Historical Mechanisms Promoting Chestnut Survival Through Hybridization



Historically, chestnuts have throughout the​ ages provided food and​ wood products in​ both European and​ Oriental cultures. Chestnuts have saved some civilizations from vanishing during famines, wars, and​ natural disasters. Native American chestnuts offered many promises and​ comforts to​ the​ early colonists, but during a​ blight that was introduced by importing nursery stock from Asia, the​ chestnut trees of​ American were almost eliminated. Certain chestnut tree colonies survived in​ isolated locations and​ because of​ plant breeding advances, chestnut trees are being reestablished throughout the​ nation. the​ original stands of​ American chestnuts were far superior to​ all other types in​ the​ world in​ respect to​ the​ sweet taste and​ vast quantities of​ lumber that was produced. Foreign types of​ chestnuts such as​ Chinese, Japanese, and​ European have been used to​ implant immunity qualities back into the​ historical genetic code contained within the​ tasty kernel of​ the​ American chestnut.

An early reference to​ American chestnuts, ‘Castanea dentata,’ was given in​ John and​ William Bartram’s seed and​ tree nursery catalog, America’s first nursery catalog that was published in​ Philadelphia, PA in​ 1783. the​ Bartram family, famous American explorers and​ botanists, were close friends of​ Benjamin Franklin and​ U.S. Presidents George Washington and​ Thomas Jefferson. the​ Bartrams supplied American chestnut trees to​ gardens at​ Independence Hall at​ Philadelphia and​ the​ personal gardens of​ George Washington at​ Mount Vernon and​ to​ Thomas Jefferson at​ Monticello, Va. President Jefferson was an​ avid plant collector and​ spent endless hours searching for​ profitable horticultural crops that were commercially suitable for​ American farmers. President Jefferson attempted and​ succeeded in​ intercrossing and​ hybridizing the​ various collections of​ Spanish or​ European species of​ chestnuts, ‘Castanea sativa.’ He also performed crosses on chestnuts forming hybrid crosses of​ the​ European chestnut, ‘Castanea sativa’ and​ the​ American chestnut, ‘Castanea dentata.’

Thomas Jefferson is​ documented to​ have personally grafted European chestnuts onto American rootstock, however, it​ is​ unclear why he did this, since the​ American chestnuts were more desirable and​ tasted better than the​ European chestnuts.

In his book, Travels, William Bartram never mentions any encounter or​ observation of​ the​ American chestnut ‘Castanea dentata,’ despite his extensive exploration of​ the​ Southeastern U.S., where the​ trees were growing in​ substantially large numbers in​ their native habitat. the​ mystery created by Bartram omitting references to​ this very significant inhabitant of​ American forests is​ a​ conundrum that may never be answered. Maps locating Bartram’s famous Philadelphia, Pa. arboretum and​ garden still actively used today as​ a​ tourist attraction documented the​ presence of​ chestnut goliaths in​ the​ garden border.

The legendary nuts harvested from the​ American chestnut had a​ superior taste and​ production capability over the​ European chestnut. These nuts were gathered and​ stored in​ the​ shade and​ coolness of​ fall, so that the​ starchy kernel could develop its spicy sweetness. the​ nuts could be shelled and​ eaten fresh, or​ they could be roasted over hot coals to​ improve the​ flavor. a​ common sight on the​ streets of​ New York City or​ Philadelphia was peddlers with mobile stoves roasting the​ fresh chestnuts in​ cast iron pans to​ offer for​ sale to​ pedestrians. the​ heavy crops of​ nuts in​ the​ native forests offered enough food for​ not only human populations, but also for​ animals such as​ bears, deer, squirrels, turkeys, and​ the​ now extinct passenger pigeons.

Chestnuts, because of​ their 42% starch content, can be ground into a​ powdered flour without deterioration for​ extended periods and​ baked into sweet, nutritious cakes. in​ Korea chestnuts are used in​ the​ diet much like potatoes are used in​ Western nations.

American chestnut trees were among the​ largest trees found in​ the​ Eastern U.S., sometimes measuring 17 feet in​ diameter, large enough to​ drive a​ carriage or​ automobile through. These nut trees were found growing from Maine to​ Florida and​ from the​ Eastern seaboard to​ middle America. Some scattered groves of​ chestnut trees could be found in​ Western States. the​ grandness and​ gracefulness of​ this amazingly beautiful tree was highly desirable in​ estate landscapes. the​ long white catkin flowers of​ the​ chestnut developed into a​ valuable food crop for​ the​ U.S. the​ tall, straight trunk of​ the​ tree was ideal for​ many uses, because it​ was easily split along the​ grain for​ timber and​ split-rail fences. the​ dense wood was strong and​ extremely resistant to​ rotting, thus making it​ perfect for​ telephone poles, fence posts, and​ other building materials.

The great gift to​ the​ New World of​ the​ American chestnut that provided food, shelter, shade, and​ wood resources, had all but vanished when the​ trees fell victim to​ a​ fungus infection, ‘Cryphonectria parasitica,’ in​ the​ year 1904. Many years earlier, a​ USDA plant explorer, Frank Meyer, noticed a​ fungal disease, later identified as​ chestnut blight, had entered U.S. ports in​ 1876 from China and​ Japan on nursery stock imported from those countries. Luther Burbank, perhaps the​ world’s greatest plant hybridizer, reported that he imported a​ number of​ chestnuts from China and​ Japan in​ 1884. the​ USDA official went before Congress in​ 1912 after the​ blight decimated American chestnut trees growing at​ the​ Bronx Zoo, and​ was personally given credit for​ his efforts to​ stop further debilitating diseases and​ plagues imported into the​ U.S. by enacting the​ Plant Quarantine Act of​ Congress.

Following the​ example of​ President Thomas Jefferson in​ crossing various species of​ chestnuts to​ obtain hybrids with vigor and​ offspring that might have, within the​ genetic material of​ the​ tree, a​ built-in resistance to​ disease, the​ USDA began hybridizing American chestnuts, ‘Castanea dentata,’ the​ Chinese chestnut, ‘Castanea ‘mollissima,’ and​ Japanese chestnuts, ‘Castanea crenata.’ Thousands of​ chestnut hybrids were obtained, however, the​ American and​ Chinese offspring were the​ most promising, whereas, the​ Japanese chestnuts were excluded. the​ European genetic types of​ chestnut trees were also omitted, because they were also struck down to​ some degree by the​ chestnut blight.

Since the​ hybrid seed of​ outcrossed chestnut trees were so widely variable and​ with such unpredictable germination results were unavailable, the​ seed of​ a​ hybrid selected tree did not demonstrate much promising consequence towards establishing profitable commercial chestnut orchards. the​ chestnut, outstanding hybrid selections, were grafted with extreme difficulty, thus the​ USDA was unfortunately forced to​ abandon its efforts on chestnuts in​ 1960.

It should be mentioned that the​ chestnut blight does not affect the​ roots of​ the​ trees and​ consequently shoots arise from the​ stumps that eventually produce a​ few scattered nuts that can be used to​ further the​ research in​ obtaining immunity in​ a​ hybrid offspring of​ the​ American chestnut ‘Castanea dentata.’ the​ chestnut blight only affects the​ Chinese chestnut trees, ‘Castanea sativa,’ in​ a​ minor superficial way. it​ became important to​ recognize that this immune quality could be transmitted into an​ American chestnut hybrid even when the​ presence of​ the​ Chinese chestnut immunity factor was only one-sixteenth of​ the​ final genetic composition of​ the​ hybrids that could be obtained from the​ cross of​ C. dentata and​ C. mollissima.

Luther Burbank reported intercrossing chestnuts from a​ resulting gene pool that involved crossing Chinese, Japanese, European (Italian), and​ American chestnuts to​ include also chinquapin trees. Out of​ this genetic blend, he managed to​ develop a​ dwarf chestnut 1 ½ ft tall that produced nuts from the​ seed after 6 months from being planted. He also managed to​ produce a​ crop of​ chestnuts from everbearing trees that involved chestnuts and​ flowers being produced month after month continuously. the​ nuts were a​ mammoth size of​ two inches in​ diameter, each weighing an​ ounce or​ more in​ clusters of​ 6 to​ 9 nuts per burr. in​ the​ natural state, the​ spiny burrs act as​ armor that protects the​ nuts from squirrels and​ birds.

More recent observations of​ the​ Italian pathologist Antonio Biraghi have shown that certain survivors of​ the​ European chestnuts, C. sativa, are believed to​ contain a​ form of​ chestnut blight that has been genetically weakened in​ virulence by an​ internal virus to​ the​ extent that the​ effect, called ‘hypovirulence,’ appears to​ demonstrate that the​ virus affected chestnut trees have acquired a​ measure of​ immunity to​ the​ deadly chestnut fungal blight. These clones are believed by many plant scientists to​ be capable of​ imparting a​ new immunity into the​ new C. dentata hybrid crosses with C. sativa and​ backcrossing onto parental genetic types and​ are being evaluated.

Many chestnut trees are offered by mail-order and​ internet companies today, offering an​ optimistic and​ productive future for​ commercial chestnut tree orchards. Some of​ these offerings are available through the​ valuable insight and​ efforts of​ the​ U.S. Department of​ Agriculture and​ its research facilities.





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