THE BREEDING PROGRAM OF THE AMERICAN CHESTNUT FOUNDATION

Charles Burnham hypothesized in 1981 that the blight resistance of Castanea mollissima could be backcrossed to C. dentate to produce a tree that would perform in the forest like the formerly dominant American chestnut. The American Chestnut Foundation (TACF) was formed in 1983 to test this hypothesis using a traditional plant breeding program. Two B1 chestnut trees, ‘Clapper’ and ‘Graves,’ produced by former breeding programs of the U.S. Department of Agriculture and the Connecticut Agricultural Experiment Station, were used to jump-start the program. These former breeding programs identified Chinese chestnut as the most promising source of blight resistance and developed most of the methods used in the current program. ‘Clapper’ and ‘Graves’ each were backcrossed into approximately thirty American chestnut from the vicinity of Sugar Grove, Virginia, backcrossed again then intercrossed twice. Prior to each cross, trees were selected for blight resistance and recurrent morphological traits. The resultant B3F3 seedlings were planted into the forest for testing beginning in 2009. Tests of the parents of these B3F3s suggested that their blight resistance will fall somewhere between an intermediate and a high level. We are midway through selection in B3F2 seed orchards and resistance among B3F3 progeny should increase as more trees are culled. The ultimate blight resistance of these trees in the forest will be influenced by their adaptation and also may be affected by hypoviruses and other factors; it may take decades for a clear picture to emerge. In addition to ‘Clapper’ and ‘Graves,’ ‘Nanking’ Chinese chestnut has been backcrossed into twenty American backgrounds. More than twenty other sources of blight resistance from Chinese chestnut have been backcrossed, to varying degrees, into American chestnut but into only a few backgrounds; by themselves they do not constitute a viable breeding population. Other Castanea species, C. crenata, C. henryi and C. sequinii have been used to a lesser extent, as well as large, surviving American chestnut trees. This breeding program has been extended throughout the range of the American chestnut in TACF’s network of seventeen state chapters. Through the efforts of those chapters, the effective size of our core population (Ne) should exceed five hundred. The chapters also plan to develop viable breeding populations from the additional sources of blight resistance mentioned above.