Garden Views: Protecting our threatened forests

by Bob Vaughn

Oak wilt, Dutch elm disease and the emerald ash borer are three serious threats to the forests of Minnesota.

Oak wilt is a fungal disease that affects oak trees. Oaks are the most plentiful and valuable shade trees in our state. Northern red oak and northern pin oak are most susceptible. Burr oak and white oak are less so. Oak wilt is much more controllable than Dutch elm disease because 90 percent of oak wilt is spread underground by root grafts. Trees up to 50 feet apart commonly have grafted roots.

Once an infection in an oak tree is identified, root graft barriers should be put in place. These barriers actually are a five-foot deep trench done by vibratory plowing around infected trees. A secondary trench should be plowed around the nearest trees without symptoms.

The other 10 percent of oak wilt is spread over land by the picnic beetle. The beetle carries fungal spores and is attracted to injured trees. These beetles are not borers so they need an injury on tree to spread disease. If we follow the “no prune” model in April, May and June we can prevent new outbreaks of oak wilt. Transporting firewood can spread this problem as fugal mats can be under bark of logs. The beetles rarely travel more than one mile per year.

Dutch elm disease is also a fungal disease. It is spread by two bark beetles: the native elm bark beetle and the smaller European elm bark beetle.

The disease took a heavy toll on the urban forest in the 1970s. Minneapolis lost thousands of stately elms. This disease is harder to control because 90 percent is spread over land by beetles, 10 percent by root grafts. Moving firewood is also a factor with this disease because fungus can grow in dead logs and beetles are attracted to dead or distressed trees.

Emerald ash borer is not a disease but is a serious pest invasion for ash trees. It is a non-native metallic wood boring beetle. The larvae of these beetles feed in the cambium layer just under the bark. This layer moves water up to the leaves and sugars down to the roots. Once this layer is damaged all the way around the trunk (girdled), the tree will die. This pest is also moved with firewood.

So the moral of this story is to buy your firewood where you use it. Don’t spread these pests at 60 miles per hour. The insects rarely travel more than one mile per year, without our help.

Bob Vaughn is an Anoka County Master Gardener.

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