Identifying novel sources of resistance to Phytophthora root rot in backcross American-Chinese hybrid chestnuts: A report to the American Chestnut Foundation External Grants Committee

This final report is in fulfillment of our grant obligations to the American Chestnut Foundation for a grant titled “Identifying novel sources of resistance to Phytophthora root rot and assessment of a quantitative trait locus for resistance in interspecific Castanea hybrids.” We will submit a manuscript based on this report for publication in Chestnut. Previous work by researchers from Clemson University and TACF’s Carolinas Chapter has resulted in American-Asian hybrid chestnuts with resistance to Phytophthora cinnamomi. While the Carolinas Chapter’s efforts to breed chestnuts for P. cinnamomi resistance have met with success, there exists a need to develop P. cinnamomi-resistant lines for American chestnut restoration efforts in other parts of the range. As a first step toward introgressing P. cinnamomi resistance into the Tennessee Chapter’s breeding population, we screened for resistance fifteen backcross families derived from local C. dentata individuals and a diverse panel of C. mollissima cultivars. Backcross families were planted in the nursery using a randomized complete block design and inoculated with a locally isolated strain of P. cinnamomi. Roots were assessed for symptom severity during the following winter. Plants with the fewest symptoms were transplanted to a P. cinnamomi-positive orchard and evaluated for survival mid-way through the following growing season. In the nursery experiment, the majority of backcross families were significantly less symptomatic than C. dentata controls. While nearly all backcross families were significantly more symptomatic than C. mollissima and C. henryi controls, two first-backcross families, derived from the same Chinese-American F1 mother, were not significantly different from the Asian chestnut control groups. In the P. cinnamomi-positive orchard, the pre-screened backcross families had a higher proportion of survivors than families that had not been prescreened for P. cinnamomi resistance. The results of this study suggest that introgression of new sources of P. cinnamomi resistance into Tennessee Chapter lines is a realistic objective. We propose a few modifications to future screening efforts; for example, screening plants in individual containers in the greenhouse, rather than large tubs in the nursery, may allow for greater control of experimental conditions. While a cost-benefit analysis was not performed, these results do raise the possibility that greenhouse/nursery pre-screening for P. cinnamomi resistance could be an effective means of reducing resource input at later stages of the TACF breeding program.