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  • Author or Editor: Gale H. McGranahan x
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Twenty-five random decamer primers were used to evaluate the level of polymorphism between Persian walnut and the Northern California black walnut. Sixty-six randomly amplified polymorphic DNA (RAPD) markers were identified using an interspecific walnut backcross population [(Juglans hindsii × J. regia) × J. regia]. Segregation data from these polymorphisms were joined to a previously published set of restriction fragment-length polymorphism (RFLP) marker data to expand the genetic map of walnut to 107 markers in 15 linkage groups.

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Walnut blight of English walnut (Juglans regia L.), incited by Xanthomonas campestris pv. juglandis (Pierce) Dowson, causes significant crop loss in California. To assess levels of resistance in walnut germplasm, leaves and nuts of mature walnut genotypes were inoculated with X. campestris pv. juglandis. Significant differences were found among cultivars in size and frequency of lesions on leaves and in frequency of abscission of diseased leaves. Cultivars also varied in frequency of abscission of nuts following infection and in marketability of infected nuts. Afthough there was considerable variation in disease levels over 2 years, leaves of PI 159568 consistently received significantly higher disease ratings than leaves of `Chandler' or `Adams'. Nuts of `Adams', `Payne', PI 18256, and `Sinensis 5' abscised less frequently following inoculation than nuts of other cultivars. In addition, the quality of infected nuts that did not abscise was consistently better for PI 18256 and `Sinensis 5'. The rank of cultivars for levels of disease in inoculated leaves was not significantly correlated with the rank of cultivars for frequency of infestation of dormant buds associated with infected foliage. The apparent resistance of walnut germplasm may be affected by the abscission or necrosis of infected tissues.

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Abstract

Genetic variation in vigor, phenology, and branching in juvenile seedlings of Juglans californica and J. hindsii was investigated. Significant differences between species were detected in traits that reflect vigor (height, diameter, volume), in phenological traits (dates of leafing out and leaf drop), and in branching. J. californica was generally more vigorous, more branched, leafed out earlier, and dropped leaves later than J. hindsii. When species were analyzed separately, seed source was a highly significant source of variation for phenological traits and branching in J. hindsii. Upper estimates of heritability for phenological traits ranged from 0.47 to 0.88. The results of this study suggest that J. hindsii may have had a history of differentiation and adaptation to latitude, elevation, or other climatic characters. Conservation of germplasm resources in J. hindsii will be essential to maintain the purity of the species and to provide resources for studying the species and breeding rootstock for J. regia, the Persian walnut.

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To determine if flavor differences could be detected among several Persian walnut (Juglans regia L.) cultivars, difference tests with eight cultivars were conducted using the duo-trio method. No differences were found when `Hartley' was compared to `Vina', `Scharsch Franquette', and `Mayette'. However, `Chandler', `Chico', `Howard', and `Sunland' were significantly different, and paired comparisons were then used to test these cultivars against `Hartley' in terms of several flavor characteristics. No differences in astringency and “walnut flavor” were detected; however, `Chandler' was judged to be sweeter than `Hartley', which was sweeter than `Howard'. `Chico' was found to be the firmest cultivar.

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RFLP markers were used to study genetic diversity among California walnut (Juglans regia L.) cultivars and germplasm collected worldwide. 16 of 21 RFLP markers were polymorphic in the 48 walnut accessions tested. Seven RFLP markers permitted unique identification of all walnut cultivars. All genotypes were heterozygous at approximately 20% of the loci for both California and worldwide germplasm. California walnut germplasm contained 65% of the worldwide allelic diversity. Cluster analysis of genetic distance between accessions and principal component analysis of allelic genotypes showed two major groups of walnut domestication. California germplasm was associated with germplasm from France, Central Europe, and Iran, and had less genotypic similarity with germplasm from Nepal, China, Korea, and Japan.

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The role of pollen in abscission of pistillate flowers of Persian walnut (Juglans regia L.) cv. Serr was investigated over a 4-year period by controlled pollinations and pollen counts. Self-pollen, pollen from other walnut selections or cultivars, or dead pollen was applied at high and low doses to pistillate flowers enclosed in pollination bags. Unbagged, open-pollinated flowers and bagged, nonpollinated flowers served as controls. In all cases, presence of pollen significantly increased the probability of pistillate flower abscission (PFA). Dead pollen resulted in as much PFA as live pollen. Counts of pollen grains confirmed that PFA-type flowers had significantly more pollen than normal flowers. In the fourth year `Serr' pollen was applied to unbagged flowers of `Serr' and ten other Persian walnut cultivars, and the amount of PFA on the artificially pollinated flowers was significantly higher than on the open-pollinated flowers, while the control flowers dusted with talc or pine pollen had almost no PFA. These results clearly indicate that excess pollen is involved in pistillate flower abscission in `Serr' walnut and suggests that other cultivars may also be sensitive to pollen load. This phenomenon may have implications in the biology of selfing and evolution.

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Seedlings from seven open-pollinated selections of Chinese wingnut (Pterocarya stenoptera) (WN) representing collections of the USDA-ARS National Clonal Germplasm Repository at Davis, CA, and the University of California at Davis were evaluated as rootstocks for resistance to Phytophthora cinnamomi and P. citricola and graft compatibility with scions of five cultivars of Persian walnut (Juglans regia). Seedlings of Northern California black walnut (NCB) (J. hindsii) and Paradox hybrid (PH) (typically J. hindsii × J. regia) were used as standards. In greenhouse experiments, potted plants of the rootstocks were subjected to intermittent flooding in soil artificially infested with the pathogens. All WN seedlings were relatively resistant to the pathogens (means of 0% to 36% of root and crown length rotted) compared with NCB (44% to 100%) and PH seedlings (11% to 100%). Negligible disease occurred in flooded control soil without the pathogens. In 9-year graft compatibility trials in an orchard, NCB and PH rootstocks supported relatively good survival and growth of all tested scion cultivars (‘Chandler’, ‘Hartley’, ‘Serr’, ‘Tulare’, and ‘Vina’; final scion survival 80% to 100%, mean scion circumference increase 292 to 541 mm), whereas results with WN were mixed. Wingnut rootstocks from all sources were incompatible with ‘Chandler’ (final scion survival 20% to 60%, scion circumference increase 17 to 168 mm). Conversely, all WN rootstocks from all sources were compatible with ‘Tulare’ and ‘Vina’ (final scion survival 80% to 100%, scion circumference increase 274 to 556 mm). Use of the WN rootstocks produced variable results in ‘Hartley’ and ‘Serr’ (final scion survival 10% to 100%, mean scion circumference increase 69 to 542 mm). There was a tendency for more rootstock sprouts on WN selections than on NCB or PH. In a commercial walnut orchard infested with P. cinnamomi, ‘Hartley’ survived and grew markedly better on WN selections than on PH. High resistance to P. cinnamomi and P. citricola was common to all of the WN selections. The results indicate that WN selections may be useful rootstocks for cultivars Tulare and Vina in soils infested with P. cinnamomi or P. citricola and that WN selections may contribute valuable resistance to these pathogens in walnut rootstock breeding efforts.

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Somatic embryos derived from walnut (Juglans regia L.) ovule tissues were evaluated to determine whether they were of zygotic or maternal origin. Molecular markers were used to permit evaluation at an early stage, before whole plant development. Somatic embryos developed from potentially apomictic `Sunland' and `Cisco' ovule tissue isolated from bagged putatively unpollinated flowers. Phosphoglucomutase (PGM) isozyme analysis showed that all of these embryos, except one from each cultivar, carry the same zymotype as the maternal tissue. However, restriction fragment length polymorphism (RPLP) analysis combined with isozyme evaluation demonstrated that the tested embryos originated from zygotic rather than maternal tissues. This study demonstrates the application of molecular marker analyses, particularly RFLPs, evaluation of somatic embryo origin.

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