American chestnut - Castanea Dentata
Common Name: American chestnut
Hardiness Zone: 4a to 8a
Height: 100 ft
Width: 100 ft
Description: The American chestnut is a large tree with brown, smooth buds and twigs. The tree can grow upwards of 100 feet in height and 10 feet in diameter. The bark, though brown like the twigs and buds, is very uneven consisting of deep furrows and flat topped ridges in maturity. The leaves are about 6” long and have points along their edges, like a saw. The tree has creamy yellow flowers in June and nuts later in the year. These nuts are edible, but inside a spiny case. The American chestnut was once a very common tree, but now is much more rare due to chestnut blight. It was found naturally as a dominant overstory species in eastern United States deciduous forests. Native trees were commonly found on well drained acidic soils as well as soils formed from shale, sandstone, and limestone parent material (Rhoades).
Chestnut blight is a disease that came into the United States in the 1900s, killing almost all the American chestnut trees in the country. The chestnut tree was an important economic species due to its strong and rot resistant lumber as well as its edible nut. The species was also an integral part of the forest ecosystem providing shelter and nutrition to a diverse number of species. The eventual decline of the American chestnut also led to a decline in species diversity and number (Powell). Remnant root systems are resilient and continue to send up new shoots that eventually succumb to the blight.
The blight is caused by the fungus Cryphonectria parasitica, a member of the Ascomycota phylum. The fungus easily colonizes damaged bark of the chestnut tree and proceeds to penetrate through to the healthy cambial tissue of the tree with a structure known as a hyphal fan (JGI). Stromata form on the bark’s exterior where the orange colored spores are released from sexual and asexual fruiting bodies (JGI).
Efforts to reestablish the American chestnut tree have been underway for some time, applying both traditional breeding and transgenic methods. Chinese chestnut (Castanea mollissima) has evolved resistance to the fungus; however, the physical characteristics of this species are much different than the American chestnut. The stature of a mature Chinese chestnut is more similar to that of a large old apple tree growing 40 to 60 feet in height with a similar spread. Though the two species have quite different physical appearances, they can be crossed with one another to make hybrid offspring. The American Chestnut Foundation has been conducting its ‘Backcross Breeding Program’ since 1989. After an initial cross between Chinese and American chestnut parents, offspring of three successive generations are backcrossed to American chestnut resulting in a tree that, genetically speaking, is almost 94% American chestnut. The offspring of each generation are tested for resistance to the fungus, and only the most resistant trees are kept for the next round of backcrosses or intercrosses. To learn more about this work and The American Chestnut Foundation see http://www.acf.org/index.php
Another hope for restoration is from a virus that naturally attacks C. parasitica. Hypoviruses enabled European chestnut (Castanea sativa), which was also decimated by C. parasitica, to begin recovery in the 1950s. So far only a single instance of a naturally occurring hypovirus has been observed in the United States, in Michigan (Jarosz). Since discovery, the hypovirus has been isolated and used to treat a survivor stand of American chestnuts in West Salem, Wisconsin (Jarosz).
Current genetic engineering methods seek to introduce single genes into the American chestnut which confer resistance to the fungus but maintain the physical appearance and properties of the tree. Genes have been taken from plant species such as Chinese chestnuts, wheat, grapes, and peppers (Powell). Researchers hope to introduce the transgenic trees back into the wild in hopes of restoration.
The influence of American Chestnut (Castanea dentata) on nitrogen availability, organic matter and chemistry of silty and sandy loam soils. 2007. Charles C. Rhoades. Pedobiologia (50): 553-562.