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  • Author or Editor: C.C. Reilly 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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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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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.

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Nearly 5000 seedling trees representing more than 100 peach [Prunus persica (L.) Batsch.] and plum (Prunus spp.) lines were planted at a 4 × 0.6-m spacing in Jan. 1983, on a site with a known history of peach tree short life (PTSL) and Armillaria root rot (ARR). Trees were arranged in a randomized complete-block with eight replicates of six trees each. Beginning in Spring 1984 and each year thereafter the cause of tree death was determined. At the end of 9 years, 50% of the trees had succumbed to PTSL and 35% had been killed by ARR apparently caused by Armillaria tabescens. Analysis of the data for trees killed by ARR showed a wide range in mortality, some peach lines appeared significantly more tolerant to ARR than others. Plum lines derived from native North American species also appeared to be a potential source of improved tolerance. We did not establish whether ARR tolerance is affected by PTSL.

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Germ tube, appressorium, and subcuticular hypha development were analyzed on host and nonhost leaves for Cladosporium caryigenum (Ell. et Lang. Gottwald), the fungus causing scab on pecan [Carya illinoinensis (Wangenh.) C. Koch]. Plant features characterized for supporting fungal growth were genotype, adaxial and abaxial leaf surfaces, and leaf maturity. Germ tubes and appressoria developed on all plant leaves, despite genotype, leaf surface, or maturity. Germ tube frequency on the susceptible host, `Wichita', was lower than on the resistant host, `Elliott', but was not significantly different from the nonhost, tobacco (Nicotiana tabacum L.). Appressoria formed with equal frequency on leaves of both pecan cultivars and tobacco. Adaxial and abaxial leaf surfaces were not different within any given genotype for supporting fungal development. Immature leaves of `Elliott', but not of `Wichita', had a higher frequency of germ tubes and appressoria than mature leaves. Subcuticular hyphal development occurred only on immature leaves of susceptible `Wichita' pecan. Hence, subcuticular hyphal development is a prime candidate for being the fungal stage specific for host susceptibility. Resistance to C. caryigenum infection appears to be expressed at the plant site beneath the cuticle as fungal hyphae did not develop in a resistant pecan genotype or on nonhost leaves. Thus, resistance to the fungus causing pecan scab likely is expressed after both germ tube and appressorium development and operates beneath, not on the surface, of the leaf cuticle. Furthermore, technology developed to make these assessments would be adaptable in pecan breeding programs to screen for scab resistance.

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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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Two field experiments were conducted to assess peach (Prurus persica L.) cultivar susceptibility to the three Botryosphaeria spp. that cause peach tree fungal gummosis. Inoculated trees were evaluated for disease severity by rating gum exudation, vascular discoloration, and fungal colonization. Each severity measurement yielded a different rank ordering of cultivars for susceptibility. However, in a greenhouse study, these same measurements gave consistent rankings for aggressiveness of the fungal species on `Blake'. Despite large differences in disease severity in the greenhouse study, none of the severity measures were correlated with tree growth after inoculation. The only factor significantly correlated with growth rate of the trees after inoculation was growth rate before inoculation.

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Long-term field trials of a wide range of peach [Prunus persica (L.) Batsch] germplasm on two peach tree short-life (PTSL) sites revealed marked differences in survival among lines. Generally, cuttings and seedlings of a given line performed similarly, as did ungrafted seedlings and their counterparts grafted to a commercial cultivar. No apparent relationship existed between a line's chilling requirement and survival. B594520-9 survived best in Georgia and South Carolina, providing significantly greater longevity than Lovell, the standard rootstock for use on PTSL sites. B594520-9 is derived from root-knot-nematode-resistant parentage, and progeny of surviving seedlings have demonstrated root-knot resistance similar to Nemaguard seedlings.

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