| Chestnut blight fungus | |
|---|---|
| Cankers caused by the fungal infection cause the bark to split. | |
| Scientific classification | |
| Kingdom: | Fungi |
| Phylum: | Ascomycota |
| Subphylum: | Pezizomycotina |
| Class: | Sordariomycetes |
| Order: | Diaporthales |
| Family: | Cryphonectriaceae |
| Genus: | Cryphonectria |
| Species: | C. parasitica |
| Binomial name | |
| Cryphonectria parasitica (Murrill) Barr |
|
The pathogenic fungus Cryphonectria parasitica (formerly Endothia parasitica) is a member of the ascomycota (sac fungus) category, and is the main cause of chestnut blight, a devastating disease of the American chestnut tree that caused a mass extinction in the early 1900s of this once plentiful tree from its historic range in the eastern United States.
Contents |
The chestnut blight was accidentally introduced to North America around 1900, either through imported chestnut lumber or through imported chestnut trees. In 1905, American mycologist William Murrill studied the disease, isolated and described the fungus responsible (which he named Diaporthe parasitica), and demonstrated by inoculation into healthy plants that the fungus caused the disease.[1] By 1940, mature American chestnut trees were virtually wiped out by the disease.
Infection of Asian chestnut trees with the chestnut blight fungus was discovered on Long Island in 1904. The blight appears to have been introduced from either China or Japan. Japanese and some Chinese chestnut trees show some resistance to infection by C. parasitica: they may be infected, but the fungus does not usually kill them. Within 40 years the near-4 billion-strong American chestnut population in Northern America was devastated[2] – only a few clumps of trees remained in California and the Pacific northwest. Because of the disease, American Chestnut wood almost disappeared from the market for decades, although American chestnut wood can still be obtained as reclaimed lumber.[3] The root collar and root system of the chestnut tree are fairly resistant to the fungal infection, so a large number of small American chestnut trees still exist as shoots from existing root bases. However, the shoots are seldom able to grow enough to reproduce before the blight attacks them.[4] So they only survive as living stumps, or "stools", with only a few growing enough shoots to produce seeds shortly before dying. This is just enough to preserve the genetic material used to engineer an American chestnut tree with the minimal necessary genetic input from any of the disease-immune Asiatic species. Efforts started in the 1930s and are still ongoing to repopulate the country with these trees, in Massachusetts[5] and many other places in the United States.[2]
In some places such as the Appalachian Mountains, it is estimated that one in every four hardwoods was an American chestnut. Mature trees often grew straight and branch-free for 50 feet (sometimes up to one hundred feet), and could grow up to 200 feet tall with a trunk diameter of 14 feet at a few feet above ground level. For three centuries most barns and homes east of the Mississippi were made from American chestnut.[6] The chestnut blight caused by C. parasitica has destroyed about 4 billion American chestnut trees,[2] and dramatically reduced the tree population throughout the East Coast. The American Chinquapin (Chinkapin) chestnut trees are also very susceptible to chestnut blight. The European chestnut and the West Asian species are susceptible but less so than the American species. The resistant species (particularly Japanese chestnut and Chinese chestnut but also Seguin's chestnut and Henry's chestnut) have been used in breeding programs in the US to create hybrids with the American chestnut that are disease-resistant.[7]
The fungus is spread by wind-borne ascospores and, over a shorter distance, conidia distributed by rain-splash action. Infection is local in range, so some isolated American chestnuts survive where there is no other tree within 10 km. Also, there are at least two viral pathogens that weaken the fungus (hypovirulence) and help trees to survive.
Surviving 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.
A small stand of surviving American chestnuts was found in F. D. Roosevelt State Park near Warm Springs, Georgia on April 22, 2006 by Nathan Klaus of the Georgia Department of Natural Resources.[8][9]
The fungus makes its entry at wounds and grows in and beneath the bark which eventually kills the cambium all the way round the twig, branch or trunk.[10] The first symptom is a small orange-brown area on the bark of a twig or branch. 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 plant cells. The canker eventually girdles the tree, killing everything above it. Distinctive yellow tendrils (cirrhi) of conidia can be seen extruding from the stroma in wet weather. [11]
There are approximately 2,500 chestnut trees growing on 60 acres 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 less than a dozen chestnuts. Planted outside the natural range of chestnut, these trees escaped the initial onslaught of chestnut blight, but in 1987, scientists found blight in the stand. Scientists are working to try to save the trees. There is a desire to bring American chestnut back to the Eastern forest and funded by the American Chestnut Foundation, Wisconsin Department of Natural Resources, USDA Forest Service, University of West Virginia, Michigan State University, and Cornell University.[12]
Removing blighted trees was first attemped when the blight was discovered, but proved to be no longer an effective solution. The scientists then set out to introduce a hypovirus into the chestnut blight. The treated chestnut responded immediately. Trees began to heal over their cankers. However, the virus was so efficient at attacking the fungus that it prevented the fungus from moving from tree to tree. Only the treated trees recovered. Scientific opinion regarding the future of the stand varies.[12]
In the years since the chestnut blight, many scientists and botanists have worked to create a resistant hybrid chestnut tree that retains the main characteristics of the American chestnut tree. In the early 1950s, a large living American chestnut was discovered in a grove of dead and dying trees in Ohio that showed no evidence of blight infection. Budwood was sent to Dr. Robert T. Dunstan, a plant breeder in Greensboro, North Carolina. Dunstan grafted the scions onto chestnut rootstock and the trees grew well. He cross-pollinated one with a mixture of 3 Chinese chestnut selections: "Kuling", "Meiling", and "Nanking". The resulting fruit-producing hybrid was named the Dunstan Chestnut.[13] Current efforts are underway by the Forest Health Initiative to use modern breeding techniques and genetic engineering to create resistant strains, with contributions from SUNY College of Environmental Science and Forestry, Penn State, UGA, and the US Forest Service. One of the most successful methods of breeding is to create a back cross of a resistant strain (such as one from China or Japan) and American Chestnut. The two species are first bred to create a 50/50 hybrid. After 3 back crosses with American Chestnut, the remaining genome is approximate 1/16 that of the resistant tree, and 15/16 American. The hope is that the resistant genes will be preserved through the back crossing, while the more wild-type traits of American Chestnut will be the dominant phenotype of the plants. Research is also being done at the State University of New York College of Environmental Science and Forestry using Agrobacterium tumefaciens to insert plasmids containing genes of interest from resistant trees. The genes chosen are present only in the resistant strain, and not in the American Chestnut, and are examined for the potential to produce a blight-resistant tree. Currently, SUNY ESF has over 100 individual events being tested, with more than 400 slated to be in the field or in the lab for various assay tests in the next several years.