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Black walnut trees are a familiar sight across Michigan — you may even have one in your own yard. But many homeowners don’t know about a serious threat the state is working to keep out.
Called thousand cankers disease (TCD), the deadly disease hasn’t been detected in Michigan yet, but it has been confirmed in nearby states.
We’ll walk through everything you need to know about this disease below, including the way thousand cankers disease spreads, what it does to black walnut trees, and what you can do about it.
What Is Thousand Cankers Disease?
Thousand cankers disease is caused by the fungus Geosmithia morbida.
It’s primarily spread by walnut twig beetles (Pityophthorus juglandis), which introduce the fungal spores while tunneling beneath the bark of walnut trees.
The fungus causes small areas of dead tissue called cankers to form beneath the bark. As beetles continue feeding and introducing the fungus in new places, more cankers develop. These cankers can eventually overlap, interfering with the tree’s ability to move water and nutrients, and ultimately leading to its death.
The History of Thousand Cankers Disease in Michigan
Both walnut twig beetles and Geosmithia morbida are thought to be native to portions of the American West.
The walnuts native to that portion of the country — including both the Arizona (Juglans major) and California walnut (Juglans californica) — are largely resistant to the beetle and its associated fungal pathogen.
However, in the early 1990s, black walnuts (Juglans nigra) growing in Utah and Oregon began dying from what would eventually be identified as TCD. The disease continued to spread, and it was eventually detected in Tennessee in 2010.
Fortunately, TCD has not been detected in Michigan yet. However, the state currently includes it on its invasive species Watch List because of the potential threat it poses to Michigan’s black walnut trees.
Michigan has taken preventive measures to reduce the risk of TCD entering the state, including restrictions on certain wood materials coming from areas where the disease occurs.
Which Michigan Trees Are at Risk?
Several walnut species are known to be vulnerable to TCD, but black walnut is the Michigan tree most at risk from the disease. The disease can also infect butternut (Juglans cinerea), which is already under threat from other pathogens in our region.
Wingnut trees (Pterocarya spp.) are also vulnerable to the disease. Though not native to Michigan, these trees are planted occasionally.
Signs and Symptoms of Thousand Cankers Disease
If your black walnut looks different than usual during the summer, start by looking at the upper crown. Early signs of trouble often appear there before becoming more noticeable as the disease progresses.
- Yellowing or Wilting Leaves: Foliage may turn yellow, then brown and wilt as affected branches decline.
- Branch Dieback: Individual branches may begin dying before larger portions of the crown are affected.
- Cankers Beneath the Bark: Small areas of dead tissue may be visible beneath the bark.
- Dark Staining or Cracked Bark: Dark amber staining or cracks may sometimes appear over cankers.
- Tiny Beetle Holes: Small entrance or exit holes may be visible on dead or dying branches.
- Epicormic Sprouts: New shoots may develop along the lower trunk or around the base as the upper crown declines.
But these symptoms don’t necessarily mean your tree has TCD. Identifying walnut twig beetles and confirming the disease requires specialized expertise.
Can a Walnut Tree Recover from Thousand Cankers Disease?
There is currently no known cure for TCD, and trees with advanced disease are unlikely to recover. Because the damage develops beneath the bark as cankers multiply and interfere with the movement of water and nutrients, decline can continue as the disease progresses.
Accordingly, the best course of action involves managing the tree (which may necessitate its removal) and monitoring other vulnerable trees for signs of an infection.
Just keep in mind that a declining black walnut in Michigan should not be assumed to have TCD. Similar symptoms can have other causes. A professional tree assessment can help determine what’s affecting the tree and whether it can be treated or preserved.
Quarantine and Regulatory Status in Michigan
Michigan maintains an exterior quarantine designed to reduce the risk of TCD entering the state. The Michigan Department of Agriculture and Rural Development (MDARD) restricts certain walnut materials and hardwood firewood from areas where TCD occurs.
Before bringing walnut wood, firewood, or other potentially regulated materials into Michigan, check current MDARD quarantine requirements.
Think You’ve Found Thousand Cankers Disease?
If a black walnut has unexplained wilting leaves or dying branches during summer, note the tree’s location and take photos of the symptoms if possible. Report the suspected case to the Michigan Department of Agriculture and Rural Development (MDARD):
Email: MDA-Info@michigan.gov
Phone: 800-292-3939
Online: Submit a report through the Midwest Invasive Species Information Network (MISIN).
Identifying the walnut twig beetle and confirming TCD requires specialized expertise, so don’t try to diagnose the disease yourself.
Treatment and Prevention Options
With no known cure for TCD, prevention is especially important in Michigan. Homeowners can help reduce the risk of introducing or spreading the disease by taking a few precautions:
- Avoid Moving Regulated Material: Buy certified heat-treated firewood or source it locally, and check current MDARD quarantine requirements before bringing walnut wood, trees, or other regulated materials into Michigan.
- Pay Attention to Unusual Decline: Unexplained wilting, crown thinning, or branch dieback in a black walnut may warrant a closer look, especially during the summer.
- Report Unusual Symptoms: If you spot possible signs, report them as outlined above so the case can be investigated.
Because other problems can cause similar symptoms, professional evaluation can help identify what’s actually affecting a declining walnut before treatment or removal decisions are made.
Thousand Cankers Disease: FAQ
Here are a few more things Michigan homeowners should know about TCD and black walnut trees.
Is thousand cankers disease in Michigan?
No. TCD has not been detected in Michigan, but the state considers it a significant potential threat to black walnut trees and maintains measures intended to prevent its introduction.
Can thousand cankers disease kill a black walnut tree?
Yes. Black walnut is highly susceptible to TCD, and repeated beetle attacks and fungal cankers can eventually disrupt the tree’s ability to transport water and nutrients.
Can thousand cankers disease be treated?
No. Prevention, not treatment, is what protects Michigan’s walnuts right now.
Does a dying black walnut mean it has TCD?
No. A declining black walnut can have many possible causes. Unusual symptoms should be professionally evaluated rather than assumed to be TCD.
Should I remove a black walnut if I suspect TCD?
Not based on symptoms alone. Get the tree evaluated first, since removal is hard to undo and something other than TCD may be at work.
Can I bring walnut firewood into Michigan?
Michigan restricts certain walnut materials and hardwood firewood from areas where TCD occurs. Check current MDARD quarantine requirements before transporting firewood or walnut material into the state.
What should I do if I suspect thousand cankers disease?
Note the tree’s location and take photos of the symptoms if possible, then report the suspected case to MDARD or through MISIN. Confirming TCD requires specialized expertise, so don’t try to diagnose the disease yourself.
Give Your Walnut Trees the Support They Need
Thousand cankers disease isn’t the only reason a black walnut may show signs of decline. If something looks wrong with your tree, Longtree Tree Service’s ISA-Certified Arborists can assess its condition, help identify the cause, and recommend appropriate next steps.
Contact Longtree Tree Service to schedule an evaluation of your walnut tree.
Sources & References
- Michigan Invasive Species Program – “Invasive Species: Thousand Cankers Disease”
Covers the current status of thousand cankers disease in Michigan, its designation on the state’s invasive species Watch List, and the threat to Michigan’s black walnut trees. It also provides information on disease development and spread, the lack of an identified effective control, and Michigan guidance for recognizing and reporting suspected cases to MDARD or through MISIN.
https://www.michigan.gov/invasives/id-report/disease/thousand-cankers-disease - Michigan Department of Agriculture & Rural Development – “Thousand Cankers Disease of Walnut Exterior Quarantine”
Details Michigan’s exterior quarantine for thousand cankers disease, including regulated walnut plants and plant parts, wood products, and hardwood firewood. It also outlines restrictions on materials originating in regulated areas and certification and treatment provisions for certain materials entering Michigan.
https://www.michigan.gov/mdard/-/media/Project/Websites/mdard/documents/pesticide-plant-pest/planthealth/thousand_cankers_of_walnut_quarantine.pdf - USDA Animal & Plant Health Inspection Service – “Thousand Cankers Disease”
Explains the relationship between the walnut twig beetle (Pityophthorus juglandis) and Geosmithia morbida, including how beetle activity introduces the fungus and leads to canker development. The resource also covers disease progression, visible signs and symptoms, and the role of infested wood and firewood in spreading TCD.
https://www.aphis.usda.gov/plant-pests-diseases/thousand-cankers - Midwest Invasive Species Information Network – “Report an Invasive Species”
Provides an online option for submitting reports of suspected invasive species through MISIN.
https://www.misin.msu.edu/report/ - Tennessee Department of Agriculture – “Thousand Cankers Disease”
Describes TCD identification and disease progression, including mid-summer flagging, crown thinning, twig and branch dieback, epicormic sprouts along the lower trunk and base, and cankers beneath the bark.
https://www.tn.gov/protecttnforests/diseases/thousand-cankers-disease.html - USDA APHIS / USDA Forest Service – “Thousand Cankers Disease Survey Guidelines for 2022”
Joint federal survey protocol for thousand cankers disease, covering monitoring and trapping methods used by APHIS Plant Protection and Quarantine and the Forest Service to detect the walnut twig beetle and its associated fungus.
https://www.aphis.usda.gov/sites/default/files/tcd-survey-guidelines.pdf - EPPO Global Database – “Geosmithia morbida — Host Plants”
International plant health database entry listing the documented host range of Geosmithia morbida, the fungal pathogen responsible for thousand cankers disease in walnut species.
https://gd.eppo.int/taxon/GEOHMO/hosts
