Thousand cankers disease (TCD) is a recently recognized disease of certain walnuts (Juglans spp.). The disease results from the combined activity of the walnut twig beetle (Pityophthorus juglandis) and a canker producing fungus, Geosmithia morbida.[1] 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. In 2013, an outbreak was found in the Veneto region of Italy, where the disease has been found on both black walnut and English walnut.[2]
Adult walnut twig beetles carry spores of the Geosmithia morbida fungus, which grows profusely around the pupal chamber of the beetles. 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 3cm 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 nutrients. As TCD progresses cankers coalesce to further girdle branches greatly restricting nutrient movement. As the tree declines, more bark beetles 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.
In end stages of the disease external symptoms 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 hosts, 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.
History
Thousand cankers is a recently described disease and its involvement in tree mortality apparently is also recent. The first published note involving black walnut die-offs that likely can be attributed 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. T
he 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 researchers, arborists 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.
Management
Thousand cankers disease can be spread by moving infected black walnut wood. Trees intended for shipment should be inspected for dieback and cankers and galleries after harvest. G. morbida or the walnut twig beetle (Pityophthorus juglandis) are not currently known to be moved with walnut seed.[3] There is currently no chemical therapy or prevention available for the disease making it difficult to control the spread of the disease from the west to the eastern United States. Wood from infected trees can still be used for commercial value, but safety measures such as removing the bark, phloem, and cambium should be followed to reduce the risk of spreading the disease with shipment.[4] Quarantines have been put in place in some states to reduce the potential movement of fungus or beetle from that region. On May 17, 2010, the Director of the Michigan Department of Agriculture issued a quarantine from affected states to protect Michigan’s black walnut ecology and production.[5] Contacting the appropriate entities about possible infections is important to stopping or slowing the spread of thousand cankers disease.
Pathogenesis
The genus Geosmithia (Ascomycota: Hypocreales) are generally saprophytic fungi affecting hardwoods. As of its identification in 2010, the species G. morbida is the first documented as a plant pathogen.[6] The walnut twig beetle (Pityophthorus juglandis) carries the mycelium and conidia of the fungus as it burrows into the tree. The beetle is currently only found in warmer climates, allowing for transmission of the fungus throughout the year. Generations of the beetle move to and from black walnut trees carrying the fungus as they create galleries, the adults typically moving horizontally, and the larvae moving vertically with the grain.[4] As they move through the wood, the beetles deposit the fungus, which is then introduced into the phloem; cankers then develop around the galleries, quickly girdling the tree.[4] The fungus has not been found to provide any value to the beetle. A study done by Montecchio and Faccoli in Italy in 2014 found that no fungal fruiting bodies were found around or on the cankers but in the galleries. Mycelium, and sometimes conidiophores and conidia were observed in the galleries as well.[7] No sexual stage of the fungus has currently been found.
Importance
Black walnut wood is valued for its use in cabinetry, furniture making, and other woodworking. In 2008, 700 trees in boulder Colorado were removed from what was thought to be a decline disease, but later discovered as Thousand Cankers Disease.[8] In 2009, the Missouri Department of Conservation issued a prediction of losses for the state due to the disease. They predicted the annual wood product loss after established infection would have a total value of $36,333,677, with a loss of 210 jobs. It was also predicted that the annual nut production loss would total $35,291,500, with 504 jobs lost in the industry and sector.[9]
Although the integrity of the wood is not compromised, the rapid tree death greatly reduces the supply of black walnut wood.[4] The seed from black walnut is also valued for its ecological importance. G. morbida does not affect the seeds, but increased tree death would lead to decreased nut production, hindering both human industry, and creating a lost food source for wildlife.[4] With widespread loss of walnut trees spreading from the western united states, to Tennessee, there is increasing importance to protect walnuts in the eastern United States, or risk losing much of the walnut production in the United States.
Origin
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 to 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 trees indicate that the beetle in this host functions as a "typical" twig beetle of the genus Pityophthorus, restricting breeding to overshaded or damaged branches and twigs. 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 inoculations 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.
Related Research Articles
Ambrosia beetles are beetles of the weevil subfamilies Scolytinae and Platypodinae, which live in nutritional symbiosis with ambrosia fungi. The beetles excavate tunnels in dead or stressed trees in which they cultivate fungal gardens, their sole source of nutrition. After landing on a suitable tree, an ambrosia beetle excavates a tunnel in which it releases its fungal symbiont. The fungus penetrates the plant's xylem tissue, extracts nutrients from it, and concentrates the nutrients on and near the surface of the beetle gallery. Ambrosia fungi are typically poor wood degraders, and instead utilize less demanding nutrients. Symbiotic fungi produce and detoxify ethanol, which is an attractant for ambrosia beetles and likely prevents growth of antagonistic pathogens and selects for other beneficial symbionts. The majority of ambrosia beetles colonize xylem of recently dead trees, but some attack stressed trees that are still alive, and a few species attack healthy trees. Species differ in their preference for different parts of trees, different stages of deterioration, and in the shape of their tunnels ("galleries"). However, the majority of ambrosia beetles are not specialized to any taxonomic group of hosts, unlike most phytophagous organisms including the closely related bark beetles. One species of ambrosia beetle, Austroplatypus incompertus exhibits eusociality, one of the few organisms outside of Hymenoptera and Isoptera to do so.
Juglans nigra, the eastern American black walnut, is a species of deciduous tree in the walnut family, Juglandaceae, native to North America. It grows mostly in riparian zones, from southern Ontario, west to southeast South Dakota, south to Georgia, northern Florida and southwest to central Texas. Wild trees in the upper Ottawa Valley may be an isolated native population or may have derived from planted trees.
Juglans cinerea, commonly known as butternut or white walnut, is a species of walnut native to the eastern United States and southeast Canada.
A bark beetle is the common name for the subfamily of beetles Scolytinae. Previously, this was considered a distinct family (Scolytidae), but is now understood to be a specialized clade of the "true weevil" family (Curculionidae). Although the term "bark beetle" refers to the fact that many species feed in the inner bark (phloem) layer of trees, the subfamily also has many species with other lifestyles, including some that bore into wood, feed in fruit and seeds, or tunnel into herbaceous plants. Well-known species are members of the type genus Scolytus, namely the European elm bark beetle S. multistriatus and the large elm bark beetle S. scolytus, which like the American elm bark beetle Hylurgopinus rufipes, transmit Dutch elm disease fungi (Ophiostoma). The mountain pine beetle Dendroctonus ponderosae, southern pine beetle Dendroctonus frontalis, and their near relatives are major pests of conifer forests in North America. A similarly aggressive species in Europe is the spruce ips Ips typographus. A tiny bark beetle, the coffee berry borer, Hypothenemus hampei is a major pest on coffee plantations around the world.
Juglans ailantifolia, the Japanese walnut, is a species of walnut native to Japan and Sakhalin. It is a deciduous tree growing to 20 m (66 ft) tall, rarely 30 m (98 ft), and 40–80 cm (16–31 in) stem diameter, with light grey bark. The leaves are pinnate, 50–90 cm (20–35 in) long, with 11–17 leaflets, each leaflet 7–16 cm (2.8–6.3 in) long and 3–5 cm (1.2–2.0 in) broad. The whole leaf is downy-pubescent, and a somewhat brighter, yellower green than many other tree leaves. The male flowers are inconspicuous yellow-green catkins produced in spring at the same time as the new leaves appear. The female flowers have pink/ red pistils. The fruit is a nut, produced in bunches of 4–10 together; the nut is spherical, 3–5 cm long and broad, surrounded by a green husk before maturity in mid autumn.
Ophiognomonia clavigignenti-juglandacearum is a mitosporic fungus that causes the butternut canker, a lethal disease of butternut trees. It is also known to parasitize other members of the genus Juglans on occasion, and very rarely other related trees including hickories. The fungus is found throughout North America, occurring on up to 91% of butternut trees, and may be threatening the viability of butternut as a species.
Ophiostoma ulmi is a species of fungus in the family Ophiostomataceae. It is one of the causative agents of Dutch elm disease. It was first described under the name Graphium ulmi, and later transferred to the genus Ophiostoma.
The plant pathogenic fungus Leucostoma kunzei is the causal agent of Leucostoma canker, a disease of spruce trees found in the Northern Hemisphere, predominantly on Norway spruce and Colorado blue spruce. This disease is one of the most common and detrimental stem diseases of Picea species in the northeastern United States, yet it also affects other coniferous species. Rarely does it kill its host tree; however, the disease does disfigure by killing host branches and causing resin exudation from perennial lesions on branches or trunks.
Tomicus piniperda, the common pine shoot beetle, is a bark beetle native throughout Europe, northwestern Africa, and northern Asia. It is one of the most destructive shoot-feeding species in northern Europe.
Fusarium circinatum is a fungal plant pathogen that causes the serious disease pitch canker on pine trees and Douglas firs. The most common hosts of the pathogen include slash pine, loblolly pine, Monterey pine, Mexican weeping pine, and Douglas fir. Like other Fusarium species in the phylum Ascomycota, it is the asexual reproductive state of the fungus and has a teleomorph, Gibberella circinata.
The European spruce bark beetle, is a species of beetle in the weevil subfamily Scolytinae, the bark beetles, and is found from Europe to Asia Minor and some parts of Africa.
Geosmithia is a genus of anamorphic fungi of uncertain familial placement in the order Hypocreales. The genus, circumscribed by Australian mycologist John Pitt in 1979, is widely distributed. A 2008 estimate placed ten species in the genus, but several new species have since been described. Thousand cankers disease, which affects economically important black walnut populations in North America, is caused by Geosmithia morbida.
Geosmithia morbida is a species of anamorphic fungus in the Bionectriaceae family that, together with the activity of the walnut twig beetle, causes thousand cankers disease in species of walnut trees. It was described as new to science in 2010 from specimens collected in the southern United States. The fungus, transmitted by the walnut twig beetle, Pityophthorus juglandis, is known from the western USA from California to Colorado. The cankers resulting from infection restrict nutrient flow and typically kill the host tree within three to four years. Based on closeness of internal transcribed spacer DNA, the closest relative of G. morbida is G. fassatiae. The specific epithet morbida refers to the deadly pathogenic effect it has on its host.
Pityophthorus juglandis, also known as the walnut twig beetle for feeding on several different species of walnut trees, Juglans, is one of only a few species in the genus Pityophthorus that is associated with hardwoods and the only one associated with feeding on walnut trees.
Ips is a genus of beetles in the family Curculionidae, the true weevils. They are bark beetles, members of the subfamily Scolytinae. Species are distributed throughout the Northern Hemisphere. Some are known as introduced species in Australia and Africa. Many species are pests of forest trees, especially pines and spruces. They are known commonly as engraver beetles, ips engraver beetles, and pine engravers.
The foamy bark canker is a disease affecting oak trees in California caused by the fungus Geosmithia sp. #41 and spread by the Western oak bark beetle. This disease is only seen through the symbiosis of the bark beetles and the fungal pathogen. The bark beetles target oak trees and bore holes through the peridermal tissues, making tunnels within the phloem. The fungal spores are brought into these tunnels by the beetles and begin to colonize the damaged cells inside the tunnels. Symptoms of the developing fungus include wet discoloration seeping from the beetle entry holes as the fungus begins to consume phloem and likely other tissues. If bark is removed, necrosis of the phloem can be observed surrounding the entry hole(s). As the disease progresses, a reddish sap and foamy liquid oozes from entry holes, thus giving the disease the name foamy bark canker. Eventually, after the disease has progressed, the tree dies. This disease is important because of its detrimental effects on oak trees and its ability to spread to several new Californian counties in just a couple of years.
Hypoxylon canker of shade trees is a weak ascomycete fungus that negatively affects growth and can eventually lead to the death of already dying or diseased host trees. There are many different species that affect different trees. For example, Hypoxylon atropunctatum, a common species, is found on oak trees, Hypoxylon tinctor affects sycamore trees, and Hypoxylon mammatum infests aspen trees.
Species which are not native to a forest ecosystem can act as an agent of disturbance, changing forest dynamics as they invade and spread. Invasive insects and pathogens (diseases) are introduced to the United States through international trade, and spread through means of natural and human-dispersal. Invasive insects and pathogens are a serious threat to many forests in the United States and have decimated populations of several tree species, including American chestnut, American elm, eastern hemlock, whitebark pine, and the native ash species. The loss of these tree species is typically rapid with both short and long-term impacts to the forest ecosystem.
Cypress canker is a disease affecting Cupressus species, caused by one of several species of fungus in the genus Seiridium. Infection causes die-back of twigs and branches in susceptible cypress trees, with rapidly increasing amounts of damage and the death of the tree.
References
↑ Kolarik, M.; Freeland, E.; Utley, C.; Tisserat, N. (2011). "Geosmithia morbida sp. nov., a new phytopathogenic species living in symbiosis with the walnut twig beetle (Pityophthorus juglandis) on Juglans in USA". Mycologia. 103 (2): 325–332. doi:10.3852/10-124. PMID20943528. S2CID35496500.
↑ Montecchio, L.; Vettorazzo, M.; Faccoli, M. (2016). "Thousand cankers disease in Europe: an overview". EPPO Bulletin. 46 (2): 335–340. doi:10.1111/epp.12301. ISSN0250-8052.
↑ "details". www.tsusinvasives.org. Retrieved December 15, 2017.
↑ Kolarik, M (Winter 2017). "Geosmithia morbida sp. nov., a new phytopathogenic species living in symbiosis with the walnut twig beetle (Pityophthorus juglandis) on Juglans in USA". Mycologia. 103 (2): 325–32. doi:10.3852/10-124. PMID20943528. S2CID35496500.
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Anonymous 2005. Forest Insect and Disease Conditions of the Rocky Mountain Region, 2004. USDA-FS Rocky Mountain Region, Renewable Resources and Forest Health Management Report R2-05-09.
Blackman, M. W. 1928. The genus Pityophthorus Eichh. in North America: A revisional study of the Pityphthori, with descriptions of two new genera and seventy-one new species. New York State College of Forestry Syracuse University Bulletin 1(3-6), Tech. Publ. 25: 183 pp.
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