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Ronald S. Revord, Sarah T. Lovell, John M. Capik, Shawn A. Mehlenbacher, and Thomas J. Molnar

Eastern filbert blight (EFB), caused by the fungus Anisogramma anomala, is a primary limitation to european hazelnut (Corylus avellana) cultivation in eastern North America. American hazelnut (Corylus americana) is the endemic host of A. anomala and, despite its tiny, thick-shelled nuts, is a potentially valuable source of EFB resistance and climatic adaptation. Interspecific hybrids (Corylus americana × C. avellana) have been explored for nearly a century as a means to combine EFB resistance with wider adaptability and larger nuts. Although significant progress was made in the past, the genetic diversity of the starting material was limited and additional improvements are needed for expansion of hazelnut (Corylus sp.) production outside of Oregon, where 99% of the U.S. crop is currently produced. Our objective was to determine if C. americana can be a donor of EFB resistance. We crossed 29 diverse EFB-resistant C. americana accessions to EFB-susceptible C. avellana selections (31 total progenies) to produce 2031 F1 plants. In addition, new C. americana germplasm was procured from across the native range of the species. The new collection of 1335 plants from 122 seed lots represents 72 counties and 22 states. The interspecific hybrid progenies and a subset of the American collection (616 trees from 62 seed lots) were field planted and evaluated for EFB response following field inoculations and natural disease spread over seven growing seasons. EFB was rated on a scale of 0 (no EFB) to 5 (all stems containing cankers). Results showed that progeny means of the interspecific hybrids ranged from 0.96 to 4.72. Fourteen of the 31 progenies were composed of at least one-third EFB-free or highly tolerant offspring (i.e., ratings 0–2), transmitting a significant level of resistance/tolerance. Several corresponding C. americana accessions that imparted a greater degree of resistance to their hybrid offspring were also identified. In addition, results showed that 587 (95.3%) of the 616 C. americana plants evaluated remained completely free of EFB. These findings confirm reports that the species rarely expresses signs or symptoms of the disease and should be robustly studied and exploited in breeding.