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Corylus americana is native to a wide area of land in eastern North America, bounded by the Rocky Mountains in the west and the Atlantic Ocean in the east, and ranging from southern Canada to the southeastern US. The species has been shown to be highly genetically diverse and is adapted to a variety of climates and soils. It is also resistant to the disease eastern filbert blight (EFB) caused by the fungus Anisogramma anomala. Unfortunately, C. americana has thick-shelled, tiny nuts that make it unsuitable for commercial production. However, it is cross-compatible with the commercial hazelnut C. avellana and can serve as a donor for genes for disease resistance, early nut maturity, cold tolerance, etc. As part of the Hybrid Hazelnut Consortium consisting of Rutgers University, Oregon State University, the University of Nebraska-Lincoln, and the Arbor Day Foundation, we have been successfully collecting C. americana germplasm since 2009 with the help of partners, colleagues, and the interested public around the USA. Today, we have a planting established in the field at Rutgers University in New Jersey that holds 1,899 seedlings obtained from 128 individual seed lots that span 23 states and one Canadian province. These bushes are being evaluated for EFB response, other diseases and insects, flowering and vegetative bud break phenology, cold hardiness, nut yield, and nut and kernel characteristics. The genetic diversity and population structure of the new collection is also being evaluated using several approaches, including genotyping by sequencing and subsequent SNP analysis, SSR allelic variation, and chloroplast haplotype determination across the large population. Using this multi-faceted approach, we hope that superior, diverse selections can be identified for use in systematic breeding efforts to develop new cultivars adapted to the eastern USA, especially in respect to consistent cropping in regions with highly variable year-to-year climates.

Eastern filbert blight (EFB) is a hazelnut disease caused by Anisogramma anomala, a fungus endemic to North America where it can be found associated with the wild hazelnut Corylus americana. While C. americana is resistant to EFB, the commercial hazelnut C. avellana is highly susceptible and typically dies within 5 years from exposure. A hazelnut breeding program was initiated in 1996 at Rutgers University in New Brunswick, New Jersey. The early objective was to identify sources of resistance to EFB for use in breeding regionally adapted, EFB-resistant plants to support the establishment of a hazelnut industry in eastern USA. By working with Oregon State University (OSU) and international colleagues, germplasm collections were made in Russia, Ukraine, Poland, Georgia, Latvia, Lithuania, Estonia, Italy, and Turkey. Over 5,000 seedlings were grown and evaluated for response to EFB. While most plants succumbed to EFB, about three percent were found to be resistant. Interestingly, the resistant plants spanned nearly all collection locations and molecular markers have shown them to be highly diverse. Today, we have access to over 100 EFB-resistant accessions selected from more than 60 locations, which represents a significant pool of germplasm to support breeding. Recent studies have shown that most resistance seems to be controlled by only one or a few major genes; R-gene mapping projects are underway at Rutgers as well as Oregon State University. We continue to study new sources of resistance for use in a focused cultivar improvement program.

The disease eastern filbert blight (EFB), caused by Anisogramma anomala, severely limits hazelnut (Corylus avellana) production in North America. In the 1970s, ‘Gasaway’, a late-flowering pollenizer, was found to be resistant to EFB in the Pacific northwestern U.S., and to transmit its resistance in a predictable Mendelian pattern. Its subsequent use in breeding at Oregon State University led to the release of a number of EFB-resistant cultivars, including Jefferson, Yamhill, Dorris, Wepster, and McDonald, which are now being widely planted across the Willamette Valley of Oregon. However, some cultivars carrying the ‘Gasaway’ R-gene develop EFB in New Jersey. Thus, the utility of this resistance source for breeding plants adapted to the eastern U.S has not been fully evaluated. In this study, we examined the EFB response of seedlings from 31 different full-sib progenies expected to segregate for the ‘Gasaway’ R-gene. Trees were exposed to the disease over at least 5 years and evaluated on a scale of 0 to 5 (0 = resistant, 5 = highly susceptible). In general, we found the number of seedlings free of EFB (class 0) to be less than expected based on prior work in Oregon, but resistant individuals were identified in each progeny, indicating the presence of heritable resistance. When disease response classes 0, 1, and 2 were combined (resistant through highly tolerant) and considered as “resistant”, the expected ratio of resistant to susceptible trees was recovered, signifying that the single dominant allele alone continues to provide a predictable level of at least tolerance. Modifying genes/factors appear to play a role in the final disease response of the individual seedling. It is thus hypothesized that some cultivars selected as resistant in Oregon that develop cankers in New Jersey may lack the necessary modifying genetic factors for full protection in the east. Until these modifying genes are elucidated, long-term field evaluation remains necessary to identify selections that carry the ‘Gasaway’ R-gene and express a high level of resistance to EFB in the eastern U.S.

In North America, the native hazelnut Corylus americana harbors an endemic fungus, Anisogramma anomala, which causes the disease eastern filbert blight (EFB). Although C. americana is tolerant of EFB, the economically important European hazelnut, C. avellana, is highly susceptible to the disease. This susceptibility greatly limits commercial European hazelnut production in North America. The breeding and subsequent utilization of resistant plants are considered the most effective means of control and efforts are underway to identify diverse sources of genetic resistance. In this study, 1374 C. avellana seedlings, spanning 47 seed lots collected in the Republic of Georgia and 3 in Azerbaijan, were inoculated with the EFB pathogen and evaluated for disease response in New Jersey, USA. After 5 years, plant responses were rated on a scale of 0–5, in which 0 represents no signs or symptoms of EFB and 5 represents all stems showing cankers. Cankers were found on over 94% of the seedlings, with the large majority being highly susceptible (91.4% = rating 4 and 5). However, 79 plants from 34 seed lots remained free of signs or symptoms of EFB (rating 0). In most cases, only one or two resistant seedlings were in each of these seed lots, making it difficult to infer any genetic control of resistance. In contrast, seed lots from ‘Gulshishvela’ and ‘Kharistvala’ yielded significant numbers of resistant plants, which may indicate that dominant genes for resistance are present. Overall, these new plants will add to the genetic resources available for breeding new EFB-resistant cultivars.

The development of new cultivars resistant to the disease eastern filbert blight (EFB), caused by Anisogramma anomala, is of primary importance to hazelnut (Corylus sp.) breeders in North America. Recently, a large number of EFB-resistant cultivars, grower selections, and seedlings from foreign germplasm collections were identified. However, for a significant number of these, little is known of their origin, relationships, or genetic background. In this study, 17 microsatellite markers were used to investigate the genetic diversity and population structure of 323 unique accessions, including EFB-resistant and tolerant germplasm of uncertain origins, in comparison with a panel of known reference accessions representing a wide diversity of Corylus cultivars, breeding selections, and interspecific hybrids. The resulting allelic data were used to construct an unweighted pair group method using arithmetic averages (UPGMA) dendrogram and STRUCTURE diagram to elucidate relationships among the accessions. Results showed 11 consensus groups with EFB-resistant or tolerant accessions in all, providing strong evidence that EFB resistance is relatively widespread across the genus Corylus. Furthermore, open-pollinated seedlings tended to group together with reference accessions of similar geographic origins, providing insight into their genetic backgrounds. The results of this study add to the growing body of knowledge of hazelnut genetic resources and highlight recently introduced EFB-resistant seedling germplasm as new, unrelated genetic pools of resistance.

Eastern filbert blight (EFB) is a serious disease of European hazelnut, Corylus avellana L., which causes economic losses in Oregon (OR) where 99% of the U.S. crop is produced. The causal organism, Anisogramma anomala (Peck) E. Müller, is native east of the Rocky Mountains where it is found in association with C. americana Marshall. While C. americana is tolerant, EFB causes cankering, branch dieback, and death of C. avellana (3). EFB was first discovered in Washington State in the late 1960s (1). Since then, it has spread throughout the Willamette Valley of OR. In OR, ‘Gasaway’, an obsolete pollinizer, shows complete resistance to EFB, conferred by a dominant allele at a single locus (4). ‘Gasaway’ has been widely used in breeding at Oregon State University (OSU) to develop resistant cultivars that are used in most new orchards. In January 2008, cankers containing rows of dark brown elliptical stroma, characteristic of EFB, were first observed on more than 25 trees of ‘Gasaway’ growing at the Rutgers University research farms in Adelphia and North Brunswick, NJ. At that time, cankers were also found on 18 trees of ‘VR20-11’ growing on the research farms. ‘VR20-11,’ an offspring of ‘Gasaway’ that carries the same resistance gene, was released by OSU for use as a pollinizer for ‘Barcelona’, an EFB-susceptible but widely grown cultivar in OR. Additional cankers were observed on the New Jersey trees in January 2009 and 2010. To our knowledge, this is the first report of EFB on either cultivar under field conditions. The cankers are smaller than those on susceptible cultivars. Of 61 cankers on 10 trees of ‘Gasaway’, the average length was 11 cm with a range of 4 to 42 cm. Canker lengths on susceptible trees are typically 20 to 100 cm. The cankers appear otherwise alike with stromata, 2 to 4 × 2 mm, up to 2 mm high; perithecia upright, in the lower part of stroma; asci ellipsoid, 35 to 45 × 9 to 12 μm; and ascospores 8 to 11 × 4 to 5.5 μm, hyaline, smooth, ellipsoid, 2-celled, with the lower cell very short (1 to 1.5 μm long and wide). Genomic DNA was isolated from ascospores excised from cankers of ‘Gasaway’ and ‘VR20-11’. ITS1F and ITS2 primers were used to amplify and sequence the internal transcribed spacer 1 region (ITS1) of the rRNA genes (GenBank Accession Nos. HM565133 and HM565132). BLAST analysis of the 238-bp segments showed 99% homology with a sequence of A. anomala (EU683064). Phylogenetic analysis also confirmed that the two isolates are A. anomala. To test viability, ‘Gasaway’ cankers were excised and ascospore suspensions (1 × 106 spores ml–1) were applied to 15 trees of susceptible ‘Barcelona’ in March 2008 following the protocol of Johnson et al. (2). In December 2009, 12 of 15 inoculated trees expressed EFB. ‘Gasaway’ has shown no signs or symptoms of infection by A. anomala over several decades of exposure in OR, which is believed to have a limited diversity of the fungus due to a single-point introduction. Our findings suggest quarantine efforts must be bolstered to prevent further introductions of A. anomala into the Pacific Northwest to protect the viability of the U.S hazelnut industry. References: (1) A. D. Davison and R. M. Davidson, Jr. Plant Dis. Rep. 57:522, 1973. (2) K. B. Johnson et al. Phytopathology 84:1465, 1994. (3) K. B. Johnson and J. N. Pinkerton. Eastern filbert blight. Page 44 in: Compendium of Nut Crop Diseases in Temperate Zones. B. L. Teviotdale et al., eds. The American Phytopathological Society. St. Paul, MN. 2002. (4) S. A. Mehlenbacher et al. HortScience 26:410, 1991.