WASHINGTON — Billions of American chestnut trees once covered the eastern United States. …But by the 1950s, this venerable tree went functionally extinct, culled by a deadly airborne fungal blight and lethal root rot. A new study provides hope for its revitalization, finding that the genetic testing of individual trees can reveal which are most likely to resist disease and grow tall, thus shortening how long it takes to plant the next, more robust, generation. …The American chestnut … had little defense against foreign-introduced blight and root rot. Another type of chestnut, however, had evolved alongside those diseases. The Chinese chestnut had been introduced for its valuable nuts and it could resist diseases. …So, the authors want a tree with the characteristics of the American chestnut and the disease resistance of the Chinese chestnut. …Breed for disease resistance alone and the trees get shorter, less competitive.