Chestnut blight

The pathogenic fungus Cryphonectria parasitica (formerly Endothia parasitica) is a member of the Ascomycota (sac fungi). This necrotrophic fungus is native to East Asia and South East Asia and was introduced into Europe and North America in the early 1900s. By 1940, most mature American chestnut trees had been girdled by the disease. Today, uninfected wild American chestnut trees are extraordinarily rare, with researchers encouraging members of the public to report sightings. The world's largest remaining stand of genetically pure, mature American chestnut trees is near West Salem, Wisconsin, where scientists have been trying for decades to save the trees while studying the efficacy of hypoviruses that target the blight fungus. Other sightings of mature American chestnut trees have been reported within and outside of its original range, including in Alabama, Kentucky, Maine, Michigan, Ohio, Tennessee, and Vermont.

Japanese and Chinese chestnut trees Because of the disease, American chestnut wood almost disappeared from the market for decades, although it can still be obtained as reclaimed lumber.

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.

Since the 1930s, there have been various efforts to repopulate chestnut trees in the United States. Surviving American chestnut trees are being bred for resistance to the blight, notably by The American Chestnut Foundation, which aims to reintroduce a blight-resistant American chestnut to its original forest range within the early decades of the 21st century. Japanese chestnut and Chinese chestnut, as well as Seguin's chestnut and Henry's chestnut, have been used in these breeding programs in the US to create disease-resistant hybrids with the American chestnut. Chinese chestnut trees have been found to have the highest resistance to chestnut blight;

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 (strains), it is likely that a second introduction of chestnut blight occurred in Georgia and Azerbaijan in 1938. 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.

Symptoms

thumb|left|A chestnut tree that has been felled, with blight on its inner bark and trunk

The fungus enters through wounds on susceptible trees and grows in and beneath the bark, eventually killing the vascular cambium all the way around the twig, branch, or trunk. 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.

Life cycle and reproduction

The primary plant tissues targeted by C. parasitica are the inner bark, an area containing the conductive tissue, and the vascular 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. Upon becoming airborne, ascospores are carried by eddies of wind to new hosts or infect other parts of the same tree. It was later discovered that this attenuated virulence was due to infection by a dsRNA mycovirus, Cryphonectria hypovirus 1 (CHV1) of genus Hypovirus. 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.

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.

Conservation efforts in North America

thumb|right|250px|Experimental trials by [[The American Chestnut Foundation at Tower Hill Botanic Garden in Massachusetts]]

thumb|upright|American chestnut field trial sapling from the American Chestnut Cooperators Foundation

There are approximately 2,500 chestnut trees growing on near West Salem, Wisconsin, which is the world's largest remaining stand of American chestnut. These trees are the descendants of those planted by Martin Hicks, an early settler in the area. In the late 1800s, Hicks planted fewer than a dozen chestnuts. Planted outside the natural range of American chestnut, these trees escaped the initial wave of infection by chestnut blight, but in 1987 scientists found blight also in this stand. There is a program to bring American chestnut back to the Eastern forest funded by the American Chestnut Foundation, Wisconsin Department of Natural Resources, USDA Forest Service, West Virginia University, Michigan State University, and Cornell University.

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 select blight-resistance genes during the backcrossing while preserving the more wild-type traits of American chestnut as the dominant phenotype. Thus, the newly bred hybrid chestnut trees should reach the same heights as the original American chestnut. Many of these 15/16 American chestnut hybrids have been planted along the East Coast, including in the Jefferson National Forest and on the Flight 93 National Memorial. Some of these sites have had researchers check on the saplings that have been planted to see their survival rate. For the hybrids to do well, they need areas with decent drainage and abundant sunlight. Meeting these needs can be hard to do, so not all restoration areas have been successful with hybrid survival.

Transgenic blight-resistant chestnut trees

A 1983 study on hypovirulence had shown that chestnut blight infected with hypovirus produced less oxalic acid when attacking the cambium. Government approval will be required before returning any of these blight resistant trees to the wild. The New York Botanical Garden has planted several of the transgenic trees for public display.

At the start, there were few such engineered chestnut trees. For seed multiplication, grafting could work. However, in December 2023, TACF withdrew its petition for use as a restorative species due to poor performance and high mortality in Darling 54 saplings.

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 of forest. 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. 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.

References

External links

American Chestnut Cooperators' Foundation - Blight Fungus
Don't Move Firewood - Gallery of Pests: Chestnut Blight
SUNY ESF Chestnut Restoration Project
The American Chestnut Foundation
Species Profile - Chestnut Blight (Cryphonectria parasitica), National Invasive Species Information Center, United States National Agricultural Library.