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Dale E. Kester and Ale E. Kester

The term “clone” is a key biological term that designates a number of horticultural situations. In breeding, many cultivars are designated as clones, originating from consecutive vegetative propagation from individuals within a seedling population, from individual plants of a clone exhibiting “bud mutations,” and, more recently, from genetic engineering and biotechnology. Extensive vegetative propagation of a limited numbers of clones in modern horticultural systems has been accompanied by systemic incorporation by serious pathogens (viruses, viroids, phytoplasmas, etc.), and in some cases by horticultural deterioration (e.g., noninfectious bud-failure in almonds). Control of these problems in clonal propagation is achieved by 1) propagation source selection 2) maintenance of the source in a registered foundation block under protected conditions and 3)multipli-cation in controlled “mother blocks” or “increase blocks” from which commercial material is distributed after a minimum of consecutive generations of vegetative propagation. This system is the basis for Registration and Certification programs and “clean stock” in general. In many crops the selected propagation source is a single plant, its progeny constitutes a “clone,” and the new entity is given a unique name or number. To distinguish this “new” clone from the “original” clone, the designation of FOUNDATION CLONE is suggested. Biological and horticultural significance is illustrated in almond (Prunus dulcis).

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Dale E. Kester and Thos. Gradziel

Almond (Prunus dulcis MIll) and peach (Prunus persica L.) are closely related species with many genetic traits in common. Variation in growth habit shows a consistent pattern among populations of peach, almond and their hybrid offspring. From this material a system of growth habit traits has been identified based upon genetically controlled processes of vegetative shoot elongation and flower bud initiation. All flowers are produced from lateral buds. The classification proposed for their characterization includes:

  • Class I. Growth from terminal buds on one year old shoots (six morphological groups),

  • Class II. Growth produced from lateral buds on 1-year old shoots (three morphological groups),

  • Class III. Combinations of Class I and II These classes cover the entire range of peach and almond phenotypes and probably all Prunus. Class I is precocious and produces flowers by the second year from growth initiation. Class II plants do not produce flowers until the third year. Expression is enhanced by increase in vigor.

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Dale E. Kester and Thomas M. Gradziel

Approximately twenty native almond species have been described. Representative germplasm from seven of these are present in UC collections and have been used in crossing. Three specific breeding lines utilizing these species are described. One (1980 series) involved increasing yield potential through selection of high blossom density following gene introgression from Prunus fenzliana. A second involved incorporation of self-fertility, late bloom, smaller tree size, early maturity, high blossom density, and desirable nut characters from Prunus webbii into commercial breeding lines. A self-fertile selection resembling `Nonpareil' has been obtained from this material. The third line involves transmission of a unique thin, netted-surfaced, hard-shell phenotype from Prunus argentea.

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Thomas M. Gradziel and Dale E. Kester

Breeding lines have been developed incorporating introgressed genes from three native almond species Prunus fenzliana, Prunus webbii and Prunus argentea. Selected traits include self-fertility and autogamy, late bloom, smaller tree size, early nut maturity, improved cropping potential, and a well-sealed shell (endocarp) with high kernel/shell crack-out percentages. Fertility barriers, while present were easily overcome though linkage to introgressed genes with undesirable phenotypes remains an important obstacle to commercial use. Current breeding results, however, support a general conclusion that the wide diversity present within the range of species related to the cultivated almond (Prunus dulcis) provides an valuable gene pool for variety improvement.

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Dale E. Kester, Tom Gradziel, and Karen Pelletreau

A model for the epidemiology of noninfectious bud-failure (Fenton, et al., 1988) predicts that BF-potcntial is universally present within specific almond cultivars with variation existing in the rate and pattern of development of BF phenotypes. Orchard surveys of Carmel in 1990 and 1991 involving four nursery sources showed a trend of 2 per cent of affected trees after one year in the orchard, increasing to 4 per cent in the second, with prospects for gradual increase with time. All four sources produced some BF trees with significant differences among sources. A study has been started to identify the source and pattern of BF-potential within the entire Carmel cultivar. It has two parts. A pedigree analysis of propagation sources from eleven commercial nurseries traces their genealogy from the original seedling plant first discovered in 1947. A propagation test of approximately 3000 individual trees representative of the propagation sources of all eleven commercial nurseries has been established. The origin of each progeny tree has been maintained in respect to source, tree, budstick and individual bud location on the stick. Expression of bud-failure symptoms in individual trees will identify the source and pattern of BF-potential within the cultivar.

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Dale E. Kester, Richard N. Asay, and Thomas M. Gradziel

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Ali Lansari, Dale E. Kester, and Amy F. Iezzoni

The mean inbreeding and coancestry coefficients were calculated for almond, Prunus dulcis (Miller) D.A. Webb, cultivars from the United States, France, Spain, Israel, and Russia. To improve cultivars to meet market demand, the recurrent use of four selections as parents in U.S. breeding programs has resulted in a mean inbreeding coefficient (F) of 0.022 in this collection. In France, a single cultivar, Ferralise, has an inbreeding value of F = 0.250, while cultivars of other almond-producing countries are noninbred (F = 0). Due to the use of common parents, U.S., Russian, and Israeli cultivars share coancestry, while coancestries also exist between French and Spanish almond germplasm. Cultivars of known parentage in the United States, Russia, Israel, France, and Spain trace back, respectively, to nine, eight, three, four, and three founding clones. Future almond-breeding programs may narrow the genetic base and thereby limit genetic gain.

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Dale E. Kester, Warren C. Micke, and Mario Viveros

`Jeffries', a mutant of `Nonpareil' almond [Prunus dulcis (Mill.) D.A. Webb], showed “unilateral incompatibility” in that its pollen failed to fertilize cultivars in the `Carmel' (CIG-V), `Monterey' (CIG-VI), and `Sonora' (CIG-VII) pollen cross-incompatibility groups (CIGs), as well as specific cultivars (`Butte', `Grace', and `Valenta') whose CIG group is unknown. `Jeffries' is not self-compatible, but produced good set when pollinated by 12 almond cultivars representing the entire range of CIGs involving `Nonpareil' parentage, as well as the parent `Nonpareil'. It was concluded that the `Jeffries' mutant—both gametophyte and sporophyte—expressed a loss of a single S allele of the `Nonpareil' genotype.

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Dale E. Kester, K.H Shackel, T.M. Gradziel, M. Viveros, and W.C. Micke

The potential for noninfectious bud-failure in propagation source material for `Carmel' almond in California has been determined in progeny tests from commercial nursery sources. Percentage BF increased with time (temporal), but decreased in severity (spatial). Analysis of variability in nursery sources showed that the key to successful selection for low BF potential is the individual tree, although variability exists among nurseries, budsticks (within trees), and individual buds (within budsticks). One-half of the individual trees of the nursery population tested have produced BF progeny so far within the test period. Future BF from the remainder was project by a BF model to be beyond the critical economic threshold. Two low BF-potential single tree sources were identified for commercial usage and progeny tests have started on an additional 19.

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Dale E. Kester, Kenneth A. Shackel, Warren C. Micke, Mario Viveros, and Thomas M. Gradziel

The spatial and temporal pattern of noninfectious bud failure (BF) expression (BFexp) was studied during seven growing seasons in a population of `Carmel' almond trees originating from twelve commercial propagation sources. All progeny trees were grown in a single experimental site with high prevailing summer temperatures. BFexp increased continuously but irregularly in each nursery population as measured as the proportion of trees showing BF and as an average BFexp rating. Populations from the 12 nurseries represented increasing clonal generations from the original seedling tree and showed increasing levels of BF, as well as a decreasing shape value and increasing scale value derived by a failure statistics model. Models for development, distribution and hazard functions were defined for each of the 12 sources studied. Only sources from the original tree and source A demonstrated potential for commercial use. A significant correlation was found between average yearly increase in BFexp and the average daytime temperature for the previous June. The June period coincides with a specific stage in the seasonal growth cycle when vegetative buds mature.