Typically Oak and Elm trimming season runs October 15th through March 15th, or first frost to first
thaw. The reason for this strict time frame is because the frost kills or forces the elm bark beetle and
insects that carry infected fungal spores to go dormant, thus inhibiting the spread of Oak Wilt and
Dutch Elm Diseases.
Not familiar with these two diseases? Read below for further information.
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 the
specified time period that the city has mandated. 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.
Symptoms of Oak Wilt
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. Owners 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.
