Thousand-Cankers Disease

General Information

Walnut killed by thousand-cankers disease

Figure 1.  This black walnut was killed by the thousand-cankers disease fungus.  Photo courtesy of Curtis Utley, CSUE, Bugwood.org.

Thousand-cankers disease (Geosmithia morbida Kolařík, Freeland, Utley, and Tisserat (Ascomycota: Hypocreales)) is a fungal pathogen affecting most of the North American species of walnuts (Juglans L.).  Some walnuts are moderately susceptible, but others such as black walnut (Juglans nigra L.) are highly susceptible (Fig. 1).  Thousand-cankers disease (TCD) is unique among invasive pests in being, in a sense, a home-grown invasive pest.  It apparently is native to western North America.  The fungus that causes TCD was first identified and reported in 2010.  Based on descriptions of symptoms, it is suspected of killing walnut trees as far back as the early 1990’s.  To date, the disease has not been found in Arkansas; but both black walnut and butternut (Juglans cinerea L.) are native to Arkansas.

The Walnut Twig Beetle

Figure 2.  The walnut twig beetle is apparently native to the Desert Southwest, but it is not native to the native range of black walnut and butternut.  Photo courtesy of Whitney Cranshaw, Colorado State University, Bugwood.org.

 

 


Walnut twig beetle size comparison

Figure 3. The walnut twig beetle is a small reddish brown beetle, about 1/16th inch (1.5 – 2.0 mm) long. Photo courtesy of Whitney Cranshaw, Colorado State University, Bugwood.org.

 

Biology

TCD is moved from tree to tree by walnut twig beetles (Pityophthorus juglandisBlackman (Coleoptera: Curculionidae)).  This beetle (Fig. 2) is not well understood, but appears to be native to the southwestern US and northwestern Mexico.  It is a small reddish brown beetle, about 1/16thinch (1.5 – 2.0 mm) long (Fig. 3), that spends most of its life inside walnut branches.  Despite its name, the walnut twig beetle (WTB) usually burrows into branches greater than 1 inch (2.5 cm) diameter and sometimes into trunks of walnut trees.

Walnut twig beetle in bark

Figure 4.  Walnut twig beetles overwinter in bark crevices as adults and tunnel into the tree the following spring.  Photo courtesy of Karen Snover-Clift, Cornell University, Bugwood.org.

Walnut twig beetle gallerys

Figure 5.  After walnut twig beetles tunnel into a tree in the spring, they excavate a chamber under the bark with egg galleries radiating from it.  Photo courtesy of Whitney Cranshaw, Colorado State University, Bugwood.org.

 

 

 

 

 

 

 

 

 

The beetles overwinter in bark crevices as adults (Fig. 4), and subsequently tunnel into the tree during mid spring.  The adult beetles excavate a chamber under the bark along with egg galleries radiating from the chamber (Fig. 5).  During the process of excavation, WTB introduce the TCD fungus.  The fungus creates a canker in the phloem around the point of inoculation (Fig 6).  As the canker develops, the bark over the canker sloughs off leaving an open wound on the branch (Fig. 7).

Thousand-Cankers Disease canker

Figure 6. The thousand-cankers disease fungus creates a canker in the phloem around the point of inoculation. Photo courtesy of Ned Tisserat, Colorado State University, Bugwood.org.

Open TCD canker

Figure 7. As the thousand-cankers disease canker develops, the bark over the canker sloughs off leaving an open wound on the branch. Photo courtesy of Curtis Utley, CSUE, Bugwood.org.

 

 

 

 

 

 

 

 

 

 

 

The cankers produced by TCD are the primary cause of symptoms in walnuts.  The cankers kill the phloem (Fig. 8) and blocks the transport of sugars down to the roots for storage and back up to the twigs in the spring.  This will cause a slow decline in the affected parts of the tree until energy reserves are exhausted.  A few small cankers produce no symptoms, but large cankers or large numbers of cankers result in the typical symptom of limbs dying in the top of the tree (Fig. 9).  Once limbs start to die in the top of the tree, the disease progresses down the stem until the entire tree is dead (Fig. 10).  For some walnut species, death is certain although it may take 3 years or longer after the onset of symptoms.  Our native black walnut is one of these.  Our native butternut is also susceptible to TCD, but little is known about its ability to resist the disease. There is no treatment or cure for TCD.

Thousand-cankers disease coalesced cankers

Figure 8.  The cankers kill the phloem and prevent the tree from transporting sugars down to the roots for storage.  This eventually results in tree death.  Photo courtesy of Ned Tisserat, Colorado State University, Bugwood.org.

Dead Walnut Branches

Figure 9.  When large numbers of cankers coalesce, they result in the typical symptom of limbs dying in the top of the tree.  Photo courtesy of Karen Snover-Clift, Cornell University, Bugwood.org.

 

dead black walnut

Figure 10.  Once limbs start to die in the top of the tree, the disease progresses down the stem until the entire tree is dead.  Photo courtesy of Karen Snover-Clift, Cornell University, Bugwood.org.

Evidence indicates that the walnut twig beetle was primarily a pest of the Arizona walnut (Juglans major (Torr.) A. Heller).  Arizona walnut is highly resistant to the TCD fungus, so infected trees often show no symptoms.  In the past, ecological barriers (the Great Plains, for example) limited opportunities for eastward expansion of the range of WTB and TCD carried by it.  Due to its desirable qualities as an ornamental tree, black walnut has been widely planted all around the US including, within the native range of Arizona walnut.  In the west, black walnut has proven highly susceptible to the TCD fungus.

No one is certain how WTB was moved east to the native range of black walnut.  It appears that WTB, and the TCD it carried, were moved into the native range of black walnut by people who have transported black walnut wood or nursery stock containing WTB from the western US.  The first known occurrence of TCD in the eastern US was near Knoxville, TN, in 2010.  Since that time, diseased trees have been found in Pennsylvania and Virginia.

The following documents and web sites provide more information about TCD.  These links will open in a new tab.

Thousand Cankers Disease & the Walnut Twig Beetle

Walnut Twig Beetle and Thousand Cankers Disease of Black Walnut

Questions and Answers about Thousand Cankers Disease of Walnut 

Walnut Thousand Cankers Disease Alert

Thousand Cankers Disease of Walnut (Geosmithia morbida)

Pest Alert:  Thousand Cankers Disease

Pathway Assessment:  Geosmithia sp. and Pityophthorus juglandis Blackman movement from the western into the eastern United States

Exotic and Invasive Pests:  Thousand Cankers Disease and the Walnut Twig Beetle in California

Thousand Cankers Disease:  A Red Alert for Walnut

Walnut twig beetle (Pityophthorous juglandis):  The Documentary

Tree Diseases

The dangers of Thousand Cankers Disease

 

Identifying Thousand-Cankers Disease

Dying black walnut

Figure 11.  The first obvious symptom of TCD is notable thinning of the walnut crown.  Photo courtesy of Whitney Cranshaw, Colorado State University, Bugwood.org.

The first obvious symptom of TCD is notable thinning of the walnut crown (Fig. 11).  As the disease progresses, branches will die in the crown (Fig. 9); and eventually the entire tree will die (Fig. 10).  These symptoms can be caused by other disease problems, as well; so if you see these symptoms, further investigation will be necessary.

Cankers of the thousand-cankers disease of black walnut.

Figure 12.  If you examine a black walnut and see these characteristic beetle holes and cankers, call the State Plant Board immediately and report your observation.  Photo courtesy of Ned Tisserat, Colorado State University, Bugwood.org.

You will need to collect a fresh branch from the affected part of the tree crown for the next diagnostic step.  Use a sharp knife to shave some of the bark off a live branch.  If you see the characteristic beetle holes and cankers (Fig. 13), call the State Plant Board immediately and report your observation.  The more quickly the disease can be identified, the more likely the disease can be isolated.

The following documents and web sites explain how to identify thousand-cankers disease.  These links will open in a new tab.

Diagnosing Thousand Cankers Disease of Walnut

Walnut Twig Beetle and Thousand Cankers Disease:  Field Identification Guide

Thousand Canker Disease Field Recognition

 

Stop the Spread

The walnut twig beetle has become established in some parts of the native range of black walnut and butternut.  As with several invasive pests, at this point there may be little we can do to eradicate it from those areas.  That genie is already out of the bottle.  However, we may be able to prevent this invasive pest from entering Arkansas.  The Arkansas State Plant Board has imposed an external quarantine on walnut nursery stock, unprocessed walnut wood, and hardwood firewood from states known to be inhabited by the walnut twig beetle.  Those items may not be brought into Arkansas or even transported through Arkansas.  Honor the quarantine.  Don’t bring those materials home (Fig. 13).  The quarantine is described on Page 54 of Circular 11, Regulations on Plant Diseases and Pests.

Spreading Thousand-Cankers Disease

Figure 13.  This driver is unintentionally spreading thousand-cankers disease by moving potentially infested walnut wood.  Photo courtesy of Whitney Cranshaw, Colorado State University, Bugwood.org.

Learn about thousand-cankers disease and how to recognize the symptoms.  If you see a walnut showing symptoms of TCD; report it to the state plant board, APHIS, or your county extension agent.  If you purchase walnut nursery stock, ask where that nursery stock came from.  Don’t purchase the nursery stock if you can’t get a good answer to the question.  If you obtain walnut wood for woodworking, especially from non-commercial sources; be absolutely sure it does not come from one of the quarantined states.

The following documents and web sites provide information about thousand-canker disease quarantines enacted by Arkansas and other states.

Circular 11 – Regulations on Plant Diseases and Pests (skip to Page 54 for TCD information)

Pest e-Alerts – Emergency Quarantine for Thousand Cankers Disease

The Indiana Quarantine for Thousand Cankers Disease (TCD) of Black Walnut Trees

Missouri Agriculture:  Thousand Cankers Disease of Black Walnut