American chestnuts have male and female flowers on each tree, but they are not self-fertile. When pollen from the male catkin travels by wind and insects to female flowers on other trees nearby (usually within 200 yards), open pollination occurs. To make controlled pollinations, breeders cover newly emerged female flowers of the mother tree with bags, collect the desired pollen, and fertilize each flower about a week later, by misting it with water and applying pollen with a paint brush or drawing the catkin across its damp surface. Fertilized flowers are bagged and marked, and the nuts are harvested with care.
Breeding for blight resistance is currently pursued by two separate foundations: the American Chestnut Foundation (ACF) is developing advanced hybrids, building on the work of earlier breeders to improve tree form while enhancing resistance; the American Chestnut Cooperators' Foundation (ACCF) is not using Oriental genes for blight resistance, but intercrossing among American chestnuts selected for native resistance to the blight.
Early breeding efforts in the United States aimed to develop fast growing, blight-resistant hybrids to replace the American chestnut as a timber tree. The USDA began the work in 1922, joined by the Brooklyn Botanical Garden which sponsored chestnut breeding at the Connecticut Agricultural Experiment Station in 1930. The first breeders were Clapper, Gravatt and Graves. Jaynes carried on their work in Connecticut, until 1984. They crossed American chestnuts which appeared to have some blight resistance with Chinese and Japanese chestnuts, but most of their progeny had poor form and did not survive in forest plantings.
One USDA hybrid tree grown in Illinois, known as the Clapper, had good form and growth rate but died in 1977, about 25 years old, following 6 years of blight. This (Chinese X American) X American hybrid survives in grafts. Among the other hybrids with promising form, only 3 had as much as 50% American parentage, while most had both Chinese and Japanese backgrounds and 25% or less American chestnut parentage.
The largest planting of hybrids was made in the Lesesne State Forest in Virginia; between 1969 and 1975, 10,000 trees were planted. These were third and fifth generation selections from the Connecticut breeding program of Jaynes: C X (J X A) and C X A hybrids, with the best combination of forest-tree form and blight resistance, backcrossed with Americans to make better forest trees. In 1981, in a survey covering half these trees, 8 were found to have desirable growth, form, and blight resistance. In 1989, following the ACF plan (below), Dierauf made crosses of these with Americans. The Virginia Department of Forestry is to evaluate these backcrosses.
Singleton and Dietz exposed seednuts to ionizing radiation to induce mutations favoring blight resistance in American chestnut. In mutation breeding, the second and subsequent generations may produce the desired attribute. Between 1962 and 1977, large plantings of the first and second generation seedlings were made in OH, WV, MD, VA, PA and NY. The largest planting is in the Lesesne State Forest. Very few of these have been evaluated for blight resistance.
The ACF breeding program continues the USDA/Connecticut work, with a new emphasis designed by Burnham: C X A hybrids are backcrossed to American chestnuts for up to three generations, then the most blight resistant progeny will be intercrossed. They expect American form and blight resistance equal to Chinese chestnut. This work is directed by Hebard at the ACF farm in Meadowview, Virginia.
"All-American intercrosses" defines the breeding strategy of the ACCF, at Virginia Tech and Concord College, WV. Elkins & Griffin think there may be several different characteristics which favor blight resistance. By making intercrosses among resistant American chestnuts from many locations, they expect to improve upon the low levels of blight resistance to make an American chestnut that can compete in the forest.
Griffin developed a scale for assessing levels of blight resistance, which made it possible to make selections scientifically. He inoculated five-year-old chestnuts with a standard lethal strain of the blight fungus and measured growth of the cankers. Chestnuts with no resistance to blight make rapid-growing, sunken cankers that are deep and kill tissue right to the wood. Resistant chestnuts make slow-growing, swollen cankers that are superficial: live tissue can be recovered under these cankers. The level of blight resistance is judged by periodic measurement of cankers.
Grafts from large survivors of the blight epidemic were evaluated following inoculations, and controlled crosses among resistant Americans were made beginning in 1980. The first all-American intercrosses are planted in Virginia Tech's Martin American Chestnut Planting in Giles County, VA, and in Beckley, WV. They were inoculated in 1990 and evaluated in 1991 & '92. Nine of them showed resistance equal to their parents, and four of these had resistance comparable to hybrids in the same test.
It appears that inheritance of resistance requires genes of two
trees with good combining ability from each source location. Thus, more
generations of controlled crosses will be required to make American chestnuts
produce blight resistance that is regularly inherited by seednuts. It takes
at least 7 years for an American chestnut to produce nuts, and new trees
must be at least 5 years old before their resistance can be tested by inoculation;
the test requires 2 years for evaluation. Some of this time may be saved
by utilizing grafts, and further progress may result from Elkins' studies
underway, to locate chemical markers for resistance. In the meanwhile,
ACCF plantings of all-American intercrosses are producing, from open pollinations,
large nut crops.
ACCF breeding update: Since 1999 we have been making small numbers of intercrosses between two selections from our first generation crosses, Miles and Ruth, which share the same parents, Floyd and Gault, with the mother of one being the father of the other. This second generation intercross has the best chance (in theory) to breed true for blight resistance and also the potential to express a higher level of resistance than its parents or grandparents. We are not distributing these trees because we must retain large numbers for testing. Since 2000 we have also been making first generation intercrosses among new resistance sources, aimed at improving the level of blight resistance and acquiring tolerance for stress at altitudes above 2,500 feet.
Meanwhile, the potential for significant blight resistance to be randomly expressed in the nuts we send out has increased in 2001, following the culling of large numbers of mature trees which did not pass resistance tests in our largest orchard. The ACCF is always in need of more good cooperating growers, to help us carry on the all-American breeding program, by raising our nuts and seedlings and reporting their progress to a central data base.
For details of ACCF research, refer to scientific papers in the second half of the Bibliography.
To learn how you can raise American chestnuts from ACCF seed, go to Growing American Chestnuts.
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