Trenching as a Root-Graft Disruption Treatment for Oak Wilt

To prevent the spread of oak wilt, it is recommended that severing the roots between infected and healthy trees be part of the overall management plan. Here, a trench between trees was dug 48 inches deep and 8 inches wide. Photos courtesy of the author.

Most arborists are familiar with oak-wilt (Ceratocystis fagacearum) ) symptoms and management guidelines. Oak wilt is a fungal vascular disease that is spread through root grafts or by nitidulid beetles carrying fungal spores from an infected oak to an open wound on another oak.

Oak wilt affects all oak species, but because oak wilt affects the red and white oak groups differently, red oaks being more susceptible, quick action must be taken when other highly susceptible red oaks are so close to one another that root grafting can occur. Root grafting commonly occurs when trees of the same species are growing within up to 100 feet of one another. Infected red oaks in the upper Midwest typically start defoliating in mid-July, causing tree death by the end of the growing season. In a situation such as this, it is recommended that severing the roots between infected and healthy trees be part of the overall management plan.

In more than three decades as an arborist diagnosing and dispensing oak-wilt-management guidelines, I had never been involved in the root-graft disruption process. How many arborists provide this service? Not many, would be my guess. Would an excavating contractor even be willing to take on this specific kind of trenching? I would rather have an arborist doing the trenching, as someone who knows and cares about the trees and who can make informed decisions for the benefit of the trees. Here is my story.

The two infected red oaks are seen just left of center here. They would get the trenching.
The two infected red oaks are seen just left of center
here. They would get the trenching.

It all started in mid-July of this year when I received a call from a client in rural Dubuque County, Iowa. She was concerned because she had lost a red oak last year and now the one adjacent to it was starting to lose its leaves. Dan Schuster with Schuster Tree Service, a local arborist and friend of mine, referred her to me to confirm his suspicion of oak wilt and to formulate a management plan. This was a big deal to the homeowner, because she also had an additional seven oak trees in her front yard.

Having looked at the tree on July 26, there was no question in my mind it was oak wilt. I identified the tree as a red oak (Quercus rubra). It was within 10 feet of a dead red oak. The leaves started dropping from the upper crown and were browning from the outer margins toward the stem. Streaking was present in the sapwood of the dying branches.

Dan Schuster operates the trencher.
Dan Schuster operates the trencher.

I presented the client with a four-part management plan in an effort to preserve the remaining red oaks in her front yard:

• Girdle the bark on both of the infected trees to facilitate the bark drying out, preventing the possible formation of spore-producing fungal mats.

• Have the remaining healthy red oaks injected with Propizol (propiconazole) to protect them from infection.

• Trench around the trees as a root-graft disruption treatment.

• Remove the two infected trees in the late fall or winter.

Not knowing who to recommend for the trenching, I decided to convince Dan this was something we could do. In recent years, the major rental companies have filled their lots with portable aerial lifts, small brush chippers, stump grinders and, yes, even small trenchers. Dan had a heavy-duty truck and trailer for transporting such a unit, so we decided to give it a try.

The finished product.
The finished product.

Dan immediately girdled the bark of the two trees, and I made arrangements to have the remaining healthy oak trees injected, which was completed on July 31. Trenching was scheduled for two weeks later. That would give the Propizol time to translocate throughout the trees, providing them protection from any possible root-graft transmission taking place.

Luck was on our side that Friday morning, August 9. The trenching went smoother than I ever would have anticipated. From start to finish, we completed the job within two hours. The front yard contained a clay loam soil with plenty of room to maneuver. There were no underground utilities to work around, and we didn’t run into as much as a single pebble.

The infected trees were close together, as I already indicated, and were separated from the nearest healthy trees by a minimum of 22 feet. All of the red oaks on her property averaged 24- to 28-inch DBH and were in good health.

The trencher we rented was a rubber-track Vermeer RXT 250, which could cut a trench 48 inches deep and 8 inches wide. The trencher teeth were very much like the stump-grinder teeth we all are familiar with and set in a similar fashion. They cut through the roots as opposed to tearing them apart. The unit wouldn’t cut a round or oval trench, so we had to cut a large rectangle around the two trees, which worked out well.

Dan started trenching 7 to 8 feet away from the infected trees. The soil was dry and became finely pulverized, which facilitated backfilling in the end. As Dan trenched to a proper depth of 3 to 5 feet, I followed along with a hand trowel, hand pruner and pruning saw, exposing the cut roots and clean cutting as many as we encountered. I wanted nice clean cuts to promote good callus-tissue formation and root initials.

To my surprise, the root sizes ranged from that of a lead pencil to 2.5 inches in diameter. Larger roots are easier to cut than branches of similar size because they have not lignified to the same degree. A tree-planting spade was just the right width to fit in the trench to clear the area where the trenches intersected. The overall trench rectangle measured 17 feet by 30 feet.

When it came to backfilling, a stiff garden rake was all that was needed to pull the pulverized soil back into the trench. The rake also fit in the trench sideways to occasionally tamp the soil back into place. When we were finished, the front yard was as neat as a pin.

As it turned out, this was probably a best-case scenario, but root-graft disruption may be a service more arborists want to provide.

Steven Pregler is an ISA Certified Arborist and retired city forester for the city of Dubuque, Iowa.

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