Identifying Host Resistance to Phytophthora cinnamomi in Hybrid Progeny of Castanea dentata and Castanea mollissima

in HortScience

Phytophthora cinnamomi Rands, the causal pathogen of phytophthora root rot (PRR) of chestnut, is one of the main obstacles to growth of american chestnut [Castanea dentata (Marsh.) Bork.] in the southern part of its distribution. To facilitate introgression of PRR resistance of chinese chestnut (C. mollissima Blume) into a C. dentata genetic background, we assessed the disease resistance of 10 interspecific hybrid families derived from potentially resistant C. mollissima cultivars. Hybrid progeny were inoculated with P. cinnamomi in the nursery and assessed for root lesion severity after 1 year of growth. Asymptomatic plants were transplanted to a P. cinnamomi-positive orchard and evaluated for survival midway through the following growing season. During the nursery experiment, 8 of 10 hybrid families were not significantly different from susceptible C. dentata controls for average disease resistance scores. However, multiple asymptomatic individuals were identified in each of the eight families. Two of the 10 hybrid families were not significantly different from the resistant C. mollissima and C. henryi controls. In the P. cinnamomi-positive orchard, the prescreened hybrid families displayed a greater proportion of survivors than backcross families that had not been prescreened for P. cinnamomi resistance. Hybrid plants that have survived 2 years of growth in P. cinnamomi-infested potting media and soils represent an important step toward the production of genetically diverse chestnut populations in the southeastern United States that combine the PRR resistance of C. mollissima with the morphology and local adaptation of C. dentata.

Contributor Notes

This work was supported by a grant from the American Chestnut Foundation and a University of Tennessee at Chattanooga Provost Student Research Award to M. Taylor Perkins. We acknowledge additional support from the Summerfield Johnston Endowment for the Restoration of the American Chestnut, and the Bettie J. Smith FLP.

We thank those who shared their time and expertise for this project: Paul H. Sisco for guidance regarding the “better backcross” and aspects of experimental design; Tatyana N. Zhebentyayeva for guidance regarding experimental design; Paola Zannini for breeding database management and assisting with implementation of the experiments and data collection; David Morris, Jack Agricola, and members of the Alabama Chapter of the American Chestnut Foundation for providing pollen and for assisting in the greenhouse; Clint Neel for providing pollen and seeds; and Thomas M. Saielli of the American Chestnut Foundation for providing American chestnut seeds. Special thanks are given to Steven N. Jeffers and Suzette Sharpe for providing guidance and resources for Phytophthora isolation, species identification, and design of disease resistance screening experiments. We thank Lisa W. Alexander and Jared W. Westbrook for reviewing the manuscript.

Corresponding author. E-mail: hill-craddock@utc.edu.

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    (A) Crossing strategy used to generate first-backcross (BC1) seedlings screened for resistance to phytophthora root rot (PRR). (B) Crossing strategy used to generate better-backcross (BB1) seedlings screened for resistance to PRR.

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    Survival quotients (SQs) of hybrid Castanea families, C. henryi seedlings (resistant controls), C. mollissima seedlings (resistant controls), and C. dentata seedlings (susceptible controls) inoculated with Phytophthora cinnamomi. Families are arranged from high to low SQ value. SQ = {[(1 × n0) + (0.5 × n1) + (0.25 × n2)]/Total number of seedlings} × 100, where n0, n1, and n2 are equal to the number of seedlings rated 0, 1, and 2, respectively.

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    Mean symptom severity of phytophthora root rot (PRR) in hybrid Castanea families, C. henryi seedlings (resistant controls), C. mollissima seedlings (resistant controls), and C. dentata seedlings (susceptible controls). Error bars represent 1 sem. Columns with the same letter are not significantly different according to Duncan’s new multiple range test (α = 0.05). Column colors represent seed type of the different groups screened for disease resistance.

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