Thousand cankers disease

What is thousand cankers disease?

Thousand cankers disease is caused by the fungus Geosmithia morbida, carried by the walnut twig beetle (Pityophthorus juglandis). Colorado State says the beetle is tiny, about 1.5 to 1.9 mm long, and Purdue says it is native to the southwestern US. Dark cankers form around each beetle tunnel in the bark. The cankers grow and merge until the inner bark dies and the branch dies.

Black walnut is the most susceptible host. In the western US, Colorado State says infected trees often die within about 2 to 3 years after leaf symptoms show, and infestations can go unnoticed for 5 to 10 years before that. The disease reached the East in 2010 and was found in Pennsylvania in 2011.

In Pennsylvania the damage has been milder than feared. The PA Department of Agriculture says surveys and research showed less severe damage than expected, and that the disease has not been shown to kill trees there. The state repealed its quarantine in 2022 but still watches for the beetle and disease.

Symptoms and signs

• Yellowing and thinning leaves in the upper crown
• Small branches "flagging" with yellow, wilted leaves
• Branch dieback that moves into larger limbs
• Tiny round beetle exit holes in the bark of branches
• Many small dark cankers under the bark that merge together

Thousand cankers disease identification: what to look for

• Symptoms: Early signs are yellowing and thinning of the upper crown and flagging of small branches. Later, larger branches die and big areas of foliage can wilt quickly.
• Damage: Small, dark cankers under the bark around beetle tunnels, which merge and kill the inner bark.
• Adult: The walnut twig beetle is about 1.5 to 1.9 mm long, yellowish-brown and about three times as long as wide.
• Timing: Colorado State says surveys are best done in July and August, sampling flagged or recently killed limbs about 2 to 5 cm thick.

Disease cycle

Walnut twig beetles carry spores of Geosmithia morbida. They bore under the bark of walnut branches to mate, and females start egg tunnels. The fungus grows around each tunnel to form a small canker. Purdue says thousands of beetles may attack one tree at a time, delivering a possibly lethal dose of the fungus. Over time the cankers merge, all the inner bark is killed, and the branch dies. Beetles spread naturally between nearby trees and over long distances in walnut logs and firewood with bark.

When to look: Look for crown yellowing and flagging in mid to late summer. July and August are best for sampling branches.

Thousand cankers disease look-alikes

A few things get mixed up with it:

• Drought and site stress: Purdue notes TCD decline can look like general stress. The tiny exit holes and many small cankers under the bark point to TCD.

How to manage thousand cankers disease

Cultural control

• Do not move walnut logs or firewood with bark from areas where TCD is known.
• Remove wood from infested trees promptly. Colorado State says chipping is the best approach; burying or holding wood in a secure, isolated place are alternatives.
• Removing trees with early symptoms may slow spread within a community.
• Milled wood without bark, finished products and nuts are generally not regulated in other states (Colorado State).

Chemical control

No effective control has been found. Colorado State says insecticides have limited value and trunk sprays did not stop progression in trees with early symptoms. If any treatment is considered, use only products labeled for this use; the label is the law, and tree treatments should be done by an ISA Certified Arborist with a pesticide applicator license (see pesticide applicator license guide).

Use only products labeled for this disease and this site; the label is the law. For trees, work with an ISA Certified Arborist. If you hire someone to apply pesticides, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Prevention

• Buy firewood locally and do not bring walnut wood with bark from out of state.
• Check walnut logs and wood products for beetle holes.
• Watch high-value walnuts for crown yellowing each summer.

When to call a pro

Call an ISA Certified Arborist or consulting forester if a walnut shows top-down yellowing and dieback. Purdue's Indiana bulletin says to have a DNR district forester or consulting forester evaluate the tree, and that landowners should not harvest black walnut just because the disease was found nearby.

Related guides: Tree database: mature size and planting distance · Pennsylvania tree identification · Firewood rules by state · All pests and diseases

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