Oak wilt was first identified in the early 1940s in Wisconsin, with subsequent discoveries in Texas (early 1960’s), and New York (mid-2000s). The prevailing thought is that oak wilt is a non-native disease, possibly originating in South or Central America. It is also believed that the disease had spread through much of its current area of infection before its discovery. As of early 2023, oak wilt was determined to be present in 24 states, ranging from Wisconsin south to Texas, east to New York, but not very far south of Tennessee. There are still large swaths of oak habitat that are unaffected but many new disease centers are reported each year.
Oak wilt is unfortunately a very generic name for a very specific disease. After all, oak trees can wilt for any number of reasons. Oak wilt disease is caused by a fungal organism known as Bretziella fagacearum. All oak species in our state are potential disease hosts and are at risk of infection if conditions are met. Once infected, the fungus moves through the vascular tissue of the sapwood (water-conducting tissue) clogging up the “pipes”, reducing water flow, and inducing a tree’s own defense systems which act to shut down vascular vessel elements that have been invaded. The result is the entire or partial wilting of an oak tree that can lead to total wilting over 3-6 weeks or a slower wilting over a period of several years. How does all this happen? Why aren’t all oak trees already infected? How does all this happen? Why aren’t all oak trees already infected?
Well, there are several other actors in this play and certain circumstances that favor infection. We will focus on a couple of major characters and the part they play.
Sap-feeding beetles are attracted to their food, and as one entomologist has said, “They will fly as far as they need to in order to find food”. In the case of sap beetles, this could be anywhere from half a mile to several miles. How far and high they can fly also depends on prevailing winds, local topography, and other factors. The important thing to note is that when there is a fresh wound (known as an “infection court”) on an oak tree, sap beetles are attracted from afar and fly toward the wounded oak. If these beetles have been feeding on a tree that was infected with oak wilt disease, they will bring the disease inoculum with them. In one of those amazing twists of nature, B. fagacearum can form sporulation mats just under the bark of certain infected trees. These mats can press against the bark from the inside and split the bark open, emitting a sweet, fruity odor that attracts oak sap beetles.
Figure 1: Oak wilt in Ohio: Counties with confirmed infections as of 2016.
Source: Oak Wilt in the Northeastern and Midwestern States: A Story Map by the USDA Forest Service Northeast Region
Other characters in this play are those who cause wounds on oak trees. Oak trees are subject to storm damage, construction damage, line clearance pruning, tree harvesting or removal, and maintenance pruning such as that performed by arborists. All of these events will cause potential infection courts that beckon to oak sap beetles for about three days, after which their attractiveness lessens. As mentioned above, the tree will likely be infected with disease if a sap beetle carrying fungal spores arrives at the infection court. This process is known as “overland spread” because the disease is able to hitch a ride through the air from one area to another via the insect vector.
Once a tree is infected at a new site, the primary method of transmission is called “underground spread” and depends on the fact that oak trees (especially, but not necessarily, of the same variety) can form root grafts that are biologically active. That is, the contents of one tree can flow into another tree, and vice-versa. This includes pathogenic fungi such as B. fagacearum. In this case, communities of trees become vulnerable and begin to succumb to infection forming a recognizable, outward-expanding pattern of dead trees. At this point, the area is known as a “disease center”.
Let’s add one more dot, and then connect them all.
Oak sap beetles are the most active in Ohio from early March to late July with the highest periods of activity being April through July. During these times infection courts are at the highest risk of being visited by oak sap beetles which may be carrying fungal spores. There is reduced beetle activity later in the summer (August through sometime in early-to-late October) and of course, there is no beetle activity expected when temperatures cool down in fall/winter. For this reason, pruning should be limited to low-to-no-risk times of the year. The safest period for oak pruning in Ohio would be November through February, with some variation at either end of the window based on how temperatures act as winter and spring approach. All Ohio residents know that pre- and post-winter seasons in Ohio are anyone’s guess from year to year regarding what the temperatures will be!
If pruning is unavoidable (such as for removal of hazardous or storm-damaged limbs) the pruning cuts must be painted with latex paint or a suitable wound dressing within five minutes of the cut being made. In the case of storm-damaged trees (unexpected infection courts), the trees should be monitored for any symptoms of oak wilt so that steps can be taken to prevent infection of any neighboring trees. One of the methods that can be used to prevent underground infection is by injecting healthy oak trees with a fungicide that will inhibit disease infection.
The most important part of managing the health of oak trees where the threat of oak wilt exists is choosing the right time to prune. This is a cultural practice that has been successful in containing oak wilt in areas where it has otherwise decimated large stands of oak. The introduction of a chemical treatment should only be considered when there is a known infected oak in the vicinity. This is an approach that should be discussed with a qualified Certified Arborist who can guide the need for such a decision. If a healthy oak tree is not within root grafting distance of other oak trees, the only management method required is cultural: prune at a time when you are confident there is little or no risk of infection. If an oak tree is within root grafting distance, pay attention (along with your qualified Certified Arborist) to neighboring trees, talk to the owners/managers of those trees, and make decisions that aid in the preservation of both your tree(s) and the neighboring tree(s).
There is much more that could be said about oak wilt, including the symptoms to look for. This is an extensive topic as there are quite a few other diseases/ conditions in oak trees that could be mistaken for oak wilt. The best option is to enlist the aid of your qualified Certified Arborist to help determine if symptoms are indeed oak wilt, or if sampling is necessary for a laboratory test. A great resource to dive deeper into the topic is Oak Wilt in the Northeastern and Midwestern States: A Story Map by the USDA Forest Service Northeast Region.
A concise summary of the disease can also be found at the Ohioline - Ohio State University Extension: (Oak Wilt: Fact sheet by Dr. Enrico Bonello, The Ohio State University).
A handy USDA resource comparing symptoms on oak trees that are similar to oak wilt is How To Recognize Common Diseases of Oaks in the Midwest: A Quick Guide.
The Ohio Chapter ISA has made a training series available specifically for arborists interested in learning more about the disease. A list of Ohio arborists who have completed this training will soon be available on this site. Check out this flyer on Oak Wilt - When to Prune.
Oak Wilt in the Northeastern and Midwestern States
OhioLine, Ohio State University Extension: Oak Wilt Fact Sheet
Oak Wilt | Ohioline (osu.edu)
Juzwik, Jennifer; Appel, David N.; MacDonald, William L.; Burks, Susan. 2011. Challenges and successes in managing oak wilt in the United States. Plant Disease. 95(8): 888-900. https://doi.org/10.1094/PDIS-12-10-0944.