Chestnut Blight - Overview

Overview

The chestnut blight was accidentally introduced to North America around 1900, probably on imported Japanese chestnut nursery stock. 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. By 1940, most mature American chestnut trees had been wiped out by the disease.

Infection of Asian chestnut trees with C. parasitica was discovered on Long Island in 1904. The fungus appears to have been introduced from either China or Japan. Japanese and some Chinese chestnut trees have some resistance to infection by C. parasitica: the infection usually does not kill these Asian chestnut species. Within 40 years the near-4 billion-strong American chestnut population in Northern America was devastated – 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 it can still be obtained as reclaimed lumber.

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 and could grow up to 100 feet tall with a trunk diameter of 14 feet at a few feet above ground level. For three centuries many barns and homes near the Appalachian Mountains were made from American chestnut.

The root collar and root system of the chestnut tree have some resistance against the fungal infection. So a large number of small American chestnut trees still exist as shoots growing from existing root bases. However, these regrown shoots seldom reach the sexually reproductive stage before being killed by the fungus. So they only survive as living stumps, or "stools", with only a few growing enough shoots to produce seeds. This is just enough to preserve the genetic material used to engineer an American chestnut tree with the minimal necessary genes from any of the disease-immune Asiatic species to confer resistance to the disease. Efforts started in the 1930s and are still ongoing to repopulate the country with these trees, in Massachusetts and many other places in the United States.

The American Chinquapin (Chinkapin) chestnut trees are also very susceptible to chestnut blight. The European chestnut and the West Asian species are also susceptible, but less so than the American species. 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. The resistant species—particularly 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.

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 through a mechanism, termed hypovirulence that helps trees to survive.

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.