Thousand cankers disease
Encyclopedia
Thousand cankers disease (TCD) is a recently recognized disease
of certain walnut
s (Juglans spp.). The disease results from the combined activity of the walnut twig beetle (Pityophthorus juglandis) and a canker
producing fungus
in the genus
Geosmithia (proposed name Geosmithia morbida). Until July 2010 the disease was only known to the western United States
where over the past decade it has been involved in several large scale die-offs of walnut, particularly black walnut, Juglans nigra. However, in late July 2010 a well-established outbreak of the disease was found in the Knoxville, Tennessee area. This new finding is the first locating it within the native range of its susceptible host, black walnut.
s of the Geosmithia fungus, which grows profusely around the pupal chamber of the beetle
s. Following emergence from trees the beetles subsequently tunnel into branches and trunks of walnut for production of egg galleries or overwintering shelters. The fungus is introduced into the tree during this wounding where it subsequently germinates and grows.
The fungal mycelium
initially colonize tissue immediately surrounding the beetle galleries. However, in less than a month black, oval-shaped, inky cankers extend considerably beyond the galleries and may reach more than 3 cm in length in susceptible hosts (e.g., black walnut). In the beginning these cankers develop in phloem
and tissues
formed by the cork cambium
. The affected area is very shallow and never show the ‘open-faced’, perennial, target-shape typical of many canker diseases of trees (e.g., Nectria
canker). Instead in TCD the bark
remains firmly attached to the canker face making the necrotic areas very difficult to observe. Branch cankers usually are not visible until the outer bark is shaved to expose the beetle tunnels, although during late stages of the disease a dark amber stain may form on the bark surface in association with the cankers.
Each time a beetle tunnels into a tree a canker is initiated. Cankers also may continue to expand and penetrate into the cambium of the tree. Each such injury destroys the phloem and robs the tree of its ability to store and move nutrient
s. As TCD progresses cankers coalesce to further girdle branches greatly restricting nutrient movement. As the tree declines, more bark beetle
s are attracted and more cankers are formed.
Eventually the enormous number of beetle attacks and subsequent canker formation overwhelms and kills the tree. Thousand cankers is a progressive disease and its effects result from the culmination of a large number of relatively small cankers over a period of time. Just as a thousand cuts was once used as a form of human execution
in Imperial China, black walnuts are subjected to death by thousands of branch and trunk cankers produced by infection from the Geosmithia fungus.
In end stages of the disease external symptom
s become visible. Leaf yellowing on the exterior of the crown is often the first symptom and may originally be restricted to a single branch. However, as the cumulative effects of the girdling progress increasingly large areas of the tree are affected. Sudden leaf wilting
, ultimately involving large limbs, characterizes end stage thousand cankers disease. In susceptible host
s, trees are almost always killed within 2–3 years after external symptoms of leaf yellowing are first observed.
The progress of thousand cankers will vary due to several factors, notably the susceptibility
of the host. There appears to be a considerable range of TCD susceptibility among various Juglans species with Juglans nigra (black walnut) being particularly susceptible. Conversely, Arizona walnut (Juglans major
) appears to be quite resistant to the disease, with bark beetle attacks largely limited to small diameter branches, the fungus growing to a very limited extent, and effects of the disease rarely, if ever, progressing to involve large areas of the tree. Similarly southern California walnut (Juglans californica
) and little walnut (Juglans microcarpa
) may show fairly high resistance. Northern california walnut (Juglans hindsii
) and the commercial nut-producing Persian (English) walnut (Juglans regia) apparently show various degrees of intermediate TCD susceptibility.
apparently is also recent. The first published note involving black walnut die-offs that likely can be contributed to TCD occurred in the Espanola Valley
of northern New Mexico
in 2001. Walnut twig beetles were associated with this unusual cluster of walnut mortality, but drought was originally thought to be the cause. A parallel situation occurred in eastern Colorado
during the early 2000s where unexplained black walnut deaths were observed in several sites (Colorado Springs
, Boulder
, Westminster
) and originally thought to be associated with drought. Walnut twig beetles were subsequently recovered in 2004, which constituted a new state record for the species.
This appears to have been preceded by unusual black walnut mortality during the mid 1990s of undetermined cause that were noted in the vicinity of Logan, Utah
. Collections of walnut twig beetles were made in 1996 in association with this outbreak. A 1988 collection of walnut twig beetle from the Provo
area, collected incidentally when sweeping vegetation
, appears to be a precedent first collection of this species in the state.
New state records of walnut twig beetle have occurred in several other western states within the past 12 years including Oregon
(1997), Idaho
(2003) and Washington (2008). Most collections from these states were associated with walnut die-offs, although the beetle has also been incidentally collected in Lindgren funnel traps used for survey of other forest pests. Walnut twig beetle records in California
date to 1959, when it was found in Los Angeles County
; known range extensions for this species within the state have increased widely in recent years and it is now known to be widely distributed within the state.
Although walnut twig beetles were repeatedly and regularly noted in association with unexplained walnut deaths since 2002 it was recognized that their activity alone appeared insufficient to produce all the effects that produce thousand cankers disease. It required discovery of the previously undescribed Geosmithia fungus - and an appreciation of its role in canker production - to complete an understanding of how the disease can develop. The initial description of the thousand cankers disease was constructed in early 2008 by researchers at Colorado State University
and subsequently information was extended to alert researcher
s, arborist
s and others with interest in tree health care. This led to numerous new TCD records in the western US during 2008 and 2009. Initial description of thousand cankers disease in a refereed journal occurred in August 2009. Symposia
dedicated to this new disease have been held at the Annual Meeting of the Entomological Society of America
in December 2008 and at a Missouri Department of Agriculture sponsored meeting in St. Louis
during November 3–4, 2009.
Several data seem support this hypothesis
. The walnut twig beetle is a normal associate of Arizona walnut and was first described in 1929 from a collection near Silver City
(Lone Mountain) in Grant County, New Mexico
. Subsequent reviews of North American bark beetles through 1992 only note walnut twig beetles from areas within the range of Arizona walnut, excepting the two 1959 captures in Los Angeles County, where southern California walnut is native.
Recent observations of walnut twig beetle activity in Arizona walnut indicate that the beetle in this host functions as a 'typical' twig beetle of the genus Pityophthorus, restricting breeding to overshaded or damaged branch
es and twig
s. The appearance of extensive colonization of trees by walnut twig beetle have only been observed in black walnut, a species native to the eastern half of the US but planted widely in the western states over the past century.
The Geosmithia fungus, although recently described, is consistently found where ever walnut twig beetles are recovered, including those associated with Arizona walnut. In Arizona walnut, laboratory inoculation
s indicate that it grows slowly, particularly compared to black walnut, and thousand cankers disease has not been observed to develop in native stands of this species that grow in Arizona and New Mexico. Optimum temperatures for development of the Geosmithia fungus are high, also suggesting a warm climate origin, a feature that is consistent with the native distribution of Arizona walnut.
Genetic analysis
of Geosmithia isolates from throughout the western United States is currently considered a high research priority. Preliminary observations indicate that there is considerable variability, indicating no genetic bottlenecks that might be expected from point introductions. Furthermore, genetically related strains may be found over wide geographic areas (e.g., California and Colorado).
Alternatively it is suggested that thousand cankers may have developed from a recent introduction of the Geosmithia fungus or some change in its pathogenicity
.
Regardless there has been a broad range extension of the walnut twig beetle within the past 15 years so that it currently is known from all western states for which it has been surveyed (AZ, CA, CO, ID, OR, UT, WA). Whether this has occurred through natural dispersal events or by human transport of twig beetle infested walnut products (e.g., firewood
, fresh cut logs, wood pieces used by woodworkers) is not known.
The most recent finding of the beetle, in the Knoxville, Tennessee area, clearly is the result of human transport of walnut twig beetle-containing walnut wood, the original event probably occurring a decade or more ago. Walnut wood is extremely infectious (i.e., contains fungus bearing bark beetles) for at least 2-3 years after trees are cut. The beetles are capable of surviving on small pieces of wood, as long as some bark is attached, and some beetles can even survive normal wood chipping.
New infestations from thousand cankers can become established with the movement of a single piece of wood. As the disease develops slowly, and external symptoms may not become visible for 10-20 years after the original establishment, it is possible that additional, undetected infestations are present in the eastern US.
Disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...
of certain walnut
Walnut
Juglans is a plant genus of the family Juglandaceae, the seeds of which are known as walnuts. They are deciduous trees, 10–40 meters tall , with pinnate leaves 200–900 millimetres long , with 5–25 leaflets; the shoots have chambered pith, a character shared with the wingnuts , but not the hickories...
s (Juglans spp.). The disease results from the combined activity of the walnut twig beetle (Pityophthorus juglandis) and a canker
Canker
Canker and anthracnose are general terms for a large number of different plant diseases, characterised by broadly similar symptoms including the appearance of small areas of dead tissue, which grow slowly, often over a period of years. Some are of only minor consequence, but others are ultimately...
producing fungus
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
in the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Geosmithia (proposed name Geosmithia morbida). Until July 2010 the disease was only known to the western United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
where over the past decade it has been involved in several large scale die-offs of walnut, particularly black walnut, Juglans nigra. However, in late July 2010 a well-established outbreak of the disease was found in the Knoxville, Tennessee area. This new finding is the first locating it within the native range of its susceptible host, black walnut.
Description of the Disease
Adult walnut twig beetles carry sporeSpore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s of the Geosmithia fungus, which grows profusely around the pupal chamber of the beetle
Beetle
Coleoptera is an order of insects commonly called beetles. The word "coleoptera" is from the Greek , koleos, "sheath"; and , pteron, "wing", thus "sheathed wing". Coleoptera contains more species than any other order, constituting almost 25% of all known life-forms...
s. Following emergence from trees the beetles subsequently tunnel into branches and trunks of walnut for production of egg galleries or overwintering shelters. The fungus is introduced into the tree during this wounding where it subsequently germinates and grows.
The fungal mycelium
Mycelium
thumb|right|Fungal myceliaMycelium is the vegetative part of a fungus, consisting of a mass of branching, thread-like hyphae. The mass of hyphae is sometimes called shiro, especially within the fairy ring fungi. Fungal colonies composed of mycelia are found in soil and on or within many other...
initially colonize tissue immediately surrounding the beetle galleries. However, in less than a month black, oval-shaped, inky cankers extend considerably beyond the galleries and may reach more than 3 cm in length in susceptible hosts (e.g., black walnut). In the beginning these cankers develop in phloem
Phloem
In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
and tissues
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
formed by the cork cambium
Cork cambium
Cork cambium is a tissue found in many vascular plants as part of the periderm. The cork cambium is a lateral meristem and is responsible for secondary growth that replaces the epidermis in roots and stems...
. The affected area is very shallow and never show the ‘open-faced’, perennial, target-shape typical of many canker diseases of trees (e.g., Nectria
Nectria
This article is about a genus of fungi. For the echinoderm genus, see Nectria .Nectria is a genus of Ascomycete fungi. They are most often encountered as saprophytes on decaying wood but some species can also occur as parasites of trees, especially fruit trees and a number of other hardwood trees...
canker). Instead in TCD the bark
Bark
Bark is the outermost layers of stems and roots of woody plants. Plants with bark include trees, woody vines and shrubs. Bark refers to all the tissues outside of the vascular cambium and is a nontechnical term. It overlays the wood and consists of the inner bark and the outer bark. The inner...
remains firmly attached to the canker face making the necrotic areas very difficult to observe. Branch cankers usually are not visible until the outer bark is shaved to expose the beetle tunnels, although during late stages of the disease a dark amber stain may form on the bark surface in association with the cankers.
Each time a beetle tunnels into a tree a canker is initiated. Cankers also may continue to expand and penetrate into the cambium of the tree. Each such injury destroys the phloem and robs the tree of its ability to store and move nutrient
Nutrient
A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...
s. As TCD progresses cankers coalesce to further girdle branches greatly restricting nutrient movement. As the tree declines, more bark beetle
Bark beetle
A bark beetle is one of approximately 220 genera with 6,000 species of beetles in the subfamily Scolytinae. Traditionally, this was considered a distinct family Scolytidae, but now it is understood that bark beetles are in fact very specialized members of the "true weevil" family...
s are attracted and more cankers are formed.
Eventually the enormous number of beetle attacks and subsequent canker formation overwhelms and kills the tree. Thousand cankers is a progressive disease and its effects result from the culmination of a large number of relatively small cankers over a period of time. Just as a thousand cuts was once used as a form of human execution
Capital punishment
Capital punishment, the death penalty, or execution is the sentence of death upon a person by the state as a punishment for an offence. Crimes that can result in a death penalty are known as capital crimes or capital offences. The term capital originates from the Latin capitalis, literally...
in Imperial China, black walnuts are subjected to death by thousands of branch and trunk cankers produced by infection from the Geosmithia fungus.
In end stages of the disease external symptom
Symptom
A symptom is a departure from normal function or feeling which is noticed by a patient, indicating the presence of disease or abnormality...
s become visible. Leaf yellowing on the exterior of the crown is often the first symptom and may originally be restricted to a single branch. However, as the cumulative effects of the girdling progress increasingly large areas of the tree are affected. Sudden leaf wilting
Wilting
Wilting refers to the loss of rigidity of non-woody parts of plants. This occurs when the turgor pressure in non-lignified plant cells falls towards zero, as a result of diminished water in the cells...
, ultimately involving large limbs, characterizes end stage thousand cankers disease. In susceptible host
Host (biology)
In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...
s, trees are almost always killed within 2–3 years after external symptoms of leaf yellowing are first observed.
The progress of thousand cankers will vary due to several factors, notably the susceptibility
Susceptibility
*In physics, the susceptibility of a material or substance describes its response to an applied field. There are many kinds of susceptibilities, for example:These two susceptibilities are particular examples of a linear response function;...
of the host. There appears to be a considerable range of TCD susceptibility among various Juglans species with Juglans nigra (black walnut) being particularly susceptible. Conversely, Arizona walnut (Juglans major
Juglans major
Juglans major is a walnut tree which grows to with a DBH of up to at elevations of 1000–7000 ft in Texas, Oklahoma, New Mexico, Arizona, and Utah. Common names include Arizona Walnut and Nogal....
) appears to be quite resistant to the disease, with bark beetle attacks largely limited to small diameter branches, the fungus growing to a very limited extent, and effects of the disease rarely, if ever, progressing to involve large areas of the tree. Similarly southern California walnut (Juglans californica
Juglans californica
Juglans californica, the California black walnut, also called the California walnut, or the Southern California black walnut, is a large shrub or small tree of the Juglandaceae family endemic to California....
) and little walnut (Juglans microcarpa
Juglans microcarpa
Juglans microcarpa, known also as the Texas Walnut and the Little Black Walnut is a large shrub or small tree which grows wild along streams and ravines in Texas, New Mexico, Oklahoma, and Kansas. It produces nuts with a width of 1/2—3/4 in. The pinnately compound leaves bear 7—25 untoothed...
) may show fairly high resistance. Northern california walnut (Juglans hindsii
Juglans hindsii
Juglans hindsii, the Hinds' black walnut, also called the Northern California walnut, is a large tree endemic to roughly circular area in California centered near Fresno and reaching the San Francisco Bay area. Some authorities Juglans hindsii, the Hinds' black walnut, also called the Northern...
) and the commercial nut-producing Persian (English) walnut (Juglans regia) apparently show various degrees of intermediate TCD susceptibility.
History of Thousand Cankers Disease
Thousand cankers is a recently described disease and its involvement in tree mortalityMortality rate
Mortality rate is a measure of the number of deaths in a population, scaled to the size of that population, per unit time...
apparently is also recent. The first published note involving black walnut die-offs that likely can be contributed to TCD occurred in the Espanola Valley
Española Valley
Española Valley may refer to:*Española Valley High School*Española, New Mexico...
of northern New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
in 2001. Walnut twig beetles were associated with this unusual cluster of walnut mortality, but drought was originally thought to be the cause. A parallel situation occurred in eastern Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
during the early 2000s where unexplained black walnut deaths were observed in several sites (Colorado Springs
Colorado Springs, Colorado
Colorado Springs is a Home Rule Municipality that is the county seat and most populous city of El Paso County, Colorado, United States. Colorado Springs is located in South-Central Colorado, in the southern portion of the state. It is situated on Fountain Creek and is located south of the Colorado...
, Boulder
Boulder, Colorado
Boulder is the county seat and most populous city of Boulder County and the 11th most populous city in the U.S. state of Colorado. Boulder is located at the base of the foothills of the Rocky Mountains at an elevation of...
, Westminster
Westminster, Colorado
Westminster is a Home Rule Municipality in Adams and Jefferson counties in the U.S. state of Colorado. Westminster is a northwest suburb of Denver. The Westminster Municipal Center is located north-northwest of the Colorado State Capitol. The United States Census Bureau that the city population...
) and originally thought to be associated with drought. Walnut twig beetles were subsequently recovered in 2004, which constituted a new state record for the species.
This appears to have been preceded by unusual black walnut mortality during the mid 1990s of undetermined cause that were noted in the vicinity of Logan, Utah
Logan, Utah
-Layout of the City:Logan's city grid originates from its Main and Center Street block, with Main Street running north and south, and Center east and west. Each block north, east, south, or west of the origin accumulates in additions of 100 , though some streets have non-numeric names...
. Collections of walnut twig beetles were made in 1996 in association with this outbreak. A 1988 collection of walnut twig beetle from the Provo
Provo, Utah
Provo is the third largest city in the U.S. state of Utah, located about south of Salt Lake City along the Wasatch Front. Provo is the county seat of Utah County and lies between the cities of Orem to the north and Springville to the south...
area, collected incidentally when sweeping vegetation
Vegetation
Vegetation is a general term for the plant life of a region; it refers to the ground cover provided by plants. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characteristics. It is broader...
, appears to be a precedent first collection of this species in the state.
New state records of walnut twig beetle have occurred in several other western states within the past 12 years including Oregon
Oregon
Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...
(1997), Idaho
Idaho
Idaho is a state in the Rocky Mountain area of the United States. The state's largest city and capital is Boise. Residents are called "Idahoans". Idaho was admitted to the Union on July 3, 1890, as the 43rd state....
(2003) and Washington (2008). Most collections from these states were associated with walnut die-offs, although the beetle has also been incidentally collected in Lindgren funnel traps used for survey of other forest pests. Walnut twig beetle records in California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
date to 1959, when it was found in Los Angeles County
Los Angeles County, California
Los Angeles County is a county in the U.S. state of California. As of 2010 U.S. Census, the county had a population of 9,818,605, making it the most populous county in the United States. Los Angeles County alone is more populous than 42 individual U.S. states...
; known range extensions for this species within the state have increased widely in recent years and it is now known to be widely distributed within the state.
Although walnut twig beetles were repeatedly and regularly noted in association with unexplained walnut deaths since 2002 it was recognized that their activity alone appeared insufficient to produce all the effects that produce thousand cankers disease. It required discovery of the previously undescribed Geosmithia fungus - and an appreciation of its role in canker production - to complete an understanding of how the disease can develop. The initial description of the thousand cankers disease was constructed in early 2008 by researchers at Colorado State University
Colorado State University
Colorado State University is a public research university located in Fort Collins, Colorado. The university is the state's land grant university, and the flagship university of the Colorado State University System.The enrollment is approximately 29,932 students, including resident and...
and subsequently information was extended to alert researcher
Researcher
A researcher is somebody who performs research, the search for knowledge or in general any systematic investigation to establish facts. Researchers can work in academic, industrial, government, or private institutions.-Examples of research institutions:...
s, arborist
Arborist
An arborist, or arboriculturist, is a professional in the practice of arboriculture, which is the cultivation, management, and study of individual trees, shrubs, vines, and other perennial woody plants...
s and others with interest in tree health care. This led to numerous new TCD records in the western US during 2008 and 2009. Initial description of thousand cankers disease in a refereed journal occurred in August 2009. Symposia
Symposium
In ancient Greece, the symposium was a drinking party. Literary works that describe or take place at a symposium include two Socratic dialogues, Plato's Symposium and Xenophon's Symposium, as well as a number of Greek poems such as the elegies of Theognis of Megara...
dedicated to this new disease have been held at the Annual Meeting of the Entomological Society of America
Entomological Society of America
The Entomological Society of America was founded in 1889 and today has more than 6,000 members, including educators, extension personnel, consultants, students, researchers, and scientists from agricultural departments, health agencies, private industries, colleges and universities, and state and...
in December 2008 and at a Missouri Department of Agriculture sponsored meeting in St. Louis
St. Louis, Missouri
St. Louis is an independent city on the eastern border of Missouri, United States. With a population of 319,294, it was the 58th-largest U.S. city at the 2010 U.S. Census. The Greater St...
during November 3–4, 2009.
Origin of Thousand Cankers Disease
The origin of thousand cankers disease remains a subject of some uncertainty and its resolution will require further studies likely based on genetic analyses. One proposal suggests that the disease is resulted from a host transfer of the walnut twig beetle and its Geosmithia associate from native, resistant Juglans (Arizona walnut and possibly southern California walnut) into susceptible species of walnut.Several data seem support this hypothesis
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
. The walnut twig beetle is a normal associate of Arizona walnut and was first described in 1929 from a collection near Silver City
Silver City, New Mexico
Silver City is a town in Grant County, New Mexico, in the United States. As of the 2000 census, the town population was 10,545. It is the county seat of Grant County. The city is the home of Western New Mexico University.-History:...
(Lone Mountain) in Grant County, New Mexico
Grant County, New Mexico
-2010:Whereas according to the 2010 U.S. Census Bureau:*84.9% White*0.9% Black*1.4% Native American*0.4% Asian*0.1% Native Hawaiian or Pacific Islander*2.8% Two or more races*9.8% Other races*48.3% Hispanic or Latino -2000:...
. Subsequent reviews of North American bark beetles through 1992 only note walnut twig beetles from areas within the range of Arizona walnut, excepting the two 1959 captures in Los Angeles County, where southern California walnut is native.
Recent observations of walnut twig beetle activity in Arizona walnut indicate that the beetle in this host functions as a 'typical' twig beetle of the genus Pityophthorus, restricting breeding to overshaded or damaged branch
Branch
A branch or tree branch is a woody structural member connected to but not part of the central trunk of a tree...
es and twig
Twig
A twig is a small thin terminal branch of a woody plant. Twigs are critically important in identification of trees, shrubs and vines, especially in wintertime. The buds on the twig are an important diagnostic characteristic, as are the abscission scars where the leaves have fallen away...
s. The appearance of extensive colonization of trees by walnut twig beetle have only been observed in black walnut, a species native to the eastern half of the US but planted widely in the western states over the past century.
The Geosmithia fungus, although recently described, is consistently found where ever walnut twig beetles are recovered, including those associated with Arizona walnut. In Arizona walnut, laboratory inoculation
Inoculation
Inoculation is the placement of something that will grow or reproduce, and is most commonly used in respect of the introduction of a serum, vaccine, or antigenic substance into the body of a human or animal, especially to produce or boost immunity to a specific disease...
s indicate that it grows slowly, particularly compared to black walnut, and thousand cankers disease has not been observed to develop in native stands of this species that grow in Arizona and New Mexico. Optimum temperatures for development of the Geosmithia fungus are high, also suggesting a warm climate origin, a feature that is consistent with the native distribution of Arizona walnut.
Genetic analysis
Genetic analysis
Genetic analysis can be used generally to describe methods both used in and resulting from the sciences of genetics and molecular biology, or to applications resulting from this research....
of Geosmithia isolates from throughout the western United States is currently considered a high research priority. Preliminary observations indicate that there is considerable variability, indicating no genetic bottlenecks that might be expected from point introductions. Furthermore, genetically related strains may be found over wide geographic areas (e.g., California and Colorado).
Alternatively it is suggested that thousand cankers may have developed from a recent introduction of the Geosmithia fungus or some change in its pathogenicity
Pathogenicity
Pathogenicity is the ability of a pathogen to produce an infectious disease in an organism.It is often used interchangeably with the term "virulence", although virulence is used more specifically to describe the relative degree of damage done by a pathogen, or the degree of pathogenicity caused by...
.
Regardless there has been a broad range extension of the walnut twig beetle within the past 15 years so that it currently is known from all western states for which it has been surveyed (AZ, CA, CO, ID, OR, UT, WA). Whether this has occurred through natural dispersal events or by human transport of twig beetle infested walnut products (e.g., firewood
Firewood
Firewood is any wood-like material that is gathered and used for fuel. Generally, firewood is not highly processed and is in some sort of recognizable log or branch form....
, fresh cut logs, wood pieces used by woodworkers) is not known.
The most recent finding of the beetle, in the Knoxville, Tennessee area, clearly is the result of human transport of walnut twig beetle-containing walnut wood, the original event probably occurring a decade or more ago. Walnut wood is extremely infectious (i.e., contains fungus bearing bark beetles) for at least 2-3 years after trees are cut. The beetles are capable of surviving on small pieces of wood, as long as some bark is attached, and some beetles can even survive normal wood chipping.
New infestations from thousand cankers can become established with the movement of a single piece of wood. As the disease develops slowly, and external symptoms may not become visible for 10-20 years after the original establishment, it is possible that additional, undetected infestations are present in the eastern US.
External links
- Species Profile- Thousand Cankers Black Walnut Disease (Geosmithia morbida), National Invasive Species Information Center, United States National Agricultural LibraryUnited States National Agricultural LibraryThe United States National Agricultural Library is one of the world's largest agricultural research libraries, and serves as a National Library of the United States and as the library of the United States Department of Agriculture...
. Lists general information and resources for Thousand Cankers Black Walnut Disease