Resistance to in American Chestnut () Backcross Populations that Descended from Two Chinese Chestnut () Sources of Resistance.

Restoration of American chestnut () depends on combining resistance to both the chestnut blight fungus () and , which causes Phytophthora root rot, in a diverse population of . Over a 14-year period (2004 to 2017), survival and root health of American chestnut backcross seedlings after inoculation with were compared among 28 BC, 66 BC, and 389 BCF families that descended from two BC trees (Clapper and Graves) with different Chinese chestnut grandparents. The 5% most resistant Graves BCF families survived infection at rates of 75 to 100% but had mean root health scores that were intermediate between resistant Chinese chestnut and susceptible American chestnut families. Within Graves BCF families, seedling survival was greater than survival of Graves BC and BC families and was not genetically correlated with chestnut blight canker severity. Only low to intermediate resistance to was detected among backcross descendants from the Clapper tree. Results suggest that major-effect resistance alleles were inherited by descendants from the Graves tree, that intercrossing backcross trees enhances progeny resistance to , and that alleles for resistance to and are not linked. To combine resistance to both and , a diverse Graves backcross population will be screened for resistance to , survivors bred with trees selected for resistance to , and progeny selected for resistance to both pathogens will be intercrossed.