Oak Wilt Control on the Chequamegon-Nicolet National Forest Using the Root Rupture Method (USFS)

State or Province
Nearest city or town
Describe the location
Numerous sites within the Lakewood/Laona Ranger District, Chequamegon-Nicolet National Forest
US Forest Service
Cover type
Plant community detail and growth stage
AVb (Acer/Viburnum)
Forest health threats
Adaptive silviculture options
Silviculture system
Estimated year of stand origin
Site index
68 feet
for species
northern red oak
Brief silvicultural objective
Cost effective and operationally feasible control of oak wilt in a forested setting.
Site preparation method
Soil texture
Soil details
Kennan fine loamy sand / Padus sandy loam

45.35, -88.433333


Stand origin: The area was historically dominated by red and white pine which was maintained by frequent surface fires (fire return interval of approximately 7 years).  Most of the pine was liquidated in the 1890’s and rafted down the nearby Oconto River.  Following the cutover, advanced understory regeneration sprang up.  This was held in check by repeated fires.  The last major fire burned through the area in 1925, resulting in an oak-dominated stand.

Stand area: variable, multiple stands- 10s to 100s of acres

Treatment area: variable, multiple treatment areas- typically 0.1 to 0.5 acres, occasionally larger

Overview: Oak wilt was first identified on the District in 1997 at Boulder Lake Campground and was effectively treated with a vibratory plow the same year. In the summer of 2001, oak wilt was discovered in an active timber sale unit near Waubee Lake. Many infection centers were subsequently found scattered over a large area in the vicinity.

Due to the uneven topography and rocky conditions at many of the sites, conventional treatment with a vibratory plow wasn’t feasible. Manfred Mielke, a pathologist with Northeastern Area State and Private Forestry, proposed an alternative method of treatment for use in a woodland setting.  It has, since, come to be known as the “root rupture method”.

This method involves cutting and removing infected and adjacent healthy trees and then using an excavator to rip out and overturn the stumps and root wads. In doing this, the root grafts are broken and diseased tissues are isolated from neighboring healthy oaks. To our knowledge, this method had never been attempted anywhere else prior to being developed on the Chequamegon-Nicolet.

We treated the infections in this manner in 2001 and 2002 and found the treatments highly effective. However, we didn’t begin annual detailed on-the-ground monitoring until 2004 and so precise figures of the effectiveness prior to that time are not available. We have continued to treat oak wilt infections in the area annually from 2004 through 2017. 

Silviculture Objective(s)

  • To contain infection centers early- before they can spread into surrounding areas.
  • To reduce the amount of oak wilt inoculum that can be spread overland by Nitidulid beetles.
  • To complete these treatments in a cost-effective manner.
  • To complete the work in a manner that is operationally feasible.
  • To prepare the sites for natural regeneration to a mixture of species.

Pre-treatment stand description and condition

Stand establishment and management history: 

Figure 1: A typical oak stand in the vicinity of the treatment areas. Note the high quality of the trees and the advanced regeneration that is present.

As noted above under Stand Origin, these stands originated in 1925, when they were released by the last large landscape-level fire to burn through the area.  The regenerating stands were a mixture of aspen, paper birch, red oak, and red maple.  In the 1970’s initial entries focused on removing aspen, which was mature and highly-valued for pulpwood.  Second entries in the 1980’s continued the removal of aspen- as well as much of the paper birch.  In the late 1990’s and early 2000’s the focus was on density management and improving stand quality by thinning the oak and culling out much of the red maple. The consequence of this management history was a landscape of high quality, high value oak sawtimber and relatively little within stand species diversity.  This has rendered the stands vulnerable to oak wilt and other pests, such as two-lined chestnut borer.

Another concern is that the stands are now over 90 years in age.  Our management has, generally, been successful in recruiting advanced oak regeneration in preparation for final rotation.  Recognizing the consequences of past management (reduced within-stand diversity), we have also begun to foster the establishment of conifers (mainly white pine) to meet long-term objectives of mixed pine-oak stands.  

Pre-treatment species composition: 

Greater than 80% of the total basal area in the treatment areas is northern red oak.

Pre-treatment growth and stocking: 

Variable.  Typically 100-130 ft2/acre, 12-18 inches average DBH, and with a normal distribution.

Pre-treatment forest health issues: 

Figure 2: An actively wilting oak resulting from an overland infection by Nitidulid beetles.  Immediate treatment of such a site will usually stop the spread of the infection.

The one common element across all treated sites was the presence of oak wilt infection centers that threatened to spread to adjacent oak trees.  Another complicating factor is the presence of two-lined chestnut borers, which can sometimes confuse the diagnosis. Local deer densities are approximately 10-15 deer/square mile.

Landowner objectives/situation: 

A landscape dominated by even-aged oak and aspen.  Oak sawtimber and aspen pulp are the main timber objectives and upland game and non-game species benefit from this management.  Oak wilt spread has been greatly limited.   In the long-term, we’d like to reestablish red and white pine throughout the area in an effort to move the landscape closer to its historic condition and increase resiliency to insect and disease.

Silviculture Prescription

Find the original infected oak and mark it for removal.

Figure 3: An infection center marked for treatment.  Harvest and stump removal normally takes place between October 15 and December 31.  

First Ring: Add the DBH of the original infected oak to the DBH of the nearest neighboring oak in question.  Using the chart below, mark the tree if it is within the limiting distance shown.  If there is a clump of 2+ trees, add each DBH to the total when figuring out your limiting distance (i.e. clumps will have a higher probability of being within the limiting distance).

Second Ring: Once the first ring is completely marked, begin a second ring.  This time instead of using a single oak tree in the middle, use multiple oaks on the outer edge of the first ring to determine limiting distances for the second ring of trees.

Only mark non-oak species for removal if they are merchantable and within 5 feet of an oak that is being removed (due to treatment of stumps as seen below in design and mitigation measures)

When marking trees, use yellow paint and paint a full ring at DBH height (2” thick) as well as a 12” long stump mark.

Below is the chart used for limiting distances (after Bruhn et al,1991):

Combined DBH (inches) Inter-tree distance (feet) Combined DBH (inches) Inter-tree distance (feet)
2 3.9 26 50.4
4 7.8 28 54.3
6 11.6 30 58.2
8 15.5 32 62.1
10 19.4 34 66.0
12 23.3 36 69.8
14 27.2 38 73.7
16 31.0 40 77.6
18 34.9 42 81.5
20 38.8 44 85.4
22 42.7 46 89.3
24 46.6 48 93.1

Severing of Root Wads (Stump Pulling)- all stumps shall be dug out of the ground, severing any root graft connections so as to prevent the spread of the Oak Wilt pathogen. All trees identified with paint as part of the included timber, are to be treated.  All stumps so identified must be completely severed from the roots remaining in the ground. All severed stumps shall be left in the hole where they were extracted, and not covered or buried.  An excavator works best for this requirement of the sale.

All merchantable wood shall be removed and processed by March 15, 2018.  It must be sawed into boards, cut and split as firewood, chipped, or otherwise processed to prevent the incubation of the oak wilt fungus.

Submerchantable wood > 4 inches in diameter must be cut into lengths no greater than 4 feet to prevent the persistence of fungal mats in the treatment sites.

What actually happened during the treatment

Between 2004 and 2016, 239 separate sites have been treated using the root rupture method.   A typical treatment area resembles a large canopy gap or a small group selection. 

About 29,000 oak stumps have been dug up as part of the treatments.  This results in notable mineral soil exposure in the treatment areas.  Surprisingly, most of the treatment areas are heavily stocked with advanced oak regeneration within a few years after the treatment.  As noted earlier, previous shelterwood preparatory cuts have been successful in accruing oak regeneration in the understories of the stands.  The removal of the overstory and soil disturbance eliminates much of the competition that would normally impede the development of the oak seedlings and saplings.

At the same time, we’ve seen robust regeneration of seed origin seedlings from a wide variety of species ranging from paper birch to white pine.  Thus, the regenerating pockets are typically quite diverse.

Post-treatment assessment

Figure 4: A treatment area immediately following treatment.  The disturbed sites are quickly revegetated with early and mid-successional tree species.

Monitoring of the treatment sites has shown that, approximately 71% of the time, no infections were found for a period of four years after the initial treatment.  On those sites in which border trees subsequently wilted, follow-up treatments were conducted.   Greater than 90% of the time, the infection was no longer active for a four year period following two treatments or less. 

Normally, within 4-5 years following the treatments, the sites are heavily stocked with a diverse assortment of early to mid-successional species > 6 feet tall.

Figure 5: Treatment sites quickly revegetate with early and mid-successional tree species.  A scene like this is typical after about ten years.

Plans for future treatments

All known sites are monitored for a minimum of five years.  As noted, some of the treatments are not successful on the first try.  Likewise, as we go about our monitoring, we find new or previously unidentified infection centers.  Most of these will be prepared for treatment and it is our intent to continue putting out these “spot fires” in the future.  Over time, the distribution of known infection centers has been gradually reduced to a core area.  We know that the goal of oak wilt elimination is unrealistic in our area, but it is our hope that we can limit the amount of contagion by limiting its distribution.  Our best success has always been in those sites that have only recently become infected. Much like firefighting, the success rate has been the greatest in the areas with the smallest perimeters.

Costs and economic considerations

Under the contracts, the purchasers have bid on the value of the timber and, at the same time, submitted a bid for the cost of the stump removal.  The value of the timber has offset the cost of the stump removal and surplus funds have been used for restoration work elsewhere on the Lakewood/Laona District.

Approximately $1.3 million in gross revenue was generated from these sales.  The cost of the service work funded by the sale of the timber has been just under $250,000.  The treatments have been completed over this 13 year period for a net gain of a little over $1.1 million.

Other notes

This treatment method has been the most effective when applied in the early onset of infection.  Most treatments have been effective in cases where there was one wilting oak at the infection center.  Other sites, with multiple dead and wilting trees at the center, often require subsequent treatments to control the infection.

At times, we’ve found oak wilt to be an elusive target. In some cases, we identify and treat an infection, monitor it for several years and find no signs of infection. Then, after several years without symptoms, a tree on the edge of the treatment site wilts. Was this a new overland infection created by beetles?  Or did the infection somehow creep across the control line?

Treating and tracking numerous treatments presents accounting challenges.  For example, two sites may be located 100 meters apart and have separate identifiers.  Both of them may be treated, resulting in two small openings that are separated by a small area of trees.  Later, one of the sites may require a follow-up treatment, which effectively creates one larger opening.  If, at a later date, it is determined that another follow-up treatment is needed, it creates a dilemma: which treatment site was effective?  Were they both ineffective?

Likewise, it can sometimes be difficult to determine whether the treatment was ineffective or if there was a new overland infection caused by beetles.  For example, if, following treatment, a wilting tree appears 50 feet from the edge of the treatment area, should it be viewed as a new overland infection- or as a failed treatment?

Situations like these complicate our ability to definitively state whether some treatments were or were not effective.  It’s not all black and white.  Still, after 15+ years of treatments, there is little question that this method is highly effective.

Summary / lessons learned / additional thoughts

Our treatment efforts are working. Over the past fifteen years, we have been successful in reducing the number and size of outlying infections. This is consistent with our ultimate goal of cordoning off infection sites into a few areas that can be more easily monitored and managed.

Early detection and treatment is the key. It is far easier and more effective to treat infection sites of, perhaps 15-20 trees than larger sites with several hundred trees. The larger the site is, the larger the perimeter and the greater the likelihood that the infection will “creep” across treatment lines.

In the future, we intend to continue monitoring and evaluation of existing sites and conduct an on-the-ground assessment of likely new infections. Controlling oak wilt is clearly a long-term proposition with periodic setbacks following summer storm events, but with our initial results we are hopeful that we can limit it to a few hotspots in the Waubee Lake Area.

Bruhn, J.N., Pickens, J.B., D.B. Stanfield. 1991.  Probit Analysis of Oak Wilt Transmission Through Root Grafts in Red Oak Stands.  Forest Science, Vol. 37, No. 1. Pp.28-44.  

This case study was developed with support from the United States Department of Agriculture's National Institute for Food and Agriculture (USDA-NIFA), Renewable Resources Extension Act (RREA). Project #MIN-44-E02, principal investigator Eli Sagor, University of Minnesota.