"One hundred years ago, magnificent American chestnut trees dominated the forested hills and mountains over much of the eastern U. S.  They made their best growth on the slopes of the Appalachian mountains where some towered up to 100 feet and had diameters greater than 10 feet. In some places, chestnut trees formed nearly pure stands. It has been estimated that one out of four trees in the Appalachian forests was an American chestnut (Castanea dentata) prior to the arrival of the lethal chestnut blight, a fungal disease which destroys the bark tissues of the chestnut."

"TACF has crossed the American chestnut with the Chinese chestnut to produce blight resistant hybrids. Their plan as envisioned by Charles Burnham is to backcross blight resistant hybrids with pure American chestnut trees for a few generations so as to develop blight resistant trees that are about 15/16 American and 1/16 Chinese; these offspring are expected to have the attributes of the American chestnut (fast-growth, straight timber form, ability to thrive in the forest, and sweet-flavorful nuts) coupled with the disease resistance of the Chinese chestnut. The latter produces nuts which are of good quality but it has the growth form of an apple tree--it does not succeed in the forest or in very cold climates. An important component of TACF's breeding program is to cross resistant hybrids with American chestnuts from different locations throughout the country so as to develop a diverse gene pool. TACF expects to have some blight resistant, forest-ready, and predominantly American chestnut trees in a few years, but many years may be needed to develop trees with blight resistance that are well adapted to the different regions of the eastern U. S. Breeding for blight resistance has been the major goal, but further breeding to develop trees resistant to ink disease and insect pests is also needed."

More here and here.