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Root rot (caused by Phytophthora cinnamomi) and chestnut blight (caused by Cryphonectria parasitica) are the two most destructive diseases affecting American chestnut, Castanea dentata. Therefore, breeding for resistance to both pathogens simultaneously is essential before the American chestnut can be restored to its full native range. Using combined genetic and genomic approaches, resistance to C. parasitica (Cp) has been mapped to three quantitative trait loci (QTLs) in chestnut. In addition, a marker set covering the chestnut genome has been generated for implementation in breeding for Cp resistance. Although P. cinnamomi was introduced to the USA far earlier than C. parasitica, an effort to breed for resistance to this pathogen has been initiated only recently. Selection of parental genotypes with a high inter-generational transmission rate of resistance to P. cinnamomi (Pc) allowed initiation of genetic studies. In pilot experiments with a limited number of progeny derived from AdairKY1 × GL158, a QTL for resistance (source Chinese chestnut, C. mollissima 'Nanking') to Pc was mapped to linkage group E (LG_E). Subsequently, three backcross families (HB1, HB2, and MK5) were selected from another source of resistance (C. mollissima 'Mahogany') for map construction and association analysis with 22 markers from LG_E. Our preliminary analyses confirmed the presence of a Pc resistance QTL on LG_E. In 2012, extended mapping populations (up to 200 individuals) representing the Mahogany and Nanking lineages of resistance were planted for phenotyping of Pc resistance and QTL mapping. Together, these materials and analyses should help resolve the location of Proc.

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