MEASURING RESISTANCE OF CHESTNUT TREES TO CHESTNUT BLIGHT

Four species of chestnut trees and four kinds of hybrids were inoculate with two virulent strains of the chestnut blight fungus (Cryphonectria parasitica (Muff.) Barr). Rate of canker expansion was measured over 114 days (June-September), and rate change was used as a quantitative assessment of individual tree resistance. Cankers on two typical American chestnut trees (Castanea dentata (Marsh.) Borkh.) expanded at a rate of 1 mm/day. Expansion rates on Chinese chestnut (Castanea mollissima Blume) ranged from 0.2 to 0.6 mm/day, and the rates on two Japanese chestnut trees (Castanea crenata Siebold & Zucc.) were 0.08 to 0.2 mm/day. Our single Henry chestnut (Castanea henryi (Skan) Reh. & Wilson) tree allowed rates of 0.3 and 0.8 mm/day. Most rates on American-Chinese and Japanese-American hybrids were from intermediate between rates on Asian and American trees to near the rates on American trees. However, a first-generation backcross of Chinese-American x American only allowed expansion rates of 0.3 and 0.4 mm/day, similar to the lowest rates on Asian trees.