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  • Author or Editor: Shawn Mehlenbacher x
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Disease resistance is an objective of most breeding programs for small fruits, tree fruits, and nuts. Often a moderate level of resistance is adequate, and must be combined with many other desirable horticultural characteristics. Classical methods (a segregating population of the host plant is inoculated with a virulent isolate of the pathogen under environmental conditions appropriate for disease development) have been used with great success and have incorporated both horizontal and vertical resistance. Molecular approaches offer new opportunities and are likely to be appropriate and cost-effective in a few situations. Transformation is not yet routine in fruit and nut crops, and there is a shortage of useful genes. Genetic maps are being constructed using RFLP and RAPD markers in several species, allowing determination of number and location of important genes as well as indirect selection based on linked markers. This presentation will include examples of both classical and molecular approaches as they are used in the genetic improvement of fruit and nut crops with an emphasis on fungal and bacterial diseases.

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Abstract

Semihardwood cuttings of 82 genotypes of peach [Prunus persica (L.) Batsch] and complex hybrids of peach with almond (P. amygdalus Batsch), P. davidiana, P. kansuensis, and P. persica vulgaris siberica were rooted under mist. Average rooting percentages ranged from 7% to 100%. Peach selections generally rooted well with the exception of hardy rootstock selections. P. davidiana rooted poorly, but most of its hybrid progeny rooted well. P. kansuensis and its hybrid progeny rooted well. Peach x almond F1 hybrids generally rooted poorly; the rooting percentage was higher in backcrosses to peach.

Open Access

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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Chilling requirements of 44 genotypes of Corylus avellana L. were estimated by cutting shoots in the field at weekly intervals and forcing them in a warm greenhouse for four weeks. The chilling requirements of catkins, female flowers, and leaf buds were assumed to have been met when development occurred on more than half of the respective plant parts. Chilling requirements were lowest for catkins and highest for leaf buds, and ranged from <100 to 860 hours for catkins, 290-1550 hours for female flowers, and 365-1395 hours for leaf buds. The lowest chilling requirements were observed for the leading cultivars of Turkey and southern Italy. The yellow-leafed ornamental C. avellana var. aure a had very high chilling requirements for all plant parts.

Free access

A chlorophyll deficiency expressed as yellowing of leaves was observed in hazelnut (Corylus avellana L.) progenies. Segregation ratios approximated 3 green: 1 yellow, indicating control by a single recessive gene designated chlorophyll deficient #1, for which the symbol c, is proposed. `Barcelona', `Butler', `Compton', `Lansing', Willamette', and the ornamental selection `Redleaf #3' are heterozygous. Pedigree analysis strongly suggests that all heteroxygotes inherited the recessive allele from `Barcelona'. A cross of `Barcelona' with the yellow-leafed ornamental Corylus avellana L. var. aurea Kirchn. produced no yellow-leafed seedlings, indicating that the chlorophyll deficiencies from these two sources are controlled by different loci. Progenies segregating simultaneously for this trait and the gene controlling presence of anthocyanin indicated that the two traits are inherited independently. Seedlings deficient in chlorophyll but with anthocyanin were able to survive under field conditions, while leaves of yellow-leafed seedlings lacking anthocyanin became scorched and the trees died.

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Seventy-seven trees representing 41 hazelnut (Corylus avellana L.) genotypes were to evaluate variance components and broad-sense heritability for 10 nut and kernel traits from 1994 to 1996. All effects in the models were assumed to be random. All traits had extremely high heritability. This indicated that nearly all of the phenotypic variation had a genetic basis. Knowledge of variance components may help us efficiently allocate resources. Broad-sense heritability estimates were larger than those in narrow sense, suggesting the presence of nonadditive genetic variation in the population.

Free access

The cutleaf hazelnut [Corylus avellana L. f. heterophylla (Loud.) Rehder] is an ornamental form with strongly dissected leaf morphology. Its stigmas express incompatibility allele S20 but none of the other 25 S-alleles was detected with fluorescence microscopy. Three seedlings from a cross of the cutleaf hazelnut and VR6-28 lacked S20 and were investigated further. Each expressed an allele from the parent VR6-28 (S2 S26), S26 in OSU 562.031 and OSU 562.048 and S2 in OSU 562.049. S2 and S26 are low in the dominance hierarchy, so we expected the new allele from the cutleaf hazelnut to be expressed in their pollen. Unexpectedly, fluorescence microscopy showed that pollen of all three selections was compatible on their cutleaf parent and on each other, and furthermore, self-pollinations showed the excellent germination and long parallel tubes in the styles that are typical of a compatible pollination. Controlled self- and cross-pollinations in the field verified the self-compatibility of two selections. Cluster set for self-pollinations was very high (75-90%) and within the range observed for compatible cross-pollinations. Furthermore, the frequency of blank nuts was low (<10%). The second allele in the cutleaf hazelnut is designated S28, and its presence in seedlings of `Cutleaf' is indicated by the absence of S20. Controlled pollinations in the field also showed that selection OSU 562.069 (S2 S28) from the cross `Cutleaf' × `Redleaf #3' was self-compatible. Fluorescence microscopy showed that two additional seedlings were self-incompatible [OSU 367.052 (S1 S28) and OSU 367.076 (S6 S28)] while a third [OSU 706.071 (S9 S28)] was self-compatible. Self-compatibility may be limited to genotypes that combine S28 with a second allele that is low in the dominance hierarchy.

Free access

Abstract

Hazelnut kernels from which the pellicle can be removed easily by dry heat are highly desirable for the international kernel market. Cultivars vary from no to complete pellicle removal after heating. Nut samples of 951 seedlings representing 62 parental combinations were roasted at 130°C for 13.5 min, rubbed, and scored for degree of pellicle removal. Narrow sense heritability, estimated by regression of progeny means on midparent values, was 48% (±10%). This moderately high heritability estimate indicates that selection of easy-to-blanch seedlings should result in rapid progress.

Open Access