, percentage of root crown length rotted and B , percentage of root rot. On x axis, cal = Juglans californica , nig = J. nigra , reg = J. regia , hin = J. hindsii , maj = J. major , mic = J. microcarpa , and ste = Pterocarya stenoptera . Fig. 2
RFLP markers were used to investigate genetic diversity among California walnut (Juglans regia) cultivars and germplasm collected worldwide. Sixteen of 21 RFLP markers were polymorphic in the 48 walnut accessions tested. RFLP markers were useful for identifying walnut cultivars. All genotypes were heterozygous at ≈20% of the loci for both California and worldwide germplasm. California walnut germplasm contained 60% 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.
In vitro propagation of mature cultivars of Juglans regia L. (Persian or English walnut) could have several major advantages over propagation by grafting on seedling black walnut [J. hindsii (Jeps.) Jeps.] rootstock. If commercial cultivars perform satisfactorily on their own roots, the expensive and time-consuming process of grafting could be avoided, genetic uniformity could be assured, and trees would not be subject to the lethal girdling of blackline disease (5). In addition, mature clones on their own roots might be more precocious than grafted seedlings. These factors are becoming increasingly important as walnut growers change to high-density or hedgerow orchards (D.E. Ramos, personal communication).
During 2 successive seasons, and several experiments, trees of Juglans regia cvs. Payne and Hartley were shaken and the walnuts were allowed to remain on the ground in sun or shade up to 72 hr before drying. Kernel temperature exceeded air temperature, sometimes by more than 10°C in those walnuts which were exposed to the sun, while those in the shade remained below air temperature. When the ambient air temperatures exceeded 35°C, extensive kernel quality loss occurred in shaded walnuts. At lower air temperature the market value of sun-exposed walnuts was decreased. Under high air temperature sun-exposed walnuts with hulls lost more market value than walnuts without hulls. Moisture content of the hull at time of exposure had no major effect on kernel quality. The greatest loss of kernel quality occurred when walnuts were exposed to midday sun.
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.
Annual applications of N, P, and K fertilizers were broadcast for 4 years around black walnut trees (Juglans nigra L.) in an upland plantation to determine their effect on nut production and foliar nutrient levels. Fertilization significantly (P = 0.05) increased nut production, treatments containing P with N and/or K being most effective. Doubling the rate of application did not produce a corresponding yield in nuts. Increases in leaf concentrations of N, P, and K were associated with increasing treatment levels of these elements. Levels of all elements tested, except P, were above deficiency levels. The modest gain in production suggests that soil fertility was not a major factor limiting nut production for trees in this study.
RFLP analysis was employed to study the inheritance of and genetic diversity identified by cloned walnut genomic probes. An interspecific backcross population of (J. hindsii × J. regia) × J. regia was used to determine the inheritance of thirty low copy number RFLP cloned probes. Of these probes, approximately 20% correspond to single copy loci, 40% correspond to single major loci with multiple minor loci, and 40% correspond to two major loci. Twenty of these probes were used to analyze variability within and between 13 walnut species (Juglans spp.). Substantial genetic variation was identified within many wild walnut species, while limited variation was identified within butternut (J. cinerea) and the widely cultivated English walnut (J. regia). Extensive polymorphism was found between walnut species, allowing a phylogenetic relationship of walnuts based upon RFLP markers to be developed. Identification of clonally propagated walnut cultivars by RFLP typing was readily performed in black walnut (J. nigra) accessions, was more difficult in English walnut accessions, and rarely possible in butternut accessions.
English walnut (Juglans regia) producers in California compete with many insect and disease pests to produce an acceptable crop. Traditional control strategies work reasonably well for most pests. However, environmental concerns, loss of certain pesticides and new or impending regulations threaten the use of many traditional techniques for control of many of the pests. Codling moth (Cydia pomonella), walnut husk fly (Rhagoletis completa), and walnut aphid (Chromaphis juglandicola) are the major insects that affect California walnut production. Control strategies that use integrated pest management programs, beneficial insects, mating disruption, insect growth regulators, improved monitoring techniques and precise treatment timing based on the insect's life cycle are leading edge techniques currently available for insect control in walnuts. Major diseases include walnut blight (Xanthomonas campestris pv. juglandis), crown gall (Agrobacterium tumefaciens) and crown and root rot (Phytophthora spp). Both copper resistant and copper sensitive strains of the walnut blight bacterium are best controlled with combinations of copper bactericides and maneb instead of copper materials alone. A new computer model, Xanthocast, used to forecast the need for walnut blight treatment is under evaluation. Crown gall is managed using a preplant biological control agent and a heat treatment to eradicate existing galls. Phytophthora crown and root rot is dealt with primarily by site selection, irrigation management and rootstock selection.
breeding studies have been relatively backward in China. To resolve this problem, we started a distant crossbreeding program, using Juglans regia , Juglans hindssii , Juglans nigra , and Juglansmajor as breeding parents, with the goal of breeding
Crown gall, caused by the common soil-borne bacterium Agrobacterium tumefaciens, can be an economic problem in walnut nurseries and production orchards in California. The principal rootstocks used for commercial walnut production in California are the native Northern California black walnut, Juglans hindsii, and “Paradox,” which are interspecific hybrids between a black walnut, primarily J. hindsii, as the maternal parent, and J. regia, the English walnut, as the paternal parent. Recent evidence has shown that some commercial black walnut trees producing Paradox hybrid seedlings are actually hybrids between J. hindsii and two other North American black walnut species, J. major and J. nigra. Here, we document that there was a higher incidence of crown gall on Paradox (J. hindsii ×J. regia) than on J. hindsii in three sites with natural soil inoculum. Paradox seedlings (with a female parent that was primarily J. hindsii with some J. nigra) inoculated with A. tumefaciens on the roots during transplanting had a higher incidence of crown gall than either J. hindsii or J. regia. When stems were inoculated with A. tumefaciens, J. hindsii ×J. regia populations had significantly larger galls than either J. hindsii or J. regia. Similarly, in stem inoculations on four out of six Paradox genotypes with a hybrid black walnut maternal parent, the progeny produced significantly larger galls than either J. hindsii or J. regia. However, two Paradox populations from black walnut hybrids that contained J. major, J. nigra, and J. hindsii produced galls that were no different in size than in the black walnut species and J. regia. Results suggest that J. regia and black walnut species are less susceptible to crown gall than most Paradox populations.