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  • Author or Editor: Bruce W. Wood x
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Long-established native tree populations reflect local adaptations. Representation of diverse populations in accessible ex situ collections that link information on phenotypic expression to information on spatial and temporal origination is the most efficient means of preserving and exploring genetic diversity, which is the foundation of breeding and crop improvement. Throughout North America, sympatric Carya species sharing the same ploidy level tend to hybridize, permitting gene flow that contributes to regional diversity and adaptation. The topographic isolation of many fragmented populations, some of which are small, places native Carya populations of United States, Mexico, and Asia in a vulnerable position and justifies systematic collection and characterization. The characterization of indigenous Mexican pecan and other Carya populations will facilitate use for rootstocks and scion breeding and will contribute to pecan culture. The Asian species, as a group, are not only geographically isolated from North American species, but also occur in disjunct, fragmented populations isolated from other Asian species. Section Sinocarya includes the members of the genus most vulnerable to genetic loss. With all species, recognition of utility based on characterization of ex situ collections may contribute to the establishment of in situ reserves. Global Carya genetic resources should be cooperatively collected, maintained, characterized, and developed. The integration of crop wild relatives into characterization and breeding efforts represents a challenging opportunity for both domestic and international cooperation. Genomic tools used on the accessible collections of the National Collection of Genetic Resources for Pecans and Hickories (NCGR-Carya) offer great potential to elucidate genetic adaptation in relation to geographic distribution. The greatest progress will be made by integrating the disciplines of genetics, botany, pathology, entomology, ecology, and horticulture into internationally cooperative efforts. International germplasm exchange is becoming increasingly complicated by a combination of protectionist policies and legitimate phytosanitary concerns. Cooperative international evaluation of in situ autochthonous germplasm provides a valuable safeguard to unintended pathogen exchange associated with certain forms of germplasm distribution, while enabling beneficial communal exploration and directed exchange. This is threatened by the “proprietary” focus on intellectual property. The greatest risk to the productive development of the pecan industry might well be a myopic focus on pecan production through the lens of past practice. The greatest limitation to pecan culture in the western United States is reduced water quantity and quality; in the eastern United States the challenge is disease susceptibility; and insufficient cold hardiness in the northern United States. The greatest benefit for the entire industry might be achieved by tree size reduction through both improved rootstocks and scions, which will improve both nut production and tree management, impacting all areas of culture. This achievement will likely necessitate incorporation of crop wild relatives in breeding, broad cooperation in the testing leading to selection, and development of improved methods linking phenotypic expression to genomic characterization. The development of a database to appropriately house information available to a diverse research community will facilitate cooperative research. The acquisition of funds to pursue development of those tools will require the support of the pecan industry, which in the United States, is regionally fragmented and focused on marketing rather than crop development.

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While nickel (Ni) deficiency occurs in certain agricultural crops, little is known regarding the influence of deficiency on metabolic or physiological processes. We studied the influence of Ni deficiency on the reduced-nitrogen (N) composition of early spring xylem sap of pecan [Carya illinoinensis (Wangenh.) C. Koch]. High-performance liquid chromatography (HPLC) analysis of sap composition found the presence of ureido-, amide-, and amino-N substances and that they are quantitatively influenced by tree Ni nutritional status. Ureido-N forms quantitatively dominated amide-N forms with respect to both molar concentration and the forms in which reduced N atoms are present; thus, pecan appears to be predominately a ureide-transporting species. The primary ureido-N substances in sap of Ni-sufficient trees are citrulline ≈ asparagine ≈ xanthine > ureidoglycolate > allantoic acid > allantoin ≈ uric acid ≈ urea. Asparagine is the primary amide-N form, while only traces of amino-N forms (e.g., tryptamine and β-phenylethylamine) are found in xylem sap. Nickel deficiency substantially increased citrulline and allantoic acid in xylem sap while decreasing the asparagine, xanthine, and β-phenylethylamine concentrations. These Ni-linked quantitative shifts in reduced-N forms indicate that Ni nutrition potentially affects intermediates of both the ureide catabolic pathway and the urea cycle as well as the nitrogen/carbon (N/C) economy of the tree. Xylem sap-associated urease-specific activity was also reduced as a consequence of Ni deficiency. These results indicate that Ni deficiency potentially disrupts normal N-cycling via disruption of ureide metabolism.

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Two shoot dieback maladies (SDM) of pecan [Carya illinoinensis (Wangenh.) C. Koch] are of unknown cause and can adversely affect tree canopy health. They occur during either early spring (SpSDM) or early summer (SuSDM). Field studies found that both maladies predominantly occur on shoots retaining peduncles from the previous crop year's fruit cluster. Isolations of transition zone (from living to dead) tissue of symptomatic shoots, of 14 cultivars, found Phomopsis sp. in 89% or greater of samples and Botryosphaeria spp. in 40% or greater of sampled shoots. Isolations occasionally found some combination of eight other apparently saprobic fungal genera with individual genera typically present in 10% or less of symptomatic shoots but were always present in association with either Phomopsis sp. or Botryosphaeria spp. when shoots exhibited either SuSDM or SpSDM. The SpSDM form was associated with 10 cm or less of the shoot's length before budbreak in early March before expanding to 30 cm or greater by late June to produce the SuSDM form, thus, providing evidence for an ongoing and expanding infection common to both SDM forms. The incidence of both “Phomopsis-associated” SDM forms was greatest on trees likely exhibiting substantial stress, some of which was crop-associated. The consistent association of these two fungi with SDM indicates a role for one or both in its development; however, further pathogenicity research is needed to determine if they are the primary cause of these shoot dieback maladies and how they interact with stress factors. Linkage of Phomopsis sp., and possibly Botryosphaeria spp., to these two SDMs raises the possibility of significant canopy damage in prolific cultivars and emphasizes the importance of management practices that minimize stress in orchard trees.

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An assessment of vegetative traits of pecan [Carya illinoinensis (Wangenh.) K. Koch] from a range-wide provenance collection indicated the existence of at least two distinct populations within the native range (i.e., provenances north of Texas vs. provenances in Texas and Mexico). Southern most provenances generally broke bud earlier, retained foliage later in the fall, grew larger in height and trunk diameter, had narrower leaflet droop angles, had greater leaflet tilt angles, wider limb angles, greater Zn deficiency, less black pecan aphid susceptibility, and less red coloration to foliage than did northern most provenances. Trees originating from Jaumaua, in northern Mexico, were especially noteworthy insomuch that they were by far the tallest, possessed the largest trunk diameters, the longest foliation period, and lowest Zn deficiency ratings of all provenances. One family within this Jaumaua population also exhibited a high level of cold hardiness. Family heritability (hf 2) estimates were ≥0.48 for trunk cross sectional area, date of budbreak, leaf redness, cold injury, leaflet droop angle, and leaflet tilt angle, and ≤0.39 for late season leaf fall, black pecan aphid susceptibility, zinc deficiency, and branch angle.

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Abstract

The high variability in physiologically different stages of leaves and susceptibility of pecan [Carya illinoensis (Wangenh.) C. Koch] cultivars to the pecan scab [Cladosporium caryigenum (Ell. et Lang) Gottwald] fungus prompted an evaluation of phylloplane-associated substances (PASs) that influence fungal conidia germination. Germination of conidia was evaluated in several TLC fractions derived from water or dichloromethane leachates of the phylloplane of pecan leaves. Reciprocal tests of pecan scab conidia isolated from ‘Schley’ and ‘Stuart’ against phylloplane leachates from both ‘Schley’ and ‘Stuart’ were conducted. Several PASs proved to have either inhibitory, neutral, or promotive effects on conidia germination. 5-hydroxy-1,4-napthoquinone (juglone) was identified as one such substance and was observed to be a strong inhibitor of conidia germination, but had no effect on colony growth or sporulation. The susceptibility of pecan foliage to pecan scab appears to be partially dependent on phylloplane composition.

Open Access

The authors have developed a computer model designed for shade-intolerant tree crops which describes the amount of intertree shading in an orchard. These data are used to formulate an optimal orchard design based on shading reduction in orchards for any tree crop during any developmental window at any global location.

Tree shape is modelled as an ellipsoid bisected about the semi-minor axis, with ellipsoid dimensions and eccentricity altered to reflect growth stages of the trees. Intertree shading is measured as the surface area of the projected shadow on the ellipsoid. Variables include crop, light extinction, ellipsoid dimensions, intertree spacing, orchard geometry, time and day of the year, and geographical coordinates. Simulations compared the sunlight-related attributes of a variety of orchard geometries for different growth phases of the trees during different parts of the year for several global locations. Results indicate extensive variability of intertree shading to be a function of latitude, regardless of other variables.

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The relative intolerance of pecan trees to low light environments suggests a need for the development of sunlight management strategies that optimize orchard productivity. Current strategies exhibit little emphasis on integrating sunlight interception, tree growth, and orchard design within this context. Therefore, there is a myriad of geometrical variation among commercial orchards and an associated difference in productivity.

We have developed a mathematical model incorporating computer simulations of pecan tree growth in a wide variety of orchard situations. Variables include tree shape, intertree spacings, geometrical pattern within the orchard, geographical coordinates, and time and day of year. This model predicts the extent of shading during the daily interval of maximum photosynthesis for any combination of these variables. It can also be used by the orchardist to establish orchards in which trees receive maximum levels of sunlight within specific windows of time; for example, during the period of nut filling or during the accumulation of dormant season assimilate reserves.

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The economic cost of pecan scab, caused by Fusicladium effusum G. Winter, can substantially limit profitability of pecan [Carya illinoinensis (Wangenh.) K. Koch] cultivation in humid environments. Laboratory, greenhouse, and field studies found nickel (Ni) to inhibit growth of F. effusum and reduce disease severity on fruit and foliage of orchard trees. Nickel was toxic to the fungus in vitro at concentrations applied to orchard trees, and Ni sprays reduced scab severity on foliage of pecan seedlings in greenhouse experiments. Host genotype appears to influence Ni efficacy with fruit tissue of cultivars of intermediate resistance (i.e., ‘Desirable’) being most responsive to treatment and those most susceptible to scab (i.e., ‘Wichita’ and ‘Apache’) being least responsive. Addition of Ni as a nutritional supplement applied in combination with fungicides applied as air-blast sprays to commercial orchards reduced severity of scab on both leaves and fruit depending on cultivar and date of disease assessment (e.g., scab severity on fruit was reduced by 6% to 52% on ‘Desirable’ in an orchard setting). Nickel-supplemented fungicide sprays to ‘Desirable’ trees in commercial orchards also increased fruit weight and kernel filling, apparently from improved disease control. Although the efficacy of Ni was typically much less than that of triphenyltin hydroxide (TPTH), a standard fungicide used in commercial orchards, Ni treatment of tree canopies for increasing tree Ni nutrition slightly lowered disease severity. These studies establish that foliar Ni use in orchards potentially reduces severity of scab on foliage and fruit in scab-prone environments. The inclusion of Ni with fungicides for management of pecan scab might reduce disease severity over that conferred by fungicide alone, especially if targeted cultivars possess at least a moderate degree of scab resistance. Similar benefit from Ni sprays might also occur in host–fungi interactions involving other crops.

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The influence of pecan [Carya illinoinensis (Wangenh.) K. Koch] leaflet bronzing, a discoloration of the lower surface, on foliar physiology and nut-meat yield is unknown. Field investigations indicate that bronzing can adversely affect foliage by reducing net photoassimilation (A), stomatal conductance (sgw ), and transpiration (E) while also altering stomatal aperture and cellular structure, and increasing temperature. Kernel weight and fill percentage are also reduced. Research indicated that foliar A declined in proportion to degree of bronze coloration, with negative A exhibited by heavily bronzed foliage. A by bronzed foliage did not increase as light levels exceeded ≈250 μmol·m-2·s-1. Within the same compound leaf, nonbronzed leaflets adjacent to bronzed leaflets exhibited greater than normal A. Bronzed leaflets also exhibited lower sgw to water vapor, less transpirational H2O loss, and higher afternoon leaf temperature. Light micrographs of bronzed foliage indicated abnormal epidermal and spongy mesophyll cells. Weight and percentage of kernel comprising the nut declined on shoots supporting foliage bronzing in July to August, but was unaffected when bronzing occurred in September to October. Bronzing of pecan foliage can therefore be of both physiological and economic significance.

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Tree crops are often planted at particular geometrical and spacing patterns with little or no quantitatively based data on how the arrangements influence sunlight interception and productivity.

We have developed a mathematical model describing intertree shading derived from computer simulations of tree growth and light extinction through the canopy. Variables include tree shape, intertree spacings, orchard geometry, geographical coordinates, season, and time of day. This model predicts the extent of intertree shading during the daily interval of maximum photosynthesis for any combination of these conditions and indicates that optimal orchard design is unique for each latitude and tree crop. It can be used by the orchardist to establish orchards in which trees receive maximum levels of sunlight within specific windows of time; for example, during the period of fruit development or during the accumulation of dormant season assimilate reserves.

Free access