Understanding oak wilt

‘Tis the season to avoid injuring oak trees. Wounds attract sap-feeding beetles that may carry spores from the oak wilt fungus.

Mid-April through mid-July is a critical time of the year to avoid injuring or pruning oak trees, especially those trees in the red oak group. It is this time of the year when certain sap-feeding beetles have the potential to spread oak wilt spores overland from oak to oak.

Should wounds occur during this period, they should be immediately coated with a latex-based paint to exclude the beetles. This is one of the few cases where tree paint is recommended. Generally, tree wounds should not be painted.

Oak wilt is a deadly exotic disease that has spread throughout much of the oak range in Michigan and across the eastern U.S. Oaks introduced to the fungus this spring are likely to die this growing season or, perhaps, the next growing season.

Oak death from oak wilt occurs rapidly, beginning at the top of tree and proceeding downward. Complete leaf loss and death can happen within two to three weeks. Leaves tend to brown from the tip to the stem.

Once a single tree is infected, the disease will spread to other oaks through their grafted root network. The impacts cease when all the grafted oaks are dead.

In a woodland situation, the spread rate varies but can move underground 100 feet or more per season. The pattern is typically concentric but that, too, can vary.

Traditional treatment involves accurate diagnosis, identification of the disease perimeter, severing root systems at that perimeter, then the removal of oaks within the perimeter. The treatment should be set-up by professionals familiar with oak wilt. The treatment and oak removal is an expensive proposition.

An alternative, less expensive treatment that has had some success is double-girdling and applying herbicide to trees along the disease perimeter. Chainsaw girdles need to extend about two inches into the wood, then the herbicide applied into the cuts.

Where a new, single tree infection is identified, the stump can be pulled. This technique has been successful across the Lake States. It is more effective when done sooner than later.

High-value trees on municipal or residential sites can be protected with a fungistat chemical that prevents spores from germinating. This treatment needs to be done by a certified pesticide applicator and must be applied every two years. Trees already infected cannot be saved.

Oak wilt fungus is an obligate parasite on oaks, meaning it must have live tissue in order to survive. Once oak wilt has run its course, the area is clear of oak wilt. Potentially, oaks can be replanted in the area, preferably as part of a mix of tree species.

Simply cutting the dead and dying oaks will not stop oak wilt. The disease exists in the root systems and will continue to spread underground, possibly a bit faster when dying oaks are cut prior to death.

Of particular importance are the oaks that have died the previous growing season. It is from these recently dead oaks that the fungus will produce spore mats to attract the beetles. These “symptomatic” oaks need to be removed and properly processed. Processing can include most anything that dries-out the wood, such as burning or making lumber, chips, or other products. Cut logs can also be tarped tightly to the ground, to exclude beetles. The following year, the tarps can be removed.

Oaks that die this year will be especially hazardous next year. Moving firewood from symptomatic oaks is one of the most common ways oak wilt is spread to new areas. For an increasing number of reasons - avoid moving firewood.

The spore mats can be found by carefully inspecting the bark, looking for cracks where the bark has separated from the wood. In the middle of the spore mats, the fungus builds a raised pressure pad that forces the crack open. It is through these cracks that the sap-feeding beetles enter and exit. A hatchet is needed to remove the bark to expose the fruiting body.

Presence of the spore mats and pressure pads is diagnostic for oak wilt. For a suspected initial infection, a properly obtained sample must be cultured in a laboratory, such as the MSU Diagnostics Lab.

If you suspect oak wilt is active on your property, contact an arborist, a consulting forester, a Conservation District, or another source of expertise. Doing “nothing” will contribute to the spread of this disease.

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