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Eastern filbert blight (EFB), caused by Anisogramma anomala, is a devastating disease of Corylus avellana, the European hazelnut of commerce, and is considered the primary limiting factor of production in eastern North America. Conversely, C. americana, the wild American hazelnut, is generally highly tolerant of EFB, although it lacks many horticultural attributes necessary for commercial nut production. Hybrids of C. americana and C. avellana combine the EFB resistance of the wild species with the improved nut quality of the European species. However, inheritance of EFB resistance from C. americana remains unclear with existing hybrids derived from a very limited selection of parents. To investigate this topic, C. americana and advanced-generation C. americana × C. avellana hybrids were crossed with susceptible C. avellana and the resulting seedlings exposed to EFB through field inoculations and natural disease spread. In the winter after their fifth growing season, plants were rated for the presence of EFB using an index of 0 (no disease) through 5 (all stems containing cankers). The three progeny related to C. americana ‘Rush’ segregated for resistance in a ratio of one resistant to one susceptible, suggesting the presence of a single dominant R gene. A wide array of disease responses was observed for the other progenies with some expressing little EFB resistance or tolerance and others showing a distribution of disease phenotypes typical of control by multiple genes. Overall, the results indicate that both qualitative and quantitative resistance is present in C. americana. They also suggest that the choice of C. americana parent as well as the C. avellana parent will play a significant role in obtaining useful levels of EFB resistance in hybrid offspring, although the degree of disease expression in the parents may not be a useful predictor of progeny performance. Thus, more research is needed to understand inheritance of resistance, especially in advanced-generation backcrosses to susceptible C. avellana.

Hazelnuts (Corylus sp.) are monoecious and wind-pollinated with reproduction limited by a sporophytic self-incompatibility system. They flower during the winter and are dichogamous with the dates of flowering ranging from December to March in New Jersey depending on the genotype, geographic location, and year. Successful, consistent nut production depends on both genetic compatibility and the appropriate timing of flowering between pollinizing and nut-producing cultivars. While the disease eastern filbert blight (EFB), caused by Anisogramma anomala, has severely limited past hazelnut production in the eastern United States, resistant and tolerant genotypes are now available for testing. However, little is known of their flowering phenology in this region. In this study, the flower and budbreak phenology of 19 different EFB-resistant and EFB-tolerant hazelnut accessions was evaluated over 4 years, and the results compared with air temperature data collected during bloom. Results showed that the accessions followed a similar progression of bloom each year (both staminate and pistillate flowers), which allowed their placement into early-, mid-, and late-flowering groups. However, the date of bloom and duration of bloom, especially for pollen shed, differed each year, largely corresponding to average air temperature trends. Confirming previous reports from other cold regions, it was shown that consistently colder average temperatures delayed bloom until later in the winter, which then led to a compressed period of flowering once temperatures warmed. In contrast, relatively warm temperatures over the season led to earlier flowering as well as a significant lengthening of the duration of bloom, similar to responses reported in Mediterranean climates. Our study documents hazelnut flowering phenology under New Jersey’s variable winter climate, and the results provide a benchmark for selecting suitable pollenizers and breeding parents for future nut production, flowering research, and/or genetic improvement in this region.

One hundred ninety clonal accessions of Corylus, including species and various interspecific hybrids of C. avellana, C. americana, C. heterophylla, C. colurna, and C. fargesii, were assessed for their response to field exposure to the eastern filbert blight (EFB) pathogen, Anisogramma anomala, in New Jersey, where the fungus is native. Plants were obtained from the U.S. Department of Agriculture Agricultural Research Service National Clonal Germplasm Repository and Oregon State University, the University of Nebraska, Lincoln, and the National Arbor Day Foundation. Additional plant material was acquired from the Morris and Holden Arboreta and from private nurseries in Amherst, NY, and Niagara-on-the-Lake, Ontario, Canada. The accessions were chosen based on their resistance to EFB in Oregon, a region where A. anomala is not native, or anecdotal reports and grower observations of tolerance or resistance to the disease. Trees were planted in the field from 2002 through 2009 in New Jersey where they were exposed to EFB yearly through field inoculations and natural spread. In Jan. 2012, they were visually evaluated for the presence of EFB. The cankers were measured, and the proportion of diseased wood was calculated for susceptible trees. Nearly all accessions reported to be resistant to EFB in Oregon maintained at least a useful level of tolerance in New Jersey with a number remaining free of cankers. However, several accessions developed small to medium-sized cankers and showed branch dieback, including offspring of C. avellana ‘Gasaway’. Most C. americana and C. heterophylla accessions remained free of EFB, although variation in EFB response was found in hybrids of these species with C. avellana, ranging from no signs or symptoms to severe EFB. Nearly half of the C. colurna × C. avellana hybrids developed cankers, whereas each of the C. fargesii accessions and most grower selections developed in eastern North America remained free of EFB. The results document the existence of a wide diversity of Corylus germplasm that expresses resistance or a high level of tolerance to EFB in New Jersey and confirms previous reports that C. americana is highly resistant to the disease. Interestingly, most C. heterophylla and the C. fargesii were also found to be resistant despite originating in Asia where A. anomala has not been found. The various interspecific hybrids show the potential for incorporating EFB resistance from wild species through breeding. The results provide further evidence of differences in disease expression in Oregon and New Jersey, where isolates differ and disease pressure may be higher.

An eastern filbert blight resistance screening technique was developed that reduces the time required to identify susceptible Corylus avellana L. seedlings from the previously reported 14 to 16 months after inoculation to 6 to 7 months. To accomplish this, hazelnuts were harvested at maturity, treated with GA₃, germinated, and grown for about 8 weeks at 24 °C day/18 °C night with 16-hour daylengths. Seedlings were then moved to a humidity chamber and inoculated with ascospores of Anisogramma anomala (Peck) E. Müller 3 times over 2 weeks by misting until run off with a solution of 1 × 10⁶ ascospores/mL in sterile distilled water. Following inoculation, seedlings were returned to the original greenhouse for 8 weeks and then were moved to a 10 to 15 °C day/5 to 10 °C night greenhouse with natural daylengths for 4 weeks. They were then moved to a 4 °C cold room for 8 weeks to receive chilling. Afterwards, seedlings were returned to a greenhouse at 24 °C day/18 °C night where stromata development was visible in 4 to 6 weeks.

Anisogramma anomala (Peck) E. Müller is the causal agent of the disease eastern filbert blight (EFB) of hazelnuts (Corylus spp.). Little is known of its genetic diversity and pathogenic variation. Most sources of host resistance have been identified in the Pacific Northwest, a region outside the native range of A. anomala believed to have limited diversity of the fungus due to a long history of quarantine and its relatively recent inadvertent introduction. In an attempt to investigate the pathogenic variation of A. anomala, 12 hazelnut genotypes that showed complete resistance in Oregon were inoculated with 12 isolates collected from across its native range. At the conclusion of the study, ‘Grand Traverse,’ ‘Ratoli’, OSU 541.147, OSU 495.072, and OSU 526.041 remained free of disease. ‘Closca Molla’, OSU 759.007, and OSU 587.044 were infected by most isolates. ‘Gasaway’ was infected by the Michigan isolate, which was also the only one to infect its offspring ‘Zimmerman’, although the lesion lacked sporulating stromata. Interestingly, ‘VR20–11’, another offspring of ‘Gasaway’, was infected by isolates from New Jersey, Minnesota, and Michigan. The Michigan isolate also caused the only signs of infection on OSU 408.040.

The perennial stem canker disease eastern filbert blight (EFB), caused by Anisogramma anomala, is devastating to most trees of European hazelnut (Corylus avellana), as genetic resistance is rare in the species. The pathogen is harbored by the wild American hazelnut (Corylus americana) found throughout much of eastern North America. Wild American hazelnut is generally resistant or tolerant to EFB, and is fully cross compatible with C. avellana, the species grown commercially for its nuts, making it a valuable resource for disease resistance breeding. The objective of this study was to identify quantitative trait loci (QTLs) associated with EFB resistance and tolerance in these two species. Three unrelated EFB-resistant C. americana selections [Oregon State University (OSU) 533.069 from Pennsylvania, OSU 403.040 from Nebraska, and OSU 557.122 from Wisconsin] were crossed with C. avellana ‘Tonda di Giffoni’ (TdG), a cultivar from Italy known to be tolerant of EFB. Their progenies, each containing 124 trees, were exposed to A. anomala through field inoculations and natural spread over 7 years, then each tree was evaluated for cumulative disease response. Results showed that disease response of all three populations exhibited a roughly normal distribution, indicating that resistance/tolerance was under multigenic control. An average of 2869 total markers were used to construct each population’s linkage map following genotyping, which included an average of 121 published simple sequence repeat markers to anchor linkage groups (LGs) to those of previous studies. Linkage maps were constructed for each parent of each population and used to map QTLs associated with EFB response. The subsequent analysis resolved five EFB-related QTLs across the three populations, highlighting three genic regions. Unexpectedly, only one QTL was identified from one of the three resistant C. americana parents, located on LG11 of the map of OSU 403.040, whereas three QTLs were found in a similar region on LG10 across the three maps of TdG, and a fifth QTL was found on LG6 of one TdG map. The lack of strong QTLs identified from the three EFB-resistant C. americana parents suggests that their resistance may be highly quantitative and not resolved within the constraints of this study. In contrast, tolerance from TdG appears to be conferred by a limited number of genes with relatively strong effects. Based on prior mapping work in European and American hazelnut where R genes have been located on LG2, LG6, and LG7, the QTLs associated with resistance/tolerance on LG10 and LG11 represent novel resistance regions. These QTLs present new targets for marker aided breeding, especially when pyramiding EFB resistance genes is a goal.

Six hundred five hazelnut (Corylus avellana L.) seedlings from a diverse germplasm collection made in the Russian Federation and the Crimean peninsula of the Ukraine were inoculated with the eastern filbert blight (EFB) pathogen Anisogramma anomala (Peck) E. Müller and their responses evaluated. Responses were rated on a scale of 0 to 5, in which 0 represents no sign of EFB and 5 represents all branches exhibiting cankers. At final evaluation, eight seedlings showed no signs of the pathogen or symptoms of the disease. Five additional seedlings expressed only very minor signs of the pathogen (rating = 1). The remainder ranged in disease expression from moderately to severely infected to dead with 89.7% (470 of 524) of the surviving seedlings rating 4 or 5. Of the 13 apparently resistant seedlings (rating 0 or 1), seven originated from nuts purchased from roadside vendors near Simferopol, Crimea, Ukraine; five from nuts purchased at an outdoor market near Krasnodar, Russia; and one from nuts obtained from the hazelnut breeding program of the Nikita Botanical Gardens, Yalta, Crimea, Ukraine. Random amplified polymorphic DNA (RAPD) markers generated by the primers UBC 152₈₀₀ and OP AA12₈₅₀, which are tightly linked to the single dominant resistance gene ‘Gasaway’, were not present in all 13 resistant seedlings, providing support, along with their geographic origins, that they represent novel sources of genetic resistance to EFB.

Stable genetic resistance to the fungal disease eastern filbert blight (EFB), caused by Anisogramma anomala, is vital for sustainable production of European hazelnut (Corylus avellana) in eastern North America. In this study, new hazelnut germplasm from the Russian Federation, Ukraine, and Poland (a total of 1844 trees from 66 seed lots) was subjected to A. anomala under field conditions over at least five years in New Jersey. Plants were then rated for the presence of EFB using an index of 0 (no disease) through 5 (all stems containing cankers). Nuts of the resistant trees were evaluated to identify plants with improved kernel characteristics. Genomic DNA of these trees was also screened with sequence-characterized amplified region (SCAR) markers generated by the primers BE-03, BE-33, and BE-68, which are closely linked to the single dominant R-gene of ‘Gasaway’, to assess the resistant seedlings for the presence of this well-known source of resistance. At final evaluation, 76 trees remained free of disease with nine expressing only minor symptoms (rating 1 or 2). The resistant trees spanned 24 different seed lots representing all three countries. The remaining trees ranged from moderately to severely infected with 81% of the total collection rating 5. Several of the resistant trees were found to produce commercial-sized (≈12 mm diameter), round kernels that blanched well. Although the results of the ‘Gasaway’ SCAR primers were inconclusive, the diverse collection origins and disease phenotypes provide evidence that novel sources of resistance were likely identified in this study. These new plants should broaden the genetic base of EFB-resistant C. avellana hazelnut germplasm available for breeding.

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.