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Resistance to Phytophthora cinnamomi in American Chestnut (Castanea dentata) Backcross Populations that Descended from Two Chinese Chestnut (Castanea mollissima) Sources of Resistance

Westbrook, Jared W., James, Joseph B., Sisco, Paul H., Frampton, John, Lucas, Sunny, Jeffers, Steven N.
Plant disease 2019 v.103 no.7 pp. 1631-1641
Castanea dentata, Castanea mollissima, Cryphonectria parasitica, Phytophthora cinnamomi, alleles, backcrossing, fungi, grandparents, inheritance (genetics), progeny, root rot, seedlings, trees
Restoration of American chestnut (Castanea dentata) depends on combining resistance to both the chestnut blight fungus (Cryphonectria parasitica) and Phytophthora cinnamomi, which causes Phytophthora root rot, in a diverse population of C. dentata. Over a 14-year period (2004 to 2017), survival and root health of American chestnut backcross seedlings after inoculation with P. cinnamomi were compared among 28 BC₃, 66 BC₄, and 389 BC₃F₃ families that descended from two BC₁ trees (Clapper and Graves) with different Chinese chestnut grandparents. The 5% most resistant Graves BC₃F₃ families survived P. cinnamomi 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 BC₃F₃ families, seedling survival was greater than survival of Graves BC₃ and BC₄ families and was not genetically correlated with chestnut blight canker severity. Only low to intermediate resistance to P. cinnamomi 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 P. cinnamomi, and that alleles for resistance to P. cinnamomi and C. parasitica are not linked. To combine resistance to both C. parasitica and P. cinnamomi, a diverse Graves backcross population will be screened for resistance to P. cinnamomi, survivors bred with trees selected for resistance to C. parasitica, and progeny selected for resistance to both pathogens will be intercrossed.