Protecting Your Black Walnut Trees from Thousand Cankers Disease

Your black walnut just isn’t looking right. The canopy is thinning, leaves are turning yellow too early, and after this summer’s extreme heat, you might assume it’s just stress. But what if it’s something more serious?

These symptoms can also point to thousand cankers disease, a fast-moving threat that works silently beneath the bark. If ignored, it could kill your tree within a few years, leaving behind a dead, brittle hazard near your home. Here’s what you need to know about thousand cankers disease, how it infects trees, and what you can do to protect your black walnuts.

Key Takeaways

• Thousand cankers disease is a fatal fungus spread by tiny beetles that has killed black walnut trees in Fairfax County since 2012, with no cure currently available.
• Infected specimens typically die within three years of exhibiting symptoms, such as yellowing leaves, dead branches, and small bark holes caused by beetle activity.
• Virginia lifted its quarantine in 2024, but this doesn’t mean the threat is gone; we’ve still found and removed infected trees in Northern Virginia.
• Prevention focuses on reducing tree stress through proper mulching, winter pruning, watering, and soil fertilization.
• A professional arborist inspection is critical at the first sign of symptoms, as early detection allows for safe removal before the tree becomes a danger to property.

Within a few years, thousand cankers disease can kill a healthy black walnut, like the one pictured here. Photo courtesy of R. A. Nonenmacher, CC BY-SA 4.0, via Wikimedia Commons.

What Is Thousand Cankers Disease?

Thousand cankers disease (TCD) is a deadly threat to our native black walnuts (Juglans nigra), caused by a fungus carried by the tiny walnut twig beetle.

The disease was initially a problem in the western part of the United States in the 1990s, killing a large portion of the black walnut population. It began spreading eastward, eventually appearing in Fairfax County in 2012.

How Thousand Cankers Disease Spreads and Attacks Northern Virginia Trees

The fungus, Geosmithia morbida, causes cankers (small patches of dead wood) to form in the tree, preventing it from transporting water and nutrients throughout the trunk. Without the ability to transport essential nutrients, the disease will girdle the tree and kill it.

But how does the fungus invade your tree? The answer is a minuscule bug called the walnut twig beetle (Pityophthorus juglandis). The beetle is about 1/16 inch long and has a reddish-brown color and carries the fungi with them when they burrow into the tree to lay its eggs. The larvae then excavate galleries in the wood (not unlike the emerald ash borer), another common problem in Northern Virginia).

Once the larvae grow into adults, they will emerge from the tree and find new hosts to continue the cycle. The beetles and fungi share a symbiotic relationship, with the fungus serving as food for the beetles, and the beetles transporting the fungus to new hosts.

Symptoms of Thousand Cankers Disease

One of the things that makes TCD such a complex disease to control is the subtlety of the symptoms. Once you detect the symptoms in your tree, it will typically die within a few years. Some of the signs of TCD include:

• Dead Branches: Branches in the canopy will initially develop yellow leaves early in the infection process. The leaves will then quickly turn brown and appear wilted, eventually killing the entire limb.
• Cankers: If you look closely at the bark, you’ll notice small dead areas. These are the cankers that give the disease its name. They are often oblong and may have dark stains near them or bark cracking.
• Bark Holes: Tiny entrance and exit holes give you evidence of beetle activity. You can find these holes on dead and broken branches.

PRO TIP: If you notice any of these signs on your black walnut tree, do not hesitate to call an arborist. They will examine the tree’s condition, diagnose the problem, and remove it if it has a TCD infection.

A black walnut tree with a dying canopy, a sign of thousand cankers disease. Photo courtesy of Jeffrey Beall, CC BY-SA 3.0, via Wikimedia Commons.

Can You Cure Thousand Cankers Disease of Black Walnuts?

Unfortunately, there’s no cure for thousand cankers disease as of 2025. Once a black walnut is infected, no treatment can stop the decline, and most trees die within three years of showing symptoms.

Researchers are still investigating possible chemical treatments, but for now, the best defense is early detection and prompt removal of infected trees to help protect nearby walnuts.

What Fairfax County Is Doing to Contain Thousand Cankers Disease

Shortly after it became apparent that TCD was present in our area and would be a problem, lawmakers enacted a quarantine in areas of Fairfax County. The goal was to slow the spread by prohibiting the movement of black walnut wood out of the quarantine zone, helping protect uninfected areas of Fairfax County and beyond.

In 2024, after studying the evidence, lawmakers determined it was time to lift the quarantine in Fairfax County. While this may seem like a cause for celebration, the reality is a bit more murky than that.

“After the quarantine was lifted, some homeowners assumed the threat was gone. But we’re still seeing active infections and removing dead black walnuts across the county. If you have one on your property, it’s important to monitor it closely and call an arborist at the first sign of decline.” – Peter Hart, Riverbend ISA Certified Arborist

How to Prevent Thousand Cankers Disease

Since there’s no cure – and no effective chemical prevention – your best defense against thousand cankers disease is keeping your black walnut trees healthy and resilient.

How? Reduce tree stress.

Researchers from the U.S. Forest Service conducted a study of stressed black walnuts and found that stressed specimens attracted bark beetles and other harmful insects. To lower the stress on your black walnut during the summer (when adult beetles are most active), we recommend:

• Adding Mulch: Organic mulch will naturally decompose to add nutrients back to the soil. It also helps keep soil and trees cooler during a hot Fairfax County summer.
• Waiting to Prune Until Winter: Save your pruning for the winter when the black walnut is dormant. Dormant pruning causes less stress and allows it to heal from the wound before insects and diseases become active.
• Watering During a Drought: Our area suffered a drought throughout the end of 2024 and the beginning of 2025. Dry spells like this and their lingering effects can increase tree stress. Provide supplemental water until your tree receives sufficient water from natural sources.
• Fertilizing as Needed: Have a professional conduct a soil test to determine any nutrient deficiencies in your soil. They can then add fertilizer to your soil to correct the problem and leave your tree with the necessary nutrients for vigor.

Frequently Asked Questions About Thousand Cankers Disease and Black Walnut Trees

There is substantial information available about TCD and black walnuts, which can be overwhelming for homeowners. To help simplify things, we’ve answered some common questions homeowners have about the disease and black walnuts.

Does thousand cankers disease only affect walnut trees?

Yes, thousand cankers disease only impacts trees in the walnut family (Juglans species). Black walnuts are the most commonly affected, but other walnut species can also be vulnerable. The disease does not spread to non-walnut trees.

Should I preemptively remove any black walnuts on my property?

No, we do not recommend preemptively removing black walnuts on your property. TCD is not prevalent enough in our area to justify removing all black walnuts. By keeping them healthy, they can stand for years to come.

Why are black walnuts important trees?

The black walnut tree is an integral part of our ecosystem. The tree provides shelter and food for squirrels, chipmunks, and many birds. The walnuts themselves also make for a healthy snack for people. Losing this species would have long-term ecological repercussions.

Don’t Let Your Tree Be a Victim – Get Expert Help from Riverbend Today

If you have a black walnut on your property, you can’t afford to ignore it as we face the threat of thousand cankers disease. TCD will kill your walnut in a matter of years, but with a little work, you can give it a fighting chance of avoiding an infection. If you’re worried about your tree, the team at Riverbend is ready to help.

Our ISA Certified Arborists can examine your trees and provide an honest appraisal of their condition. If they aren’t infected, we’ll suggest ways to keep them unstressed and safer from TCD. And if we find a sick walnut, we’ll perform professional removal to protect your property. Call us today at 703-402-9366 or request an estimate online.