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  • Author or Editor: Shawn A. Mehlenbacher x
  • Journal of the American Society for Horticultural Science x
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The european hazelnut (Corylus avellana L.) is native to most of Europe and nearby areas in Asia Minor and the Caucasus Mountains. Cross-pollination is enforced by sporophytic incompatibility under the control of a single locus with multiple alleles (haplotypes). Fluorescence microscopy is routinely used to determine if a pollination is compatible or incompatible, and use of an array of known testers allows identification of the alleles of cultivars and selections. Both alleles are expressed in the stigmas, but often only one is expressed in the pollen because of dominance. Cultivars are highly heterozygous diploids (2n = 2x = 22) and clonally propagated. Most of the world's leading cultivars were selected from local wild populations near where they are now planted on a commercial scale. Genetic improvement efforts are recent and, although tremendous genetic variability is available, such efforts have had little impact outside of Oregon and France. Studies of genetic diversity using simple sequence repeat markers have placed most cultivars in one of the four main groups: Spanish-Italian, Central European, English, or Black Sea. This study presents 17 years of data on incompatibility in hazelnut, including the discovery of six new S-alleles and determination of the dominance relationships among 105 new pairs of alleles. The total number of alleles now stands at 33. The S-alleles of 284 cultivars, 13 interspecific hybrids, and 522 selections of diverse origin are presented. The S-alleles identified in hazelnut cultivars is information that should be useful to breeders in the planning of crosses, to curators of germplasm collections, and to growers and nurseries as they choose cultivars and pollenizers when designing orchards. Differences in S-allele frequency seen in the cultivars and selections are related to geographic origin. The most common alleles of cultivars in the major geographical groups are S 2 in the Spanish-Italian group, S 5 in the Central European group, S 3 in the English group, and S 4 in the Black Sea group. Most selections belonged to the Black Sea group, and S 4 was by far the most common allele. Differences in allele frequency were also observed among seed lots within a country.

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The relationship between dormancy of seeds and buds of apple (Malus × domestica Borkh.) might provide breeders with an early opportunity to select for delayed development. Seeds of late-flowering genotypes require much longer exposure to chilling temperatures than those of early flowering” genotypes, and they germinate over a much longer period. In three progenies that exhibit much variation for the two traits, seed germination time was correlated with time of leafing-out of the resulting seedlings, and could be used to select for delayed budbreak. However, selection would be ineffective when little genetic variation for seed germination and budbreak is present. Leafing-out ratings in the nursery in the 2nd year were highly correlated with those in the 3rd year, indicating that selection for late leafing in the nursery during the 2nd year would be more effective than selection based on seed dormancy, especially in progenies exhibiting little genetic variability for this trait. Breeders can effectively use both relationships by first eliminating early germinating seeds and then eliminating early leafing seedlings.

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Eight Corylus L. (hazelnut) species were intercrossed in all possible combinations to reveal genetic relationships. Pollinations were made on either individually bagged branches or trees covered entirely with polyethylene using mixtures of pollen of five genotypes to minimize low cluster set due to single incompatible combinations. Percent cluster set, seed germination, and hybrid seedling survival were determined. Hybridity of seedlings was verified by inspection of morphological traits. Based on percent cluster set, seed germination, and hybrid seedling survival along with observed morphological similarities, Corylus species were placed in three groups: 1) the tree hazels C. colurna L. (turkish tree hazel) and C. chinensis Franchet (chinese tree hazel), 2) the bristle-husked shrub species C. cornuta Marshall (beaked hazel), C. californica (A.DC.) Rose (california hazel), and C. sieboldiana Blume (manchurian hazel), and 3) the leafy-husked shrub species C. avellana L. (european hazel), C. americana Marshall (american hazel), C. heterophylla Fischer (siberian hazel), and C. heterophylla Fischer var. sutchuensis Franchet (sichuan hazel). The two tree hazel species crossed with each other readily, as did the three bristle-husked shrub species. The frequency of blanks was low (<20%) for crosses of the tree hazels, and <50% for interspecific crosses within the group of bristle-husked species. The leafy-husked shrub species could be crossed with each other in all directions, although cluster set on C. heterophylla was low. For crosses of species belonging to different groups, set was generally low and the frequency of blanks high. Nevertheless, a few hybrid seedlings were obtained from several combinations. When used as the female parent, C. californica set nuts when crossed with all other species, indicating possible value as a bridge species. Crosses involving C. avellana were more successful when it was the pollen parent. In crosses with C. avellana pollen, cluster set on C. chinensis was better than on C. colurna and the frequency of blanks was much lower, indicating that it might be easier to transfer nonsuckering growth habit from C. chinensis than from C. colurna. Reciprocal differences in the success of crosses was observed. The following crosses were successful C. californica × C. avellana, C. chinensis × C. avellana, C americana × C. heterophylla, C. cornuta × C heterophylla, C. californica × C. colurna, and C. americana × C. sieboldiana, but the reciprocals were not.

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The effect of parentage, spacing, and sucker removal on precocity of hazelnut (Corylus avellana L.) seedlings was investigated. Wider spacing (1.2 vs. 0.6 m) within the row doubled the number of nuts per seedling in the 5th year but had no effect on nut count in the 3rd or 4th year, nor did it affect the percentage of seedlings bearing nuts in any of the three years. Differences among the four progenies were highly significant for number of clusters, number of nuts, and percentage of seedlings bearing nuts in all years and for number of years to first fruiting. The progeny `Barcelona' × OSU 55.097 had the most bearing seedlings in, the 3rd year but was outperformed by `Casina' × OSU 55.129 in the 4th and 5th years. Number of years to first fruiting varied from 4.3 for `Casina' × OSU 55.129 to 5.2 for `Tombul' × `Tonda di Giffoni'. Sucker removal increased both the percentage of seedlings bearing nuts and the number of nuts per seedlings, but the difference was not significant until the 5th year. Sucker removal reduced the length of the juvenile phase by 3 months. The use of precocious parents was more effective than sucker removal in shortening the juvenile period, while sucker removal and wide spacing within seedling rows increased the number of nuts produced by seedlings in the 5th year. Selection of seedlings for early initiation of bearing will shorten the breeding cycle, and the resulting new cultivars will be precocious when planted in commercial orchards.

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From the genome sequence of hazelnut (Corylus avellana), 192 new polymorphic simple sequence repeat (SSR) markers were developed, characterized, and used to investigate genetic diversity in 50 accessions. Next-generation sequencing allows inexpensive sequencing of plant genomes and transcriptomes, and efficient development of polymorphic SSR markers, also known as microsatellite markers, at low cost. A search of the genome sequence of ‘Jefferson’ hazelnut identified 9094 fragments with long repeat motifs of 4, 5, or 6 base pairs (bp), from which polymorphic SSR markers were developed. The repeat regions in the ‘Jefferson’ genome were used as references to which genomic sequence reads of seven additional cultivars were aligned in silico. Visual inspection for variation in repeat number among the aligned reads identified 246 as polymorphic, for which primer pairs were designed. Polymerase chain reaction (PCR) amplification followed by agarose gel separation indicated polymorphism at 195 loci, for which fluorescent forward primers were used to amplify the DNA of 50 hazelnut accessions. Amplicons were post-PCR multiplexed for capillary electrophoresis, and allele sizes were determined for 50 accessions. After eliminating three, 192 were confirmed as polymorphic, and 169 showed only one or two alleles in each of the 50 cultivars, as expected in a diploid. At these 169 SSRs, a total of 843 alleles were found, for an average of 4.99 and a range of 2 to 17 alleles per locus. The mean observed heterozygosity, expected heterozygosity, polymorphism information content, and the frequency of null alleles were 0.51, 0.53, 0.47, and 0.03, respectively. An additional 25 primer pairs produced more than two bands in some accessions with an average of 6.8 alleles. The UPGMA dendrogram revealed a wide genetic diversity and clustered the 50 accessions according to their geographic origin. Of the new SSRs, 132 loci were placed on the linkage map. These new markers will be useful for diversity and parentage studies, cultivar fingerprinting, marker-assisted selection, and aligning the linkage map with scaffolds of the genome sequence.

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Eastern filbert blight (EFB) is a serious fungal disease of european hazelnut (Corylus avellana) in North America. The causal agent is the pyrenomycete Anisogramma anomala, which is native in the eastern United States where it occasionally produces small cankers on the wild american hazelnut (C. americana). However, most commercial cultivars of european hazelnut are susceptible. Infection leads to perennial cankers, girdling of branches, and premature tree death. Cultural practices including scouting, pruning out infected branches, and fungicide applications are recommended to slow disease spread but are expensive and not completely effective. EFB resistance from ‘Gasaway’ is conferred by a dominant allele at a single locus and has been extensively used in the Oregon State University hazelnut breeding program, but there is concern that this resistance could be overcome by isolates now present in the eastern United States or that a new race of the pathogen could arise in Oregon. Segregation for EFB resistance from ‘Uebov’, a new source from Serbia, was studied in three progenies by a combination of structure exposure and greenhouse inoculation. The frequency of resistant seedlings following structure exposure was about 20% in all three progenies. The ratios failed to fit the expected 1:1 ratio but did fit a ratio of 1 resistant:3 susceptible, which would be expected if resistance were conferred by dominant alleles at two independent loci. Seedlings from a cross of susceptible selection OSU 741.105 and ‘Uebov’ were used to study correlation of disease response and presence of alleles at microsatellite marker loci. Resistance was highly correlated with the presence of alleles at marker loci on linkage group 6 (LG6), and these markers also showed segregation distortion. We conclude that EFB resistance from ‘Uebov’ maps to a single locus on LG6 in the same region as resistance from ‘Gasaway’, although only about 20% of the seedlings are resistant because of segregation distortion. ‘Uebov’ has large, well-filled, round nuts and is suitable as a parent in breeding for the in-shell market, but its low nut yields and a high frequency of shells with split sutures are the drawbacks. Its use would expand options for breeding and ‘Uebov’ resistance could be combined with other resistance alleles with an expectation of more durable EFB resistance. Durable resistance would not only sustain the hazelnut industry in Oregon but would also allow expansion of plantings to new areas.

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Eastern filbert blight (EFB), caused by Anisogramma anomala (Peck) E. Müller, is a devastating disease to european hazelnut (Corylus avellana L.) orchards in the Willamette Valley of Oregon. Selection OSU 408.040 showed no symptoms or signs of the fungus following greenhouse inoculations, and enzyme-linked immunosorbant assays (ELISAs) were negative. Segregation ratios in three progenies indicate that a single dominant gene controls the resistance. A total of 64 amplified fragment length polymorphism (AFLP) primer combinations were screened using three resistant and three susceptible individuals as well as the parents of the cross OSU 245.098 × OSU 408.040. Primer combinations that showed no more than one recombinant in these six seedlings were investigated in 30 additional seedlings. Markers that showed <15% recombination with resistance were amplified in the remaining seedlings of the population. Five AFLP markers linked in coupling to resistance were identified. B2-125 was located on one side of the resistance locus at a distance of 4.1 centimorgans (cM), while A4-265 (9.2 cM), C2-175 (5.9 cM) and D8-350 (2.5 cM) were on the other side, and A8-150 cosegregated with resistance. Three of these markers (B2-125, C2-175, and D8-350) were also linked in coupling in a similar order in seedlings from a second progeny. These markers may be useful in marker-assisted selection for eastern filbert blight resistance from hazelnut selection OSU 408.040.

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Eastern filbert blight is an economically significant disease in European hazelnut (Corylus avellana L.) production in the United States. Since genetic resistance is the only viable disease control strategy to this fungal disease caused by Anisogramma anomala (Peck) E. Müller, greenhouse and field screening of germplasm was undertaken to study the inheritance from known resistant sources and to identify new sources for inclusion in the breeding program. We confirmed that `Gasaway' resistance to this disease is conferred by a single dominant gene. No major gen was identified in the field-resistant cultivar Gem. Representatives of six Corylus species were screened to identify new resistant germplasm. Corylus cornuta Marshall var. cornuta, C. cornuta var. californica (A.DC.) Sharp, C. heterophylla Fischer, and C. sieboldiana Blume were highly resistant, as were most C. americana Marshall genotypes and one C. colurna L. clone tested, but C. jacquenontii Decaisne was highly susceptible. In several cases, hybrids of these species with susceptible C. avellana were also resistant. These new sources of resisstance are being incorporated in the resistance breeding effort.

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The hundred-year history of the european hazelnut (Corylus avellana L.) industry in the Pacific northwestern United States is threatened by eastern filbert blight (EFB) caused by the fungus Anisogramma anomala (Peck) E. Müller. Marker-assisted selection has been extensively used for ‘Gasaway’ resistance in the hazelnut breeding program at Oregon State University. Concern over possible breakdown of this single resistance gene provides an incentive to look for new sources of resistance. OSU 759.010, a selection from the Republic of Georgia, has remained free of EFB after inoculations over several years. Random amplified polymorphic DNA (RAPD) markers linked to resistance were identified by screening primers against three resistant seedlings, three susceptible seedlings, and the parents of a segregating seedling population. For the progeny OSU 759.010 × OSU 653.068, 13 linked markers were identified. The markers most closely linked to resistance were 695-1800 on the proximal side and H12-640, 373-700, 349-450, and F08-700 on the distal side. Four of the five markers also segregated in the progeny OSU 759.010 × OSU 665.076, whereas H12-640 was monomorphic. Segregation for disease response in the first population showed a surplus of resistant seedlings, approaching a 3:1 ratio, with closely linked RAPD markers showing similar ratios. In the second population, the observed segregation for disease response and associated markers did not deviate from the expected 1:1 ratio. Based on cosegregation with simple sequence repeat (SSR) markers, resistance from OSU 759.010 was assigned to linkage group 2. Resistance to EFB from ‘Gasaway’ and ‘Ratoli’ was previously mapped to linkage groups 6 and 7, respectively. Therefore, OSU 759.010 provides a novel source of EFB resistance and markers 695-1800, 373-700, 349-450, and F08-700 have potential for use in marker-assisted selection to pyramid EFB resistance alleles.

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Eastern filbert blight (EFB), caused by the fungus Anisogramma anomala (Peck) E. Müller, is an important disease of european hazelnut (Corylus avellana L.) in the Pacific northwestern United States. In 1989, a chance seedling free of EFB was discovered adjacent to a severely diseased orchard near Troutdale, Ore. This selection, subsequently named `Zimmerman', was crossed with three susceptible selections. Based on morphological characters and incompatibility alleles, we speculated that `Zimmerman' (S1 S3) was a hybrid between `Barcelona' (S1 S2) and `Gasaway' (S3 S26). The three seedling populations were inoculated with spores of the pathogen in a greenhouse test and assayed by indirect enzyme-linked immunosorbent assay (ELISA) and by observation of canker incidence. The observed segregation fit a 3 resistant : 1 susceptible ratio in all three progenies, in contrast to the 1 : 1 ratio found when the resistant pollinizer `Gasaway' was crossed to susceptible genotypes. Random amplified polymorphic DNA (RAPD) marker UBC 152800 linked to the resistance gene in `Gasaway' co-segregated with the resistant phenotype in all three populations with 2%, 4%, and 6% recombination, respectively. Seed germination and transplanting records did not provide evidence of selection in favor of resistant seedlings. Pollen germination was 71% in `Gasaway', 29% in `Zimmerman', and 18% in `Barcelona', indicating possible selection at the gametophytic level. Subsequently 16 resistant seedlings of `Zimmerman' were crossed with the highly susceptible selection OSU 313.078. Segregation fit a 3 : 1 ratio in 14 of the 16 progenies, and showed a surplus of resistant seedlings in the other two. None showed a 1 : 1 segregation. Resistance co-segregated with two RAPD markers that flank the `Gasaway' resistance allele. To test allelism of resistance from `Gasaway' and `Zimmerman', VR 6-28 with resistance from `Gasaway' was crossed with `Zimmerman'. Eight resistant selections from this progeny were crossed with OSU 313.078. Five of the eight progenies segregated 3 : 1, two progenies segregated 1 : 1, and OSU 313.078 × OSU 720.056 gave only resistant offspring. The ratios indicate that OSU 720.056 is homozygous resistant and that `Zimmerman' and `Gasaway' share a common resistance allele. Reciprocal translocations have been reported in hazelnut cultivars, including `Barcelona', the leading cultivar in Oregon. `Zimmerman' appears to be a hybrid of `Barcelona' and `Gasaway', but because of cytogenetic abnormalities, `Zimmerman' may have inherited two copies of the chromosome region that contain the resistance locus and flanking RAPD markers. If the region containing the resistance were attached to two independent centromeres, a 3 : 1 segregation ratio for disease response and flanking markers would be expected, and we propose this as the most likely explanation. Resistance from `Gasaway' and `Zimmerman' has been called “immunity” or “complete resistance.” However, we noted a few seedlings with small cankers, nearly all of which lacked sporulating stromata. Flanking RAPD markers indicate that the resistance allele is present in these seedlings. Although not “immune” or “completely resistant,” `Gasaway' and `Zimmerman' transmit a very high level of resistance.

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