I recently came across a big patch of live oaks infected with oak wilt in Richardson, Texas, and was reminded of how little common knowledge there is about such a prevalent and insidious disease.
Oak wilt is a disease lots of people have heard of but don’t know much about, except that their sense memory of it is very bad. So we’ll start with the basics.
What Is Oak Wilt?
Oak wilt is a vascular fungal disease that affects oak trees. The fungus clogs the vascular system of the tree, blocking the transportation of water and nutrients to the canopy. Because it invades the vascular system, it is impossible for only one part of the tree to have the disease.
How Does It Spread?
Oak wilt spreads in two main ways: root grafting and beetles. Many people are aware of a prescribed season for trimming oaks, roughly July-January. The reason February-June is not an ideal time to trim oak trees is the nitidulid beetles that can carry the fungus from infected trees to uninfected trees are active during this time. The beetles are attracted to any wounds on an oak, so if trimming must be done during this time-frame, all wounds should be sealed with pruning paint. Although cleaning tools between trimming different trees is part of most guidelines on the subject, it is thought by some experts that it would be nearly impossible to spread via trimming tools.
Root grafting is by far the most common method of transmission. It is when root systems from two different trees coming together and join, which is very common for trees in close proximity to each other.
Wood from a tree that has died of oak wilt cannot spread the disease unless the tree is a red oak. Fungal mats of oak wilt can only form on red oaks, so any wood from a red oak killed by the fungus should be burned.
Trenching can sometimes be used as a preventive measure in conjunction with other treatments, but it is usually only viable in field settings. The trench must be at least four feet deep and there must be at least 100 feet between trench and the first symptomatic trees, which will rarely work well in a city.
Fungicide injections, which are the only method of treatment, can also be applied preventively. Healthy trees shouldn’t be treated until oak wilt is identified within 200 feet, so effectively not until it’s on your block. Even preventive treatment doesn’t always work, but if your tree is a white oak or a bur oak, as opposed to a red oak, it is very likely they will survive.
Oak wilt only affects oaks, but it can invade all species of oak. Some species weather the disease better than others, with red oaks faring the poorest and white oaks faring best. Red oaks usually die within six weeks, while live oaks usually die within six months to a year. Post oaks, chinkapins, and bur oaks almost never die of oak wilt. In a natural stand of trees, about 15% of live oaks will be unaffected although we don’t understand why.
The only real treatment for oak wilt is injection of a fungicide called Propiconazole. The best-known brand is Alamo, but there are other brands as well. The injections should be made via macro-injection in the root flares for best results. The fungicide arrests the progress of the fungus in every part of the tree with working vascular tissue above the injection sites. The fungicide can’t flow downward, so the roots can still transmit the fungus to other trees. Treatment doesn’t always mean the tree will survive. If there are symptoms in more than thirty percent of the canopy, the tree has a greatly decreased chance of survival, as it means that at least that much of the vascular system is already compromised.
According to recommendations by the Texas A&M Forest Service, injections should be done twice, with a two year interval in between. Any other treatments won’t affect the survival of the tree.
Hopefully this post is informative for you, but contact an arborist (preferably one who is oak wilt certified) with any other oak wilt questions or concerns!