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Bruce W. Wood

Pecan growers often receive substantially higher prices for nuts if they can be marketed early in the harvest season, sometime doubling their profit as a result. Time of nut ripening (shuck dehiscence) is the primary limiting factor to the realization of early harvesting. It is now possible to advance shuck dehiscence by 10 days or more using hydrogen cyanamide (formulated as Dormex). A three year study using young 'Cheyenne' trees (6th leaf) indicated that budbreak, flowering, and shuck dehiscence could be advanced when treated with hydrogen cyanamide during the winter dormant season. The degree of advancement varied with the application date and concentration utilized. Results we most desirable when treatments were applied about 60 days before budbreak and application was at the 2% and 4% (480 and 960mM) levels. Hydrogen cyanamide had no detectable adverse effects on any growth, quality or production parameter.

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Bruce W. Wood

The cyclic, alternate bearing and correlative aspects of U.S. produced pecan [Carya illinoinensis (Wangenh.) K. Koch] nuts are characterized. An attempt to forecast production using stepwise autoregressive techniques identified a national level biennial cycle for cultivar (CV) and seedling (SC) class nuts and a novemennial (9 year) cycle for SG class nuts. The intensity of the biennial cycle at the national level has generally been low to moderate over the last 50 years for CV and SG class nuts with no clear time trend being expressed. During the most recent years (1979-1991), national production of CV class nuts has not exhibited pronounced bienniality, whereas that of SG class nuts exhibited a moderate bienniality. The nature of the the irregularity of cycling of U.S. and state production appears to nullify the use of univariate polynomial equations as a practical tool for accurately forecasting nut production. Nut production within individual states was also cyclic, with 2-, 3-, 5, 6-, 10-, 12-, 14-, 15-, and 16-year cycles, depending on state and nut class. The most intense contemporary biennial cycles for CV class nuts were from Oklahoma, South Carolina, and North Carolina, whereas cycling of SG class nuts was most intense in Texas and Oklahoma. Correlations of production within and among states indicated that most interrelationships are relatively weak; however, national production of CV class nuts are highly correlated (r = 0.96) with the production of CV class nuts in Georgia, whereas that of SG class nuts is most correlated with that of Louisiana.

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Bruce W. Wood

Alternate bearing (AB) by individual trees is a major biological problem faced by pecan [Carya illinoinensis (Wangenh.) K. Koch] nut producers. The linkage between flowering and subsequent cropload with xylem sap characteristics at the time of floral bud swelling and expansion is unknown. Multiyear field studies of mature ‘Cheyenne’ and ‘Moneymaker’ trees, in “on” or “off” phases of AB, were evaluated regarding this linkage. Xylem sap flowing from trunks of ‘Cheyenne’ trees just before, and at the time of, budbreak (i.e., “late winter/early spring”) consisted of a variety of simple sugars. These were hexoses (fructose and glucose), a disaccharide (sucrose), polysaccharides (raffinose and stachyose), and sugar alcohols (xylitol and sorbitol). Sucrose was the overwhelmingly dominant simple carbohydrate at this growth stage, comprising 55% to 75% of the total molar composition, regardless of tree bearing status or sampling time during the seasonal transition from late winter to early spring as buds swell, break, and begin to produce shoots and flowers. Both sap flow volume and concentration of individual carbohydrates were much greater in “on” phase than “off” phase trees. “On” phase xylem sap contained ≈19.9-fold more sucrose than sap from “off” phase trees. The concentration of all sap carbohydrates was much greater at flow inception, declining quickly as buds transition from “bud swell” to “budbreak” and subsequent “shoot growth.” Depending on crop year, individual “on” phase ‘Cheyenne’ trees (≈25 years old) exhibited flow volumes 5.5- to 20.2-fold greater than “off” phase trees. In-shell nut yield by both ‘Cheyenne’ and ‘Moneymaker’ trees (110 years old) increased hyperbolically with increasing “late winter/early spring” sap flow volume. Sap flow from ‘Cheyenne’ and ‘Moneymaker’ resulted in near maximum nut yield when flow volume per xylem tap peaked was at ≈10 L/tree and ≈15 L/tree, respectively, over a 16-day sampling period. These findings are suggestive that sucrose, and possibly other simple carbohydrates, moving acropetally toward axillary bud meristems of shoots during “late winter/early spring” at about the time of “bud swelling” influences the final phase of floral development and therefore subsequent cropload.

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Bruce W. Wood

The United States pecan [Carya illinoinensis (Wangenh.) K. Koch] industry is based on about 10,107,170 trees (about 15% nonbearing) comprising about 492,137 acres (199,168 ha) of orchards (34% in Texas, 27% Georgia, and 17% Oklahoma) dispersed among about 19,900 farm operations (36% in Texas, 16% Georgia, and 7% Oklahoma) in 24 states. Fifty-six percent of this acreage is on farms with ≥100 acres (40.5 ha) of trees (i.e., 5% of total farms). An evaluation of production related changes over the last decade indicate fundamental changes occurring in the nature of the U. S. industry. These include a) movement toward agricultural industrialization as reflected by fewer small-farms and more large-farms; b) reduced percentage of young (i.e., nonbearing) trees in most major producing states; c) substantial decline in number of farms and acres in the southeastern regionhistorically the primary production area-yet substantial growth in the northern region of production; d) a national 3% increase in the number of pecan farms and 14% increase in acreage; and e) substantial demographic changes, such as the enhanced importance of the southwestern region including New Mexico with diminished importance of many southeastern states. States also drastically differ in degree of biennial bearing, as measured by the biennial bearing index (i.e., K = 0.04 - 0.73; where 0 = no production variation and 1 = maximum variation), average production efficiency of both orchards [Epa = 192 - 1,224 lb/acre (215 - 1,374 kg·ha-1)] and trees [Ept = 19 - 60 lb/tree (8.6 kg/tree)], variation in grower prices (cv = 18 - 36%), and relationship between price and national supply of pecan (r 2 = 0.94 - 0.03). For the pecan industry as a whole, average price received for nut-meats is as closely associated with national supply of pecan nut-meats as that of almond and pistachio and is far better than that of walnut-pecan's primary competitor. The supply of pecan meats on-hand at the beginning of the season, plus supply from the current season's crop, plus the price of walnut meats accounts for 80% of price variation in average United States pecan meat price.

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Bruce W. Wood

Zinc (Zn) deficiency is common in commercial pecan [Carya illinoinensis (Wangenh.) C. Koch] orchards. Correction by multiple annual foliar spray applications is expensive but effective in eliminating Zn deficiency. Correction by soil application is also expensive and is usually impractical or noneffective. There is a need for more economical and long-lasting methods for satisfying tree Zn nutritional needs. It is reported here that tree foliar Zn needs [(i.e., 50 μg·g−1 dry weight (dw) or greater] are potentially met through one-time “banding” of Zn sulfate (ZnSO4·7H2O) or Zn oxide (ZnO) onto orchard floors. Zinc needs of 4-year-old ‘Desirable’ trees growing on acidic soil were satisfied over a 4-year period by a single-banded soil application of either Zn sulfate or ZnO over underground drip irrigation lines at a Zn rate of 2112 g Zn per tree (giving foliar Zn concentrations of 60–115 μg·g−1 dw). Rates of Zn at 264 to 1056 g per tree are occasionally efficacious, but rates less than 264 g Zn per tree (0, 33, 66, and 132) were always ineffective for meeting a leaf sufficiency threshold of 50 μg·g−1 dw. Sulfate and oxide Zn forms were equally effective in meeting tree Zn needs. Foliar Zn concentrations increased quadratically with increasing soil-banded Zn treatments; however, foliar Zn concentrations did not necessarily increase over the 4-year period within each Zn rate treatment. Increasing amounts of banded Zn per tree also increased foliar Mn concentration (from ≈150 to 269 μg·g−1 dw) of treated trees the fourth year posttreatment but did not affect foliar concentration of other key micronutrients (i.e., Fe, Co, Cu, or Ni). This fertilization strategy offers an efficacious alternative to annual foliar Zn sprays for orchards established on acidic soils and provides a means of ensuring rapid and long-term Zn absorption through soil application. The approach indicates that soil banding of Zn on certain acidic soils can satisfy the nutritional needs of pecan trees for several years after a single application.

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Bruce W. Wood

Dormant season sprays of hydrogen cyanamide applied to pecan [Carya illinoinensis (Wangenh.) K. Koch] trees advanced budbreak, flowering, and shuck dehiscence. Hydrogen cyanamide was applied to dormant branches at ≈60, 45, 30, and 15 days before normal vegetative budbreak at rates of 0, 120, 240, 480, and 960 mm (corresponding to ≈0%, 0.5%, 1%, 2%, and 4%, solutions for 3 years). Depending on treatment, hydrogen cyanamide advanced budbreak by as much as 17 days, female and male flower maturity by up to 15 days, and nut ripening by as much as 14 days without reducing nut yield or causing phytotoxicity. Hydrogen cyanamide applied at 480 to 960 mm ≈60 days before expected budbreak possibly may be used commercially to advance ripening, manipulate time of pollen dispersal, and substitute for chilling when pecan is grown in mild environments.

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Bruce W. Wood

Economic loss resulting from nickel (Ni) deficiency can occur in horticultural and agronomic crops. This study assesses whether excessive iron (Fe) can induce Ni deficiency. Both chelated Fe and diethylenetriaminepentaacetic acid (DPTA; a commonly used Fe-chelant) induces Ni deficiency in pecan [Carya illinoinensis (Wangenh.) K. Koch]. Foliar sprays of Fe [Fe-DPTA (1.1995 g·L−1)] during early post-budbreak shoot growth can trigger, or increase in severity, Ni deficiency symptoms in the emerging pecan canopy. Deficiency is also inducible in greenhouse-grown ‘Desirable’ seedlings at budbreak by Fe-DPTA application to soil and to a much lesser extent by DPTA alone. Endogenous Fe, just after budbreak, triggers Ni deficiency-associated distortions in pecan seedling leaf growth and morphology when the Fe:Ni is ≈150 or greater with subsequent severity being proportional to the Fe:Ni ratio and Fe:Ni ≈1200 or greater triggering extreme dwarfing of canopy organs. Timely treatment of symptomatic organs with foliar-applied Ni-sulfate restores normal growth, whereas foliar treatment with salts of other transition metals (titanium, vanadium, chromium, cobalt, copper, zinc, and molybdenum) of possible metabolic significance is ineffective. Results indicate that excessive endogenous Fe, and DPTA to a lesser extent, in organs and tissues during early post-budbreak growth can trigger Ni deficiency. A similar Fe on Ni antagonism may also occur with the Ni-associated nutritional physiology of other crops; thus, excessive exposure to chelated Fe not only triggers Ni deficiency in pecan, but may also occur in other horticultural and agronomic crops.

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Bruce W. Wood

Pecan is wind pollinated, exhibits heterodichogamy and are either protandrous (I) or protogynous (II). Orchards are typically established using two complimentary flowering types but with no further scrutiny as to the degree of compatibility of these two types. Additionally, orchards are sometime established with a very low frequency of pollinator. An evaluation of several orchards revealed that yield losses are due to poor pollination is likely common. Data indicate that trees beyond about 46 m (150 feet) from a complementary pollinator exhibit substantial reductions in fruit-set; therefore, large block-type plantings are disadvantaged. Flowering data over several years show that Type I and Type II cultivars are often functionally noncomplementary, suggesting that pecan cultivars should also be identified with a seasonal identification (i.e., early, mid, and late). Data also indicate that dichogamy patterns substantially change as trees age or with abnormally warm or cool springs; hence, pollination patterns will vary depending upon orchard age. Data indicate that orchards should be comprised of 3+ cultivars. RAPD-DNA analysis of “hooked-nuts” indicates that this trait is not reliable as an indicator of selfing.

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Bruce W. Wood

Canopy morphology of 83 pecan [Carya illinoinensis (Wangenh.) K. Koch] cultivars differed in structural, size, and form characteristics. Cluster analysis identified two to five distinct classes for canopy height and diameter and their ratio, inclination angles for both major limbs and young shoots with lower-order structures, branch types, and canopy form and volume. Cultivar-related variability in these traits may have the potential for the improvement of pecan cultivars for factors such as light interception, cooling, air movement, and fruiting; thus, there is potential for identifying the development of canopy characteristics adapted to specific site conditions or cultural/management strategies.

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Bruce W. Wood

Inadequate cross-pollination of pecan [Carya illinoinensis (Wangenh.) K. Koch] occurred in block-type orchards generally thought exempt from pollination-related crop losses because of an abundance of nearby potential pollinizers. “Off-genotypes” appeared to be potentially major assets in such orchards due to their role as backup pollinizers; hence, their presence insures against crop losses due to poor pollination. Fruit-set in `Desirable' main crop rows declined sigmoidally as distance from 'Stuart' pollinizer rows increased. For 15.4-m row spacings, rate of decrease was maximum between 49 and 78 m, depending on crop year. Maximum fruit-set was in rows immediately adjacent to the pollinizer. Tree age/size and spring temperature influences on the characteristics of flower maturity windows are probably primary factors contributing to pollination-related fruit-set losses in block-type orchards relying upon pollen from a single complementary pollinizer or from neighborhood trees. For example, flower maturity was earlier in older/larger trees, and higher spring temperatures accelerated catkin development relative to that of pistillate flowers. Maximum fruit-set occurred when pistillate flowers received pollen around 1 day or less after becoming receptive, whereas no fruit-set occurred when they were pollinated around four or more days after initial receptivity. These findings indicate that many block-type orchards in the southeastern United States are exhibiting pollination-related crop reductions and that future establishment of such orchards merits caution regarding the spatial and temporal distribution of pollinizers.