Search

The stem canker disease eastern filbert blight (EFB), caused by Anisogramma anomala, is a major impediment of European hazelnut (Corylus avellana) production in the United States. While most European hazelnut cultivars are highly susceptible to the pathogen, which remains confined to North America, EFB resistant and tolerant genotypes occur in the gene pool at low frequency. At Rutgers University, New Brunswick, NJ, USA, 5,226 trees were grown from open pollinated seeds collected from Russia, Crimea, Poland, Turkey, Estonia, Latvia, Lithuania, Moldova, Azerbaijan, Italy, and the Republic of Georgia between 2002 to 2010. The trees were field planted, exposed to A. anomala under high pathogen pressure, and evaluated for disease response 5-6 years after their establishment. At this point, around four percent were found to be EFB resistant totaling 216 accessions that spanned a wide diversity of seedlots from most countries and regions. However, recent observations show many of these once-resistant selections have since succumbed to EFB. In this study, the long-term disease response of this germplasm was evaluated to identify trees remaining resistant and tolerant and document changes in EFB response over time in relation to their origin. All trees were rated for presence of EFB according to a scale of 0 to 5 where 0 = no EFB and 5 = all stems have cankers. Data were assembled from three sets: first reports from 5-6 years after each planting year, a reassessment in 2017, and a final evaluation in January 2024. Overall, the results showed a significant reduction in resistant individuals from the original reports. By 2017, the population of 216 trees was reduced to 154 and by 2024 it decreased further to 91. Notably, this shift from resistant to susceptible phenotype was severe and abrupt and patterns were observed within related seed lots. These patterns were also apparent in trees where their resistance (R) genes were mapped. Specifically, all selections with R genes mapped to linkage group (LG) 6 now expressed severe EFB, while those with R genes mapped to LG 2 or 7 remained free of disease. These results strongly suggest pathogenic variation present over time played a role in the breakdown of resistance. Fortunately, despite loss of some of the germplasm, a wide variety of trees spanning most collection origins still remain free of EFB. These 91 trees from 56 distinct seedlots originating from 7 countries are formally documented in this manuscript to facilitate their long-term preservation, continued evaluation and sharing, and to increase global awareness of this valuable genetic resource for future research and breeding.

Eastern filbert blight (EFB), caused by Anisogramma anomala, is the primary limiting factor for hazelnut (Corylus sp.) production in the United States. In this study, 82 cultivars and selections shown to be resistant or tolerant to EFB in Oregon were field planted in New Jersey in 2017 and 2019 and evaluated for their EFB response under high disease pressure. The trees carry known single resistance (R) genes with most mapped to their respective linkage groups (LG), including LG2, LG6, and LG7, or they express quantitative resistance (QR, horizontal resistance). Disease incidence and severity was documented, stem cankers counted and measured, and the proportion of diseased wood calculated. The EFB disease response of some cultivars/selections varied considerably between New Jersey and Oregon while others were consistent. Trends were observed in relation to resistance source origin and LGs, which provide insight into durability and usefulness of resistance. In striking contrast to Oregon, nearly all selections with R-genes mapped to LG6, including those carrying the ‘Gasaway’ resistance allele, exhibited severe EFB infections (232 of 266 [87%]). This finding is of consequence since the U.S. hazelnut industry currently relies solely on LG6 resistance for EFB resistance. Further, for the first time, EFB was observed on several selections carrying LG7 resistance, specifically offspring of ‘Ratoli’ from Spain. Interestingly, selections carrying LG7 resistance from origins other than ‘Ratoli’ remained free of EFB, with one exception, all selections carrying LG2 (n=9) resistance also remained free from EFB. Interestingly, the EFB responses of selections expressing QR (n=26) more closely resembled the disease phenotypes they exhibited in Oregon. Overall, the divergence in EFB response between Oregon and New Jersey, where pathogen populations differ, supports the presence of pathogenic variation in A. anomala and highlights potential limitations of using single R-genes to manage the disease. Results also suggest trees expressing QR may be more stable across pathogenic populations.

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.