The American chestnut (Castenata dentata ) trees seen to the right were photographed in Tennesse in the early 1900s. Before blight struck in the first half of the 20th century, the chestnut was the dominant tree on more than 200 million acres of forest land in the eastern United States. Those in Appalachia were commonly 12 feet in diameter and 120 feet tall, and one tree's trunk is recorded as having measured 17 feet across.  Some refer to the American Chestnut as the Redwood of the east.

(From American Chestnut.)

By 1940, chestnut was destroyed as a commercial species. Today, incredibly, chestnut still survives in much of its former range, but only as sprouts from the old root systems. The roots and root collar are resistant to the pathogen. In many places, various oaks have replaced it. In the oak stands, you can hardly find chestnut. When the oaks are cut, fairly dense sprouts of chestnut pop up, trying to do their thing, as we see in this location. But before they can get big enough to sexually reproduce, the disease cuts them down.

The pathogen that causes this blight is Cryphonectria parasitica. It is an ascomycete, whose ascospores are forcibly ejected from conidia at various times of year under varying conditions and are wind-dispersed. The conidia ooze out in a tendril after rains. They are quite small, as small as 4 x 1 ┬Ám wide. In that little conidium is all the information and machinery necessary to wipe out one of the most important tree species in North America. Conidia may be carried by rain splash or catch a ride on an insect or bird.

Cryphonectria parasitica, formerly known as Endothia parasitica, is a member of the Ascomycota, producing its sexual spores (meiotic spores) in a sac called an ascus. To the left is a stroma that contains several perithecia, which bear asci with ascospores inside them. The spores re released through the long neck of the perithecium. Many other forest pathogens are member of the Ascomycota, including Ophiostoma ulmi, the cause of Dutch elm disease, and Ceratocystis fagacearum, cause of oak wilt.

Hope for the Chestnut's future?

The American Chestnut Foundation (ACF) has been working with others to bring about resistant chestnuts bhrough crossing our American Chestnut with Asian species that are naturally resistant to the pathogen that originated in the same location. The following information from the ACF is about how the backcrosses may help us restore this once grand species to our forests. One of their test orcahrds is nearby in Foster, RI

The Backcross Method

Chestnut blight was first introduced to North America in 1904. Like many other pest introductions, it quickly spread into its new - and defenseless - host population. American chestnut trees had evolved in the absence of chestnut blight, and our native species lacked entirely the genetic material to protect it from the fungus.

In Asia, however, where the pathogen originated, most native chestnut species and particularly Chinese chestnut are well defended against the blight. Over the course of their millennia of coexistence with the fungus, Chinese chestnut trees acquired the genetic material that confers resistance. Blighted North American chestnut species die, while blighted Chinese chestnuts suffer only cosmetic damage. Since all chestnut species can be crossed with relative ease, Chinese chestnut offers a potential solution to the American tree's susceptibility to chestnut blight.

But Chinese chestnut lacks many of the characteristics of the American. Most obvious is stature: the Chinese species is low-growing and spreading, much like an old apple tree - an American chestnut can grow straight and strong to a hundred feet or more. This habit of growth combined with the quality of wood makes the American a dominant forest tree species.

Less obvious is the role the American chestnut played in its native forests. The blight is a very recent introduction to the chestnut ecosystem. In those thousands of years preceding the blight's arrival, an enormously complex set of relationships evolved which tied the chestnut to innumerable bird, mammal, and insect species and other organisms, as well as to rocks, water, soils and fires. Essentially, chestnut was tied to the very shape of the hills and mountains on which the trees were found. This history of co-evolution on the North American continent is carried in the genetic material only of the American, not the Chinese chestnut.

The goal of TACF's breeding program is therefore two-fold: to introduce into the American chestnut the genetic material responsible for the blight resistance of the Chinese tree, and at the same time, preserve in every other way the genetic heritage of the American species.

Confirming Resistance
Although the Chinese genes for resistance are only incompletely dominant, they nonetheless usually express themselves clearly when present in seedlings purposely inoculated with a virulent form of the blight fungus. And that is how each backcross generation is tested - by inoculation with blight. Only those seedlings that show the greatest resistance are used for further backcrossing to an American parent.

But every backcross, although necessary to recover desirable American traits, also reintroduces the genes for blight susceptibility from the American parent. In order to remove those genes, the next steps at TACF are intercrosses. In the first intercross, the most blight-resistant 15/16ths American trees are crossed with other blight-resistant 15/16ths American trees. Again, only resistant seedlings are saved.

At the first intercross, it may prove difficult to select inoculated seedlings which have only inherited genes for blight resistance from their Chinese ancestor and no genes for blight susceptibility from their American ancestors. Testing in subsequent generations or a test cross back to an American parent will confirm that first intercross trees contain only the Chinese genes for resistance. Most or all the progeny of parents containing only genes for blight resistance should show blight resistance, whereas some progeny of parents with genes for susceptibility should show susceptibility to blight.

When crossed with each other, these highly blight-resistant parents will breed true for resistance, since they will have no American genes for susceptibility to blight. This second intercross will yield nuts for restoration.

Inbreeding We cannot reforest the Appalachian Mountains with progeny from only one or two chestnut trees, because this would result in a tremendous amount of inbreeding, which chestnut probably could not tolerate. To avoid inbreeding, we have been backcrossing into 20 different American chestnut trees from each of our sources of blight resistance. And we have been doing this at multiple locations throughout the native range of the chestnut tree.

Guaranteeing Regional Adaptability and Long-term Resistance
Across the range of the American chestnut, traits such as stature and timber quality probably differ little. But regional conditions such as temperature, day length, soils, moisture, elevation and others differ greatly from Maine to Mississippi, and there probably are unique combinations of traits for adaptation to different regions. In order to preserve that wide array of genetic diversity and adaptability, TACF's breeding program uses American parents from a number of populations of trees. Our Connecticut, Georgia, Maine, Maryland, Massachusetts, Indiana, Pennsylvania, Kentucky, Tennessee and Carolinas chapters have active breeding programs that are extending the range of regional adaptation in backcross trees.

Plant pathogens frequently evolve to overcome plant defenses. Although the blight fungus is not known to have overcome the defenses of the numerous Chinese chestnut trees planted in the U.S, a future "breakdown" of resistance in blight-resistant chestnut trees is possible. To minimize this possibility, TACF's breeding program uses genetic material from different Chinese chestnut trees. Our most advanced breeding lines, which are entering the first intercross of third backcross trees, are derived from two Chinese chestnut trees known as 'Mahogany' and P.I. #34517. The 'Nanking, 'Kuling' and 'Meiling' cultivated varieties are the parents of a set of trees entering the second backcross stage. Other Chinese chestnut trees are being used to a lesser extent. It is necessary to choose Chinese chestnut parents with care, as some are more resistant than others.