Oak wilt is a fungal disease that affects all species of oaks (Quercus). In recent decades, it has been popping up at an increasing rate. As with most serious plant diseases, it is caused by an exotic i.e. non-native, fungus (Ceratocystis fagacearum). It is found predominantly in the Northeast United States but has also been found in Texas and parts of Minnesota across the Midwest.

Oaks may generally be divided into two large groups of species. The red oaks are those that have points on top of the leaf lobes such as northern red oak, scarlet oak, pin oak etc. The other group are the white oaks that have rounded leaf lobes. This includes the white oak, bur oak and many others.

Although it has been found in many different oak species, it appears that those trees in the red oak group are far more susceptible than the white oaks.

It appears that infected red oaks usually die within a few months of infection. White oaks infected with the disease seem to linger on and may take several years to eventually die.

As with all "wilt" diseases, oak wilt acts by "clogging up" the water transmission vessels of the tree. This causes the foliage to wilt and, when they are deprived of water for some time, they die. The twigs and branches then die leading to the eventual death of the tree itself.

The organism that causes this disease is of the same genus (but a different species unique to oaks) as the fungus which causes Dutch elm disease. It appears to be spread the same way for both diseases:

1. Sap Feeding Beetles - Beetles are a key vector for this disease. As they feed on the sap of an infected tree, some of the fungal spores are attached to their bodies. As they visit healthy trees, they carry the fungus with them.

2. Root Grafts - Trees that are growing in close proximity to each other will have their root systems become intertwined. Over time, some of the roots will actually graft together and transmit sap between the two trees. This can transmit the disease from one tree to another.

As with Dutch elm disease, oak wilt is extremely difficult, if not impossible, to control once it has infected a tree. Here are the alternatives for dealing with oak wilt:

1. Avoid Pruning in the Growing Season - The beetles are attracted to fresh wounds that are actively losing sap. Therefore, one of the key ways to prevent the spread of oak wilt is to avoid pruning oak trees during the time when they have leaves or when the sap is flowing in the early spring. If at all possible, prune oaks (especially red oaks) in the cold of winter when the leaves have dropped. This is also the time when the insects will not be active since they are cold-blooded creatures.

2. Remove Dead Trees - Sanitation may help to minimize the spread of this disease. That means getting the proper diagnosis for oak trees that die in your landscape to be sure that it was oak wilt. Submit samples to your land grand university through their Extension Service to know for sure. Once it is confirmed that your tree(s) died of oak wilt, cut them down and remove them from the vicinity. A local arborist or the Extension office can advise you on how best to dispose of the wood.

3. Cut Root Grafts - If a properly diagnosed tree is surrounded by other oaks, it is recommended that you disrupt the roots to break any grafts that have formed. Specialized equipment will be needed to do this properly. Again, check with your local Extension office for the current recommendations.

4. Fungicide Treatments - Occasionally, a particularly valued specimen tree might be saved by injecting it with systemic fungicides every year. This treatment works sometimes on elm trees so it appears that it could work on oaks since the disease organism is very similar. The downside, of course, is that this can be a very expensive course of treatments and there is no guarantee that it will work.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

 

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