How to Manage Oak Decline | By Kirsten Stewart, Village Forester

Several residents have contacted the Village with concern about the decline of oak trees they are observing throughout the Village.

The oak decline and death residents are noticing is likely from a combination of causes that are all contributing to the problem, including the age of the trees, wet soils/fluctuations in soil moisture, wounds and/or breakage from storm damage creating entry points for bacterial and fungal pathogens and diseases such as bur oak blight and oak wilt, construction impact and environmental stress. The trees are spreading their resources thin to combat these various issues, and it takes a toll over time. As a result, the trees release stress pheromones and other compounds which are picked up on by various insects and other trees.

When oaks are stressed, they attract the 2-lined chestnut borer, which is an opportunistic native insect that damages the plumbing system of oaks in much the same way ash trees are damaged by the emerald ash borer. (Both insect species have similar life cycles, the larvae feed under the bark destroying the structures that conduct sugars and water up and down the trunk, and 2-lined chestnut borers leave D-shaped exit holes in the bark of oaks, just like EAB does with ash trees.) While the 2-lined chestnut borer causes internal damage to oaks which results in limb and crown dieback, and ultimately finishes off a stressed tree, it is really a symptom of other underlying issues.

There are a couple of steps you can take to assist and protect your trees:

1. General supportive care is recommended if trees are stressed: Fertilizing every few years is an option, coupled with insecticide treatment to control the 2- lined chestnut borer. Most of these trees are rather old and fertilizers should be used sparingly. We always recommend soil testing prior to applying any fertilizers to determine what (if any) deficiencies exist. If there are no nutrient deficiencies, fertilizing is a waste of money and effort. Various tree and lawn care companies can conduct soil testing, or you can easily collect soil samples yourself and submit them to a local County Extension Service that assists residents with soil testing or to a neighboring State University that has a soil testing lab (The University of Illinois has a Plant Clinic, but not a Soil Clinic); a link to these various State University testing labs is below, and detailed information on how to collect and submit samples can be found on their websites. https://extension.illinois.edu/soil/soil-test-labs

2. Trees can be treated for 2-lined chestnut borer with various insecticides, some are available to homeowners at hardware and big-box stores (these are mostly products containing imidicloprid). Licensed Applicators have access to products with higher imidicloprid concentrations as well as several other products with different active ingredients which have a range of application types and re-application times (i.e. annual, every 3 years, etc).

If you are applying insecticide yourself (imidicloprid), it is important to keep in mind this will most likely be a soil drench, or sometimes a trunk spray, and the insecticide is non-specific so it will kill any insects (beneficial insects, pollinators, etc.) it comes in contact with. Licensed applicators have more options, and may apply the chemicals by soil drench, soil injection, trunk injection, bark spray or foliar spray. Trunk injections keep the product contained within the tree and reduce the likelihood of the product migrating or translocating to unwanted areas. If the potential for drift or translocation or impacting beneficial insects is a concern, it is important to discuss the product, type of application, timing of application and how the product targets the insects with your applicator so you have a clear understanding of the process and you can make sure the treatment methods align with your property management objectives.

Trunk injections should only be conducted by a Certified Arborist or under the direct supervision of a Certified Arborist to avoid unnecessary damage to trees during the injection process.

When assessing oaks for the possibility of insecticidal treatment for 2-lined chestnut borer, trees that are exhibiting less than 20% or so crown dieback will likely have enough viable plumbing to distribute the product throughout the tree. If the insect infestation is localized in some branches, those branches can be pruned out and property disposed of as well. If there is more than 25% dieback in the crown, there is likely too much internal damage to the tree’s plumbing system for the product to be effectively distributed, but you may want to try treatment to see if it is helpful if you have some trees that you really want to try and save.

3. Oak wilt is a problem that is known in Lake County. It will kill red oaks in 1 growing season. White oaks can survive many years with the problem, but the stress of battling the disease makes them a target for 2-lined chestnut borer, and this can hasten their decline as well.

We recommend if oak wilt is suspected, testing should be done to confirm this is the case, so neighbors with adjacent oaks can be informed and take action as necessary. Your arborist can collect samples from suspected trees and test them if their company has that capability or submit the samples to the University of Illinois Plant Clinic for confirmation. The plant clinic website has information on how to collect and submit samples. Generally, you want samples that are at least 1” in diameter and 6” – 10” in length. Branches with oak wilt exhibit black streaking under the bark when it is peeled back. It is important to take samples from suspected branches to avoid false negative tests. In addition, samples should be taken early in the week to avoid sitting over the weekend so the quality of the samples doesn’t degrade, and the samples should be packed in a Styrofoam cooler with a couple of cooling packs to preserve the quality of the samples.

Here is a link to the Plant Clinic for more info: https://extension.illinois.edu/plant-clinic

Oak wilt is a fungus that is transmitted aerially by beetles, and below ground via root grafts. Depending on your particular site and accessibility issues, your arborist may recommend various treatment and containment options. If there are nearby trees with confirmed oak wilt, a fungicide containing propiconizole can be administered every other year by a Certified Arborist to help prevent infection in stand-alone trees, and also help manage the issue and support infected trees with grafted roots. It is important to note that treatment will not prevent infection when roots are grafted with neighboring oaks, but it will provide support to the tree to fight the infection. Roots of oaks with oak wilt can be severed via root pruning to prevent or limit transmission to nearby healthy trees. Your arborist can devise a trenching plan to sever the grafted roots of oaks you are concerned about. Roots of nearby oaks should be severed prior to cutting down a tree that is known to be infected so the nearby healthy trees are less likely to become infected if their roots are grafted. In addition, your arborist may recommend the removal of oaks that appear healthy which are adjacent to infected trees to create a transmission barrier. As always, oaks should not be pruned from May-November to avoid creating wounds which will attract the beetle that transmits oak wilt, as well as providing entry points for pathogens. If pruning or cutting cannot be avoided due to storm damage, breakage, etc., wounds and breakage should be cleaned up and addressed as necessary, and a pruning sealant should be applied to fresh cuts immediately.

Below is a link to a Forest Service reference guide explaining and identifying various oak diseases: https://hortnews.extension.iastate.edu/files/qa/files/howtorecognizecomm...