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B.W. Wood

Alternately bearing `Cheyenne' pecan [Carya illinoensis (Wangenh.) K. Koch] trees were studied to assess the temporal aspects of previous season fruit development on several reproductive and vegetative traits of horticultural importance. Action spectra were generated and used to identify the relative sensitivities of these traits to the temporal aspects of fruiting. Based on date of maximum rate of change in sigmoidal models fitted to these action spectra, the relative sensitivity of certain important growth and developmental parameters to fruit removal time was number of distillate flowers per terminal shoot > number of distillate flowers per flower cluster on lateral shoots> length of terminal shoots > percentage of lateral shoots with fruit= catkins per terminal shoot at top of the tree> percentage of terminal shoots with fruit > catkins per standard terminal shoot> shoots produced per l-year-old branch> percentage of l-year-old shoot death. Maximum rates of change for these reproductive and vegetative parameters were typically during the dough stage of ovule development; however, substantial change also occurred for several parameters over a much wider developmental window. No evidence was found for a hormone-like translocatable factor from developing fruit that either promotes or inhibits flowering. Extending the time from nut ripening to leaf drop increased production of staminate and distillate flowers the following year and appeared to increase fruit set.

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B.W. Wood and C.C. Reilly

Semi-parasitic evergreen mistletoe (Phoradendron flavescens Nutt.) is an increasingly serious weed causing loss of nut yield and tree vigor in pecan [Carya illinoinensis (Wangenh.) K. Koch] orchards of the southeastern United States. Several herbicides and growth regulators were evaluated for efficacy against mistletoe. The dimethylamine salt of 2,4-D proved to be an effective control agent. Ethephon, glyphosate, paraquat dichloride, and polyborate exhibited little or no long-term efficacy. The dimethylamine salt of dicamba also killed mistletoe, but exhibited potential for harming host trees. Dormant season treatment of mistletoe clusters with 2,4-D reduced photosynthesis by about one-third soon after treatment, and by ≈90% from 6 to 16 weeks posttreatment, but clusters did not die until ≈4 months posttreatment. Host limbs, less than ≈3 cm in diameter at the site of mistletoe attachment, usually died within 12 months of 2,4-D treatment of the associated mistletoe cluster. Treatment of entire host trees with 2,4-D did not harm trees if applied prior to ≈1 week of budbreak. Spot treatment of mistletoe clusters, with 2,4-D at 1.2 to 2.4 g·L-1 a.i. (plus 2% crop oil), ≈2 to 3 weeks before budbreak, gave effective long-term control of mistletoe. The inclusion of a crop-oil in the 2,4-D spray greatly increased efficacy. Chemical names used: (2-chloroethyl) phosphonic acid (ethephon).

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C.C. Reilly, B.W. Wood, and M.W. Hotchkiss

Zonate leaf spot (ZLS) [Cristulariella moricola (Hino) Redhead (C. pyramidalis Waterman and Marshall)] on pecan [Carya illinoinensis (Wangenh.) K. Koch.]—associated with unusually wet weather during June, July, and August—occurred across much of Georgia during Summer 1994. Scott–Knott cluster analysis indicated that 27 of 36 evaluated genotypes exhibited little or no field susceptibility to ZLS. `Moneymaker' exhibited the greatest susceptibility of all cultivars studied, with `Cape Fear', `Elliott', `Sumner', and `Sioux' segregating to exhibit moderate susceptibility. An evaluation of commercial orchards indicated susceptibility of major southeastern cultivars as `Desirable' < `Stuart' < `Schley' < `Moneymaker'. Control of ZLS in commercial orchards using standard fungicide spray strategies appeared to be generally ineffective.

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B.W. Wood, C.C. Reilly, and W.L. Tedders

Fungal leaf scorch, a potentially devastating disease in pecan [Carya illinoinensis (Wangenh.) K. Koch] orchards, was influenced substantially by irrigation and genotype. Three years of evaluating 76 pecan cultivars revealed that all cultivars exhibited scorch symptoms and that at least three classes of scorch susceptibility existed. Severity of symptoms was also much greater in nonirrigated than irrigated trees, and there were substantial differences in the concentrations of free nitrogenous compounds and free sugars in leaves between irrigated and nonirrigated trees.

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A.P. Nyczepir, B.W. Wood, and C.C. Reilly

Pecan [Carya illinoinensis (Wangenh.) K. Koch] trees exhibit nickel (Ni) deficiency in certain orchard situations. The symptoms are manifest as either mouse-ear or replant disorder and in certain situations are associated with nematode parasitism. A field microplot study of pecan seedlings treated with either Meloidogyne partityla or Criconemoides xenoplax or both found that parasitism by M. partityla can result in enhancement in the severity of mouse-ear symptoms and a reduction in foliar Ni concentration. The Ni threshold for triggering morphological symptoms in young developing foliage was between 0.265 and 0.862 μg·g–1 dry weight, while the threshold for rosetting was between 0.007 and 0.064 μg·g–1 dw. Results indicate that parasitism by M. partityla is a contributing factor to the induction of Ni deficiency in pecan and raises the possibility that nematode parasitism and Ni nutrition can be contributing factors to many plant maladies.

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S. Abou Taleb, I. Yates, B.W. Wood, and M.M. Fouad

A study was conducted to develop protocol for the preservation of pecan genetic variability by cryogenic storage of zygotic embryos and subsequent in vitro plant regeneration. Parameters evaluated for their influence on embryo survival included the amount of intact kernel, liquid nitrogen (LN) treatment, desiccation, and genotype specificity. Optimum germination with minimum contamination occurred with 12% of the kernel intact. Treatment of explants with LN reduced the percentage of embryos developing into intact plants. `Curtis' and `Shoshoni' had a significantly higher morphogenic response in shoots only than all other cultivars. In summary, cryogenic storage of pecan zygotic embryos was determined to be a feasible means for preservation of pecan germplasm. However, the procedures used in the current study should be altered to increase the probability of embryo survival.

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S. Abou-Taleb, B.W. Wood, M.M. Fouad, and I.E. Yates

At present, a clonal rootstock for pecan is unavailable. Studies were initiated to evaluate the effectiveness of air-layering, stooling and trench layering techniques for propagation and field survival of clonal pecan rootstocks and to obtain an estimation of the relative responsiveness of genotypes.

These studies demonstrated that clonal rootstocks and scion materials can be produced for pecan genotypes by either of the air-layering, stooling, or trench layering techniques. Survival and health of clones were generally best from ramets produced by the stooling and trench layering methods if girdling plus IBA is used to induce rooting. Of the three methods evaluated, stooling would appear to be the most practical method of producing rootstock materials since it produces the most healthy clones, although its yield per tree is probably a little less than that of trench-layering. The influence of the `girdling plus IBA' treatment on overall physiology of the rooted shoot appears to be relatively minor when compared to that of the `check'.

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M.M. Fouad, S. Abou-Taleb, B.W. Wood, and I.E. Yates

Certain key experiments on the clonal propagation of pecan by stem and root cuttings are described. Juvenile cuttings of pecan cv. `Stuart' produced more roots and a larger percentage of rooted cuttings than adult cuttings when treated with 1% solution of (IBA) in Feb. and Aug. Successful propagation from `Stuart' stem cuttings was high from July to September. This study also fails to support the hypothesis that auxins are necessary to increase the level of rooting by pecan hardwood cuttings. Pecans were propagated using root cutting techniques and successfully transplanted to containers. Root cutting sections collected in either March or August from two-year-old `Curtis' seedling, responded yielding high rooting success and survival. The data also provides evidence that IBA could enhance the production of roots of root cuttings but BA (Benzyl Adenrine) offers no benefit. Girdling the sprout shoots from root cuttings with the use of IBA doubled the percentage of shoots producing roots and increasing root length.

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R.C. Gueldner, I.E. Yates, C.C. Reilly, B.W. Wood, and M.T. Smith

Polyphenols were analyzed in expanding buds and developing leaves of pecan [Carya illinoensis (Wangenh.) C. Koch] cultivars with varying responses to Cladosporium caryigenum (Ell. et Lang. Gottwald), the organism causing scab. Plant tissue extracts were examined by high-performance liquid chromatography using a water: methanol gradient to separate polyphenolic components on a C-18 reversed phase column. A diode-array detector was used to identify profile components by retention times and computer matching of ultraviolet spectra to standard compounds in a library. Concentrations of these polyphenols were compared throughout the growing season in leaves of pecan cultivars with low (`Elliott'), intermediate (`Stuart'), and high (`Wichita') susceptibility to scab; during susceptibility to infection by Cladosporium caryigenum from 16 cultivars; and in `Wichita' leaf discs with and without scab lesions. The major polyphenolic constituent of tissues for all cultivars was identified as hydrojuglone glucoside, which was detected in intact buds and leaves throughout the growing season. Hydrojuglone glucoside concentration increased concomitantly with leaf expansion and then declined slowly. Juglone was barely, if at all, detectable, regardless of leaf age. No correlation was found between cultivar susceptibility to pecan scab and the levels of either juglone or hydrojuglone glucoside in the healthy leaves of 16 cultivars. Leaf tissue with scab lesions had significantly higher juglone and hydrojuglone glucoside levels than leaf discs without scab lesions. Chemical names used: 4-8-dihydroxy-1-naphthyl b-d-glucopyranoside (hydrojuglone glucoside); 1,5-hydroxy-naphthoquinone (juglone).

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I.E. Yates, E.A. Carter, T.A. Wilkins, and B.W. Wood

Polypeptides from pecan [Carya illinoensis (Wangenh.) C. .Koch] leaves were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and visualized by silver staining. Pecan leaf protein profiles were similar irrespective of cultivar (Desirable and Stuart), leaflet position, reproductive status of the allied shoot, or seasonal leaf age relative to fruit development. The large subunit of ribulose l,5-bisphosphate carboxylase and the majority of the other polypeptides were consistently present. However, the most striking change in the polypeptide composition was the seasonal decline of a polypeptide with an approximate molecular mass of 24.5 kDa. This leaf polypeptide was present in leaves collected in June and July, coinciding with the periods of initial fruit elongation and rapid increase in fruit volume. A detectable decrease occurred by mid-August, when kernel development was initiated. Changes in the abundance of this polypeptide relative to other polypeptides were observed over two growing seasons. Cells of young leaves collected early in the growing season contained more ribosomes and starch granules, but fewer vesicles and smaller electron-dense osmophilic granules than old leaves collected late in the growing season.