Chestnut blight
Chestnut blight fungus | |
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Cankers caused by the fungal infection cause the bark to split. | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Fungi |
Division: | Ascomycota |
Class: | Sordariomycetes |
Order: | Diaporthales |
Family: | Cryphonectriaceae |
Genus: | Cryphonectria |
Species: | C. parasitica
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Binomial name | |
Cryphonectria parasitica (
M.E.Barr (1978) | |
Synonyms | |
|
The
Overview
Cryphonectria parasitica is a parasitic
The
The fungus is spread by wind-borne
History
Infections in North America
In 1904, the chestnut blight was accidentally introduced to North America. Cryphonectria parasitica was introduced into the United States from East Asia via import of Japanese chestnut trees. Commercial breeding purposes motivated these imports.[12][13] Infection of American chestnut trees with C. parasitica simultaneously appeared in numerous places on the East Coast, most likely from Japanese chestnuts, which had become popular imports.[14]
Herman W. Merkel, a forester at the New York Zoological Garden (Bronx Zoo) first found infected chestnut trees on the grounds of the zoo. In 1905, American mycologist William Murrill isolated and described the fungus responsible (which he named Diaporthe parasitica), and demonstrated by inoculation into healthy plants that the fungus caused the disease.[15] By 1940, most mature American chestnut trees had been girdled by the disease.[14] It took about 40 years to devastate the nearly four-billion-strong American chestnut population in North America.[16] Only a few clumps of uninfected trees remained in Michigan, Wisconsin, and the Pacific Northwest.
Japanese and Chinese chestnut trees[17] may resist an infection from C. parasitica.[18] Because of the disease, American chestnut wood almost disappeared from the market for decades, although it can still be obtained as reclaimed lumber.[19]
It is estimated that in some places, such as the Appalachian Mountains, one in every four hardwoods was an American chestnut. Mature trees often grew straight and branch-free for 50 feet and could grow up to 100 feet tall with a trunk diameter of 14 feet at a few feet above ground level. The reddish-brown wood was lightweight, soft, easy to split, very resistant to decay, and did not warp or shrink. For three centuries many barns and homes near the Appalachian Mountains were made from American chestnut.[20] Its straight-grained wood was ideal for building furniture and caskets. The bark and wood were rich in tannic acid, which provided tannins for use in the tanning of leather.[21]
Chestnuts were an important cash crop and food source. Many native animals fed on chestnuts, and chestnuts were used for livestock feed, which kept the cost of raising livestock low.[22]
Efforts started in the 1930s and are still ongoing, in
Hypovirulence is not widespread in the US and attempts to commercially introduce CHV1 virus have not been widely successful. Though CHV1 persists in applied trees, it does not spread naturally as it does in Europe, preventing it from being an effective form of biocontrol.
Infections in Europe
In 1938, chestnut blight was first identified around Genoa. Infection quickly spread and was identified in France in 1946, Switzerland in 1951, and Greece in 1963. It has most recently been found in the UK. Due to genetic differences between the fungal populations, it is likely that a second introduction of chestnut blight occurred in Georgia and Azerbaijan in 1938.[29][30] The fungal infections initially caused widespread tree death in Europe. However, in the early 1950s trees were identified in Italy that survived fungal infection. On these trees, the fungus caused more superficial cankers, that appeared to be healing. The milder infection outcome was due to the presence of CHV1, an RNA virus that infects C. parasitica. CHV1 spread naturally throughout Europe but is also spread artificially as a biocontrol measure (particularly in France). CHV1 is currently not present in the UK, Northern France, or Eastern Georgia but an introduction for biocontrol is being considered. [citation needed]
Symptoms
The fungus enters through wounds on susceptible trees and grows in and beneath the bark, eventually killing the cambium all the way around the twig, branch, or trunk.[31] The first symptom of C. parasitica infection is a small orange-brown area on the tree bark. A sunken canker then forms as the mycelial fan spreads under the bark. As the hyphae spread, they produce several toxic compounds, the most notable of which is oxalic acid. This acid lowers the pH of the infected tissue from around the normal 5.5 to approximately 2.8, which is toxic to cambium cells. The canker eventually girdles the tree, killing everything above it. Distinctive yellow tendrils (cirrus) of conidia can be seen extruding in wet weather.[32]
Life cycle and reproduction
The primary plant tissues targeted by C. parasitica are the inner bark, an area containing the conductive tissue, and the cambium, a layer of actively dividing cells that give rise to secondary vascular tissues. In these tissues, the pathogen forms diffuse cankers in which the mycelium overwinters.
If cankers continue to form and expand, the fungus can girdle the stem, severing the flow of nutrients and water to the vital vegetative tissues. The absence of nutrient dispersal will result in above ground tree death. However, the root system may survive. As a result, American chestnuts exist mainly as shrubs sprouting from the old, surviving roots.[34] These sprouts usually die of infection by C. parasitica before reaching sexual maturity.
Management: hypovirulence, sanitation, and chemical control
In Europe, during the late 1960s, it was found that a strain of C. parasitica was less virulent, only able to produce shallow cankers that the tree's callus tissue could eventually limit and isolate. The trait of hypovirulence could be transferred from an avirulent strain to a lethal strain through
Considering the nature of hypovirulent strains, there has been a strong interest to use them to manage lethal C. parasitica strains. In Europe, natural dissemination of hypovirulence in pathogen populations resulted in the restoration of economically valuable chestnuts.[35] Unfortunately, this was not the case in the United States. Compared to Europe, the US has a greater diversity of C. parasitica strains.[37] Thus, the spread of the mycovirus in American C. parasitica populations is inhibited by vegetative incompatibility, an allorecognition system that inhibits the fusion of hyphae between individuals that are genetically distinct at specific loci.[36][35] In 2016, however, "super mycovirus donor strains" of C. parasitica were engineered to overcome this incompatibility system. This could potentially be employed as a method of biological control.[38]
As mentioned above some soil microorganisms suppress C. parasitica. This can be used to treat the cankers, by using a soil compress, a quantity of soil held against the trunk itself with plastic wrap and some adhesive tape around that.[4]
In addition to biocontrol, chestnut blight can also be managed by sanitation practices and chemical control; however, such management strategies are only feasible on a small scale, such as in an orchard. Sanitation practices like the pruning of symptomatic limbs and removal of infected trees can serve to eliminate sources of inoculum and limit the spread of the pathogen.[33]
Additionally, some fungicides may be effective at controlling this fungal disease. A study on the chemical control of chestnut blight in Castanea sativa, may have found that the external application of both
Conservation efforts in North America
There are approximately 2,500 chestnut trees growing on 60 acres (24 ha) near
Removing blighted trees to control the disease was first attempted when the blight was discovered, but this proved to be an ineffective solution. Scientists then set out to introduce a hyperparasitic hypovirus into the chestnut blight fungus. The trees infected with virus-treated fungus responded immediately and began to heal over their cankers. However, the virus was so efficient at attenuating fungal growth that it prevented the spreading of the virus from an infected fungus growing on one tree to that growing on another tree. Only the virus-treated trees recovered. Scientific opinion regarding the future of the stand varies.[40]
Hybrid chestnut trees
Current efforts are underway by the Forest Health Initiative to use modern breeding techniques and genetic engineering to create resistant tree strains, with contributions from
The two species are first bred to create a 50/50 hybrid. After three backcrosses with American chestnut, the remaining genome is approximately 1/16 that of the resistant tree and 15/16 American. The strategy is to
Transgenic blight-resistant chestnut trees
A 1983 study on
A few years later this line of research culminated in the final product: Powell
The American Chestnut Foundation (TACF) once worked close with SUNY ESF to utilize the Darling 58 in their mission to restore the American chestnut to its native range in the eastern United States.[56] However, in December 2023, Darling 58 was discontinued due to poor performance and high mortality in saplings. As a result TACF withdrew its petition for use as a restorative species.[57]
Economic and ecological impact of disease
In less than fifty years after its emergence, C. parasitica virtually eliminated American chestnut as a canopy species in 8.8 million acres (3.6×10 6 ha) of forest.[58] The chestnut fruit was a major food source for animals in the low elevation Appalachian forests. This loss resulted in a drastic decrease in the squirrel population, the extinction of seven native moth species, and the slowed recovery of deer, Cooper's hawk, cougar, and bobcat populations.[59] The effects of this disease also rippled further through the ecosystem, being linked to a decrease in the abundance of cavity-nesting birds and to a decrease in river water quality which negatively affected aquatic invertebrate populations.[58]
In 1912, standing chestnut timber in just three states was estimated to be $82.5 million ($1.9 billion in current dollars) in value.[58] Therefore, in addition to ecological impacts, C. parasitica potentially caused a devastating loss in economic welfare for communities dependent on the chestnut tree. Mountaineers, residents of Appalachian Mountain communities, had to drastically alter their lifestyles to cope with the effects of this disease.[59]
Economic effects have also been considerable in Europe, particularly before CHV1 spreads naturally to a region. In Greece for example, the disease forced the migration of people who could no longer afford to live off chestnut trees. It has also led to a 40% decline in Greek chestnut production.[60]
See also
- Phytophthora cambivora
- Phytophthora katsurae
- American Chestnut Cooperators Foundation
- Darling 58
- Forest pathology
- Lists of invasive species
- Forest disturbance of invasive insects and diseases in the United States
- The Weeping Beech
References
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- ^ a b c "A Brief Summary of Chestnut Canker Biocontrol". The American Chestnut Foundation. 2019-08-12. Archived from the original on 2021-12-05. Retrieved 2021-03-24.
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- ^ Hypovirulence-Infecting the infection Archived February 27, 2012, at the Wayback Machine
- ^ Robin, C (2001). "Chestnut blight in Europe: Diversity of Cryphonectria parasitica, hypovirulence and biocontrol". Forest Snow and Landscape Research. 76 (3): 361–367.
- ^ Garden, Missouri Botanical. "Chestnut Blight". www.missouribotanicalgarden.org. Retrieved 2016-11-13.
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Further reading
- Roane, Martha K.; Griffin, G. J.; Elkins, J. R. (1986). Chestnut blight, other Endothia diseases, and the genus Endothia. OCLC 14139066.
External links
- American Chestnut Cooperators' Foundation - Blight Fungus
- Don't Move Firewood - Gallery of Pests: Chestnut Blight
- SUNY ESF Chestnut Restoration Project
- Forest Health Initiative
- The American Chestnut Foundation
- Species Profile - Chestnut Blight (Cryphonectria parasitica), National Invasive Species Information Center, United States National Agricultural Library.