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  • Author or Editor: Bruce W. Wood x
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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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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

Abstract

The heavy levels of sooty mold commonly present on pecan [Carya illinoensis (Wangenh.) C. Koch] foliage in the autumn prompted an evaluation of its influence on net photosynthesis (Pn) of pecan leaves. Extra heavy sooty mold levels were observed to block light penetration to the leaf surface by up to 98%. Heavy mold levels suppressed leaflet Pn by up to 70% with suppression due to a blockage of photosynthetically active radiation (PAR). An observed 4°C increase in abaxial leaf surface temperature may also contribute to this suppression. The results indicate a possible need to introduce sooty mold control methods into orchard management programs.

Open Access

Overcrowding in young high-density pecan [Carya illinoensis (Wangenh.) C. Koch] orchards has prompted a study of tree transplanting and evaluation of survival and tree performance. Shoot growth and nut production characteristics of 13-year-old `Stuart' and `Farley' pecan trees subjected to different stubbing and pruning treatments and then transplanted with a large tree spade indicated that transplants can survive with little or no pruning if moved when dormant. Shoot regrowth was proportional to the degree of pruning, and nut production was inversely proportional to the degree of pruning.

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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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Alternate bearing is a major economic problem for producers of pecan nuts [Carya illinoinensis (Wangenh.) K. Koch], yet a fundamental understanding of alternate bearing remains elusive. Nut yields (over a period of up to 78 years) from a commercial-like orchard of 66 cultivars was used to calculate alternate bearing intensity (I). Best-fit regression analysis indicates no association between I and fruit ripening date (FRD) or nut volume; although, there was moderate association with post-ripening foliation periods (PRFP) in that I tends to decrease as the length of the PRFP decreases. Multiple regression models indicated that FRD and nut volume were poor predictors of I: however, PRFP possessed significant inverse predictive power. Late-season canopy health, as measured by percentage of leaflet retention, decreased as FRD approached early-season ripening. Late-season photoassimilation rate was high er on foliage of trees with late FRDs than those with mid- or early-season ripening dates. These data provide new insight into the complex nature of alternate bearing in pecan and provide evidence for modifying the existing theories of alternate bearing of pecan.

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A 4-year field study on pecan [Carya illinoinensis (Wangenh.) K. Koch] provided indirect support of the supposition held by some U.S. pecan growers that air-blast foliar sprays of potassium nitrate (KNO3) plus surfactant enhances nut yield. While these treatments did not measurably influence yield components, foliar K nutrition, or net photosynthesis, they did suppress “yellow-type” aphid populations. While air-blast sprays of water alone suppressed aphid populations, the inclusion of KNO3 plus surfactant provided an additional level of suppression.

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Mouse-ear (ME) is a potentially severe anomalous growth disorder affecting pecan [Carya illinoinensis (Wangenh.) K. Koch] trees. It is especially severe in second generation sites throughout much of the Gulf Coast Coastal Plain of the southeastern U.S., but can also occur in potted nursery trees. Orchard and greenhouse studies on trees treated with either Cu or Ni indicated that foliar applied Ni corrects ME. ME symptoms were prevented, in both orchard and greenhouse trees, by a single mid-October foliar spray of Ni (nickel sulfate), whereas nontreated control trees exhibited severe ME. Similarly, post budbreak spring spray applications of Ni to foliage of shoots of orchard trees exhibiting severe ME prevented ME symptoms on subsequent growth, but did not correct morphological distortions of foliage developed before Ni treatment. Foliar application of Cu in mid-October to greenhouse seedling trees increased ME severity the following spring. Post budbreak application of Ni to these Cu treated MEed seedling trees prevented ME symptoms in post Ni application growth, but did not alter morphology of foliage exhibiting ME before Ni treatment. Thus, high leaf Cu concentrations appear to be capable of disrupting Ni dependent physiological processes. Foliar application of Ni to ME prone trees in mid-October or soon after budbreak, is an effective means of preventing or minimizing ME. These studies indicate that ME in pecan is due to a Ni deficiency at budbreak. It also supports the role of Ni as an essential plant nutrient element.

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Mouse-ear (ME) is a potentially severe anomalous growth disorder affecting young pecan [Carya illinoinensis (Wangenh.) K. Koch] trees in portions of the Gulf Coast Coastal Plain of the southeastern United States. A survey of its incidence and severity found it to be commonly exhibited by replants on second-generation orchard sites, or where mature pecan trees previously grew. While most frequently observed as a replant problem, it also occasionally occurs at sites where pecan has not previously grown. The disorder is not graft transmissible and is only temporarily mitigated by pruning. Degree of severity within the tree canopy typically increases with canopy height. Several morphological and physiological symptoms for mouse-ear are described. Important symptoms include dwarfing of tree organs, poorly developed root system, rosetting, delayed bud-break, loss of apical dominance, reduced photoassimilation, nutrient element imbalance in foliage, and increased water stress. The overall symptomatology is consistent with a physiological deficiency of a key micronutrient at budbreak, that is influenced by biotic (e.g., nematodes) and abiotic (e.g., water and fertility management strategies) factors. A comparison of orchard soil characteristics between ME and adjacent normal orchards indicated that severely affected orchards typically possessed high amounts of soil Zn, Ca, Mg, and P, but low Cu and Ni; and were acidic and sandy in texture. The Zn: Cu ratio of soils appears to be a major factor contributing to symptoms, especially since ME severity increases as the Zn: Cu ratio increases. However, Ni may also be a factor as the Zn: Ni ratio is also larger in soils of ME sites. It is postulated that the “severe” form of mouse-ear is primarily due to the physiological deficiency of copper at budbreak, but may also be influenced by Ni and nematodes.

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