Thousand Cankers Disease of Walnut Trees

Frequently Asked Questions

What does a walnut tree look like?
• Leaves: The long leaves of the black walnut are arranged alternately on the stem, have 11 to 23 leaflets per leaf and emerge later in spring than most other trees. Each leaflet has fine serrations on its margin, and the terminal leaflet at the end of the long leaf is frequently absent on leaves from mature trees. While most parts of this tree are pungent when rubbed or bruised, its leaves are especially so; crushed leaflets and stems have a distinct odor similar to turpentine.

• Fruit: The fruit ripens in fall into a dark brown nut with a brownish-green, semi-fleshy husk. The whole fruit, including the husk, falls in October; the seed is relatively small and very hard.

• Bark: The bark of the black walnut is flaky when young, but becomes ridged and deeply furrowed with age, forming a diamondback pattern as the ridges interlace. Bark color is gray-black.

• Twigs and Buds: The dormant twig of the black walnut is characterized by being stout, having large leaf scars without hairy twigs, and with a prominent terminal bud.

What is killing the walnut trees in Boulder?
There are a significant number of black walnut trees, dead and/or dying around Boulder. The rapid decline and tree mortality is the result of the walnut twig beetle (Pityophthorus juglandis) and a Geosmithia fungus. This previously unidentified beetle/fungus combination on black walnut trees has now officially been named, "Thousand Cankers Disease" by staff at Colorado State University (CSU).

• Black Walnut Twig Beetle - Parks and Recreation Urban Forestry staff first identified the arrival of this new insect pest in 2004. Native to the Southwest, the beetle has apparently migrated north and made itself at home in Boulder (and other Front Range communities with concentrations of walnut trees). This is a very aggressive beetle that will attack both healthy and stressed trees. When trees are under additional stress due to lack of supplemental water or construction damage, they are more vulnerable to attack. The beetles have multiple generations per year and are capable of attacking both tree branches and trunks so populations can build rapidly if dead/dying walnut trees are left standing.

• Geosmithia canker fungus - a pathologist at CSU has confirmed the presence of the Geosmithia canker fungus in the dead/dying walnut trees in Boulder. The fungus is transmitted to healthy trees by the walnut twig beetle and can accelerate the decline of an otherwise healthy tree.

How can I tell if my black walnut tree is affected?
The beetles and fungus girdle the twigs and trunk restricting water and food movement within the tree so symptoms often mimic that of drought stress. Symptoms include yellowing, browning and wilting foliage, usually starting at the top of the tree and progressing downward. The brown leaves often remain attached to the branch after the branch dies. Trees showing symptoms in the fall may not leaf out at all in the spring or will leaf out but quickly collapse once temperatures increase.

Walnut trees throughout Boulder have been attacked however the largest concentrations of dead/dying trees are in the Goss/Grove, Whittier, Iris/Broadway and University Hill neighborhoods.

How long does it take the walnut twig beetle to kill my walnut trees?
The Thousand Cankers Disease can kill a mature black walnut tree in one year or less.

How did the walnut twig beetle get to Boulder/CO?
The walnut twig beetle (Pityophthorus juglandis) is native to the southwest United States (California, Arizona, and New Mexico) and northern Mexico. The beetle either moved northward naturally, or was transported to the area in firewood. In any case, the beetle unfortunately appears here to stay. Even if we are able to reduce beetle populations through swift and proper removal of diseased trees and wood, total eradication is not likely.

What does the walnut twig beetle look like?
The dark brown beetles are hard to see because they are very small; most are approximately 1.5 mm in length. The exit holes are again very small and best seen with a magnifying glass when looking at smaller twigs.

Will the walnut twig beetles attack trees other than walnut?
No, the walnut twig beetles attack only walnut trees. Most walnut trees in Boulder are black walnuts; the disease will also attack Persian or English walnuts but those are not prevalent in Boulder.

Can you recommend a company to prune, remove or treat my walnut trees?
You are allowed to contract with any tree service contractor at any time to perform maintenance on your trees. Removing trees can be difficult and dangerous, we recommend that you hire a reputable tree service company that is insured and licensed (in order to reduce the risk of injury or property damage).

The following tree service companies have done work recently for the City of Boulder. They can dispose of the walnut wood properly, are all insured and have a City of Boulder business license.
1) Berkelhammer Tree Experts, 303-443-1233
2) Davey Tree Experts, 303-449-2525
3) Ironwood Earthcare, 303-366-3020
4) Taddiken Tree Company, 303-554-7035
5) The Green Plan, 303-938-8230

My walnut tree still has some green leaves - will it recover?
Any portion of your tree that is dead or has brown leaves will not recover or releaf back out again. Trees attacked by walnut twig beetles often sprout from the base of the tree. Generally, basal sprouts are not well attached to the trunk and should not be left to grow into large trees.

Thousand Cankers Disease is very aggressive. Once your tree is symptomatic, it will not recover. If your tree is not symptomatic, there is a chance you might prevent attack if you follow the guidelines below. The disease is often found in the tree long before symptoms appear however so we highly recommended that you contact a tree care professional immediately to determine if your tree is already infected and determine the best option for preventing further damage to your tree.

What can I do to save my walnut trees?
Good sanitation - If you have several walnut trees, remove all dead/dying walnut trees from your property and dispose of the wood properly to reduce the chance of having your other trees attacked. Proper disposal of wood is mechanical chipping of all branches and smaller diameter wood as soon as possible.

Walnut wood may not be kept for firewood; if wood is kept to be milled or used in woodworking projects, it must be debarked to prevent the beetles from emerging and attacking other walnut trees.

Maintain overall tree vigor - All efforts should be made to keep your walnut trees as healthy as possible.

• Water the trees - Trees should be watered at the rate of at least 15 gallons per inch of trunk diameter every two to three weeks through the summer months (example: 10-inch diameter tree should receive 150 gallons of water every 2-3 weeks). Roots can continue to grow during winter months so supplemental watering should be continued monthly on warmer days through the winter.
• Avoid damaging the trunk or roots around your walnut tree during construction projects and lawn maintenance. Avoid using your walnut tree for a child's swing or other recreational use. If you are planning a construction project, contact a certified arborist to get advice on how to best protect your trees.
• Pesticide treatment - soil injections of a chemical pesticide - imidacloprid - may be effective at preventing attack by the walnut twig beetle only if the product is applied prior to any attack. Local tree service companies can inject the product directly into the trunk or soil so there is no direct public contact. The product can also be purchased through local garden centers and be applied by a homeowner as a soil drench. We recommend that you consult with a tree-care professional to determine the best care for your tree(s). Preventive sprays of permethrin, used extensively to protect trees against ips and mountain pine beetle, have not proven to be effective.

What is the city doing for public walnut trees?
The City of Boulder Urban Forestry staff conducts an annual Tree Health Survey to locate trees on both public and private property that have a potentially epidemic insect or disease problem. These are problems where prompt tree removal is the best method to prevent widespread infestation. In past years staff has looked for Dutch elm disease, ips beetle in spruce, pine wilt in Scotch pine, etc.

Any public trees that are affected are removed promptly. All the public trees identified during our survey are now on a contract to be removed by September 15.

What is the city doing to stop the spread of this complex?
The best way to reduce the spread of this deadly complex is to remove dead or dying walnut trees to prevent new infestations. Property owners of known affected trees will be sent a letter requiring tree removal and proper wood disposal. Our city ordinance typically requires property owners to remove an infested/infected tree within 15 days of notification; due to the large number of affected trees the city is giving property owners 30 days to remove their trees and properly dispose of the wood.

The city will continue to monitor and remove all public walnut trees known to be infested. We will also continue to monitor private trees through our annual Tree Health Survey, issuing new letters requesting private tree removal as needed.

Can I keep my walnut wood? Where can I get some walnut wood?
All walnut wood must be disposed of properly otherwise beetles can still emerge from the cut wood and infest nearby walnut trees.

If you have received a letter requiring you to remove a walnut tree on your private property, the wood must be disposed of or extra steps taken to prevent emergence of the beetles. If you are a wood worker and want to keep smaller sections of wood, you must debark all wood, place all bark and wood waste in the trash and contact our office (City Urban Forestry at 303-441-4406) to have an inspector inspect the wood to ensure the beetles cannot complete their life cycle.

Walnut wood cannot be kept for firewood within the City of Boulder or transported to other communities. If walnut wood is moved to another city, walnut trees there are at risk of attack.

Where can I bring my walnut wood?
Because tree removal can be difficult and dangerous, we recommend you hire a reputable tree service company to do the removal. Hiring a company that is insured and licensed will both ensure proper disposal of all wood and reduce the risk of personal injury or property damage. The companies listed above have been notified about the walnut twig beetle/canker complex problem and understand all available wood disposal options.

If you have any further questions, please contact the City Urban Forestry office at 303-441-4406.

Last Updated on Wednesday, 26 September 2012 13:11