minimizing their spread. Although it is recognized that sanitation alone
will not abate these diseases, without sanitation control efforts, the
instances of tree loss throughout the city would be greatly magnified.
Dutch Elm Disease
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 of
their responsibility to remove it, usually within three weeks. This time
period is essential because the entire tree is quickly colonized by the
Elm Bark Beetles between the bark and the wood. These Beetles mature
quickly and are able to further spread the disease to other elms in the
area.
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 prior to and as soon as possible before the infected tree is
removed.
As stated earlier, sanitation of infected elms protects all the
remaining healthy elms. Individual elms can also be protected with
fungicide injections. Many years of 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 choices to the most desired
specimens.
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. But Oaks
-- in the White Oak group -- are moderately susceptible. Northern White
Oaks are least susceptible. To distinguish between the two groups, Red Oak
leaves have pointed leaf margins and lobes. White Oaks have rounded
margins and lobes.
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 by a vibratory plow. This is the same
method mentioned earlier for Dutch Elm Disease. This also must be done
before the diseased tree is removed.
The other way Oak Wilt can be spread is by insect
vectors transmitting 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 of each year. 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 in bringing in diseased oak wood to their property. This
presents a high probability of starting new infection centers on their
property or throughout the neighborhood.
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.
Oak Wilt is not the only disease affecting oaks in
this area. Therefore, proper disease identification is necessary prior to
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.
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