Historical mechanisms that promote chestnut survival through hybridization
Historically, chestnuts have provided food and wood products throughout the centuries in both European and Eastern cultures. Chestnuts have saved some civilizations from disappearing during famines, wars, and natural disasters. Native American chestnuts offered much promise and comfort to early settlers, but during a plague introduced by importing nursery stock from Asia, the American chestnuts were all but wiped out. Certain chestnut colonies survived in isolated locations, and due to advances in plant breeding, chestnuts are re-establishing across the country. The original stands of American chestnuts were far superior to all other types in the world with respect to sweet taste and the large amounts of wood produced. Foreign types of chestnuts, such as Chinese, Japanese, and European, have been used to implant qualities of immunity into the historic genetic code contained in the tasty American chestnut nut.
One of the earliest references to American chestnuts, ‘Castanea dentata’, was given in John and William Bartram’s Catalog of Tree and Seed Nurseries, America’s first catalog of nurseries which was published in Philadelphia, Pennsylvania, in 1783. The Bartram family, famous American explorers and botanists, were close friends of Benjamin Franklin and US Presidents George Washington and Thomas Jefferson. The Bartrams supplied American chestnuts to the gardens of Independence Hall in Philadelphia and to the personal gardens of George Washington at Mount Vernon and Thomas Jefferson at Monticello, Virginia. President Jefferson was an avid plant collector, spending endless hours searching for profitable vegetable crops that were commercially suitable for American farmers. President Jefferson tried and succeeded in crossing and hybridizing the various collections of the Spanish or European species of chestnuts, ‘Castanea sativa’. He also made crosses on chestnut trees forming hybrid crosses of the European chestnut, ‘Castanea sativa’ and the American chestnut, ‘Castanea dentata’.
It is documented that Thomas Jefferson personally grafted European chestnuts onto American rootstocks, however it is unclear why he did so, as American chestnuts were more desirable and tasted better than European chestnuts.
In his book Travels, William Bartram never mentions any encounters or observations of the American chestnut ‘Castanea dentata’, despite his extensive exploration of the southeastern US, where the trees grew in substantially large numbers in their native habitat. The mystery created by Bartram’s omitting references to this significant inhabitant of America’s forests is one that may never be answered. Maps locating the famous Bartram Arboretum and Garden in Philadelphia, Pennsylvania, which is still actively used today as a tourist attraction, documented the presence of chestnut goliaths at the edge of the garden.
The legendary nuts harvested from the American chestnut had a superior flavor and production capacity than the European chestnut. These nuts were collected and stored in the shade and coolness of autumn, so that the starchy seed could develop its spicy sweetness. The nuts can be shelled and eaten fresh, or they can be roasted over coals to enhance the flavor. A common sight on the streets of New York City or Philadelphia was hawkers with mobile stoves roasting fresh chestnuts in cast-iron skillets to offer for sale to pedestrians. The abundant nut crops in the native forests offered enough food not only for human populations, but also for animals such as bears, deer, squirrels, turkeys, and the now-extinct homing pigeons.
Chestnuts, due to their 42% starch content, can be ground into a powdered flour without spoiling for long periods of time and baked into sweet, nutritious cakes. In Korea, chestnuts are used in the diet much like potatoes are used in Western nations.
American chestnuts are among the largest trees found in the eastern US, sometimes measuring 17 feet in diameter, large enough to drive a carriage or automobile. These nut trees were found growing from Maine to Florida and from the East Coast to Central America. A few scattered groves of chestnut trees can be found in the western states. The grandeur and grace of this astonishingly beautiful tree made it highly desirable in farm landscapes. The long white catkin flowers of the chestnut tree became a valuable food crop for the US. The tree’s tall, straight trunk was ideal for many uses, easily splitting along the grain to make lumber and rail fences divided. The dense wood was strong and extremely resistant to rot, making it perfect for telephone poles, fence posts, and other building materials.
The great gift to the New World of the American chestnut, which provided food, shelter, shade and timber, had all but vanished when the trees fell victim to a fungal infection, ‘Cryphonectria parasitica’, in 1904. Many years earlier, a USDA plant explorer Frank Meyer noted that a fungal disease, later identified as chestnut blight, had entered US ports in 1876 from China and Japan in nurseries imported from those countries. Luther Burbank, perhaps the world’s greatest plant breeder, reported importing several chestnuts from China and Japan in 1884. The USDA official appeared before Congress in 1912 after the blight decimated American chestnut trees growing at the Zoo. of the Bronx, and was personally given credit for his efforts to stop more debilitating diseases and pests imported into the US by enacting the Congressional Plant Quarantine Act.
Following the example of President Thomas Jefferson in crossing various species of chestnut trees to obtain vigorous hybrids and offspring that might have, within the tree’s genetic material, built-in resistance to disease, the USDA began hybridizing American chestnuts, ‘Castanea dentata’, the Chinese chestnut, ‘Castanea ‘mollissima’, and the Japanese chestnuts, ‘Castanea crenata’. Thousands of chestnut hybrids were bred, however, American and Chinese descendants held the most promise, while Japanese chestnuts were excluded. European genetic types of chestnut trees were also omitted, because they were also affected to some extent by chestnut blight.
Since hybrid seeds of cross chestnuts were so variable and with such unpredictable germination results that they were not available, seed from a selected hybrid tree did not show very promising consequences for establishing profitable commercial chestnut orchards. Chestnuts, prominent hybrid selections, were grafted with extreme difficulty, so the USDA was forced to abandon its chestnut efforts in 1960.
It is worth mentioning that chestnut blight does not affect the roots of the trees and consequently sprouts arise from the stumps that eventually produce some scattered nuts that can be used to advance research in obtaining immunity in hybrid offspring of the American chestnut. Castanea dentata. Chestnut blight only affects Chinese chestnuts, ‘Castanea sativa’, superficially. It became important to recognize that this immune quality could be passed on to 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 crossing chestnuts from a resulting gene pool that involved crossing Chinese, Japanese, European (Italian), and American chestnuts to also include chinquapin trees. From this genetic mix, he was able to develop a 1 ½ foot tall dwarf chestnut tree that produced nuts from seed 6 months after planting. He also managed to produce a chestnut crop from evergreen trees involving chestnuts and flowers that were produced month after month on a continuous basis. The nuts were a gigantic size of two inches in diameter, each weighing an ounce or more in groups of 6 to 9 nuts per burr. In its natural state, the spiny burrs act like armor, protecting the nuts from squirrels and birds.
More recent observations by Italian pathologist Antonio Biraghi have shown that certain survivors of the European chestnuts, C. sativa, are believed to contain a form of chestnut blight whose virulence has been genetically weakened by an internal virus to the extent that the This effect, called ‘hypovirulence’, appears to show that chestnut trees affected by the virus have acquired a measure of immunity to the deadly fungal chestnut blight. These clones are believed by many plant scientists to be capable of imparting new immunity to new hybrid crosses of C. dentata with C. sativa and backcrossing to parental genetic types and are being evaluated.
Today, many chestnut trees are being offered by mail-order and Internet companies, offering an optimistic and productive future for commercial chestnut orchards. Some of these offerings are made available through the valuable knowledge and efforts of the US Department of Agriculture and its research facilities.