Thousand canker disease (Geosmithia morbida)

Common Names: TCD, thousand cankers disease of walnut, walnut thousand cankers

Description:

Thousand cankers disease is a deadly disease of walnut trees caused by the fungus Geosmithia morbida working together with a tiny insect, the walnut twig beetle [1][3]. The beetles tunnel under the bark and carry the fungus, which forms a small dead spot (canker) around each tunnel; over time, thousands of these cankers build up, girdle the branches and trunk, and kill the tree [1][3]. MISIN tracks it because it kills black walnut ? a valuable timber, nut, and wildlife tree ? and it has been spreading eastward, partly on moved walnut wood [1][2].

Identification:

Thousand cankers disease is a disease, so you identify it by its effects on walnut trees rather than by something you can pick up. The first thing people notice is the crown: leaves yellow, wilt, and thin, and branches die back, usually starting in the upper canopy and spreading downward over a year or more [1][3]. Under the bark of affected branches are the telltale signs ? many small, dark, oval "inky" cankers (dead patches), each often more than an inch long, that you can see by shaving off the outer bark, along with the tiny tunnels and pinhole-sized entry holes of the walnut twig beetle [3]. Any one canker is small, but a heavily attacked tree has thousands of them ? the source of the name [1][3].

Crown dieback is easiest to notice in summer, when healthy walnuts are in full leaf and a sick one stands out [1]. The cankers and beetle holes can be found year-round on infested branches [3]. Once a tree shows clear crown symptoms, it often dies within two to three years [3].

At-a-glance cues:

1. Walnut trees with yellowing, thinning crowns and branch dieback from the top down [1][3]
2. Many small, dark, oval "inky" cankers in the inner bark, seen by shaving the outer bark [3]
3. Tiny pinhole entry holes and short tunnels of the walnut twig beetle on branches [3]
4. Rapid decline ? death often within two to three years of clear symptoms [3]

Habitat:

Thousand cankers disease lives in walnut trees, so its "habitat" is its host. It affects walnuts (the genus Juglans), and black walnut (Juglans nigra) ? the prized eastern timber and nut tree ? is especially susceptible [1][3]. Wherever walnuts grow, in yards, woodlots, plantations, and forests, the disease and its beetle can take hold [1].

The walnut twig beetle and the fungus are thought to be native to the western United States, where they long lived on western walnut species without causing much harm [3]. The disease emerged as a serious killer of black walnut in the early 2000s, spread through many western states, and has since been found in several eastern states as well [1][3]. In the MISIN region it is treated as a watch species ? the goal is to catch and report it early, before it establishes [1][2].

Reproduction:

Thousand cankers disease spreads as a partnership between the fungus and the walnut twig beetle. The beetles fly to walnut trees, tunnel under the bark, and carry spores of the fungus with them, so new cankers form wherever they bore in [1][3]. The beetles fly only short distances on their own, so the dangerous long-distance jumps are caused by people ? moving walnut logs, firewood, and other walnut wood that still has live beetles and fungus inside [1][2]. A walnut far from any known outbreak often turns out to have the disease because infested wood was brought in [1].

There is no "seed-set" window to target here as with a weed. The key steps are prevention and watching: do not move walnut wood, keep walnut trees healthy, and watch for declining black walnuts (see Management) [1][3].

Impact and Damage:

Thousand cankers disease is not a danger to people or pets ? it harms walnut trees [1]. For a homeowner, it can kill a yard or woodlot walnut within a few years once the crown starts to decline, and the dead tree becomes a removal cost and a hazard [3]. There is no cure once a tree is heavily infected [1].

The larger concern is for black walnut, one of the most valuable hardwood trees in North America, prized for its timber and veneer, its nuts, and its value to wildlife [1]. The disease has killed many black walnuts in the western United States and threatens the tree across its native eastern range, where black walnut is far more common [1][3]. Losing mature walnuts removes a high-value timber resource and a fall nut crop that squirrels, other wildlife, and people use, which is why keeping the disease out of new areas is a priority [1][2].

Similar species:

Walnut twig beetle damage (from the beetle Pityophthorus juglandis) is the condition most tied to thousand cankers disease, since the beetle and the fungus occur together. Beetle tunneling alone leaves the pinhole entry holes and galleries, but it is the fungus that adds the many dark inky cankers around each tunnel that define the disease.

Butternut canker (caused by the fungus Ophiognomonia clavigignenti-juglandacearum) is another canker disease of trees in the walnut family, mainly attacking butternut. It is distinguished by larger, sunken, often sooty cankers with dark ooze on the trunk and branches, rather than the thousands of small beetle-associated cankers of thousand cankers disease.

General walnut decline from drought or other stresses can also thin a walnut's crown and cause branch dieback that resembles thousand cankers disease. It is told apart by the absence of the small inky cankers and the walnut twig beetle's tunnels under the bark.

Monitoring and rapid response:

Signs to watch for:

1. Walnut trees with yellowing, thinning crowns and branch dieback from the top down, most obvious in summer [1][3]
2. Many small, dark, oval "inky" cankers in the inner bark, visible when the outer bark is shaved [3]
3. Tiny pinhole holes and short tunnels of the walnut twig beetle on branches and the trunk [3]

What you can do:

1. Do not move walnut firewood or logs ? buy and burn firewood locally, since moving walnut wood is the main way the disease and its beetle reach new areas [1][2].
2. Keep your walnut trees healthy and avoid wounding them, since stressed trees are more vulnerable [3].
3. If a walnut shows crown dieback with cankers and beetle holes, have it checked, and remove and properly handle (chip, debark, or burn on site) infested wood rather than hauling it elsewhere [1][3].
4. Watch for and report declining black walnuts so new occurrences can be confirmed and tracked (below) [1].
5. Land managers and crews should monitor walnut stands, follow any walnut-wood movement guidance for their area, and remove and destroy infested wood on site to slow local spread [1][2].

When to call a professional:

1. Any suspected thousand cankers disease, or a valuable or hazardous walnut tree ? contact a certified arborist, forester, or your state plant-health program, since confirming the disease requires laboratory testing [1][3].
2. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. USDA Animal and Plant Health Inspection Service (APHIS). Thousand Cankers Disease. aphis.usda.gov
2. USDA Forest Service, Research & Development. Geosmithia morbida, the causal agent of thousand cankers disease, found in Indiana. research.fs.usda.gov
3. Colorado State University Extension. Thousand Cankers Disease of Walnut. extension.colostate.edu

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