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  • Author or Editor: Bruce W. Wood x
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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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There is increasing evidence of substantial pollination related crop losses by pecan [Carya illinoensis (Wangenh.) K. Koch] orchards. These most likely occur in block-type orchards consisting of only one or two cultivars, but can also occur at locations with a great number of different genotypes nearby. Main crop cultivars should generally be within about two rows of pollinizers to ensure cross-pollination. Thus, block widths exceeding about four rows between pollinizers are especially likely to exhibit serious pollination problems. Scattered trees of off-type genotypes are potentially of major importance as backup orchard pollinizers. Tree age/size and spring temperatures influence the characteristics of flower maturity windows and are probably primary factors contributing to pollination-related fruit-set losses in many block-type orchards. Flower maturity tends to be earlier in older/larger trees while warmer springs accelerate catkin development relative to that of pistillate flowers. Because of substantial variability in relative differences associated with initiation and duration of flower maturity windows within either protandrous or protogynous flowering types (i.e., Type I or II), selection of complementary pollinizers should be based on the relatively high resolution 30-class flowering classification system rather than the traditional low resolution 2-class system. Other factors sometime causing pollination related crop losses are either abnormally wet weather or strong dry winds during the pollination period or abnormally warm or cool springs. Pollination problems can be visually detected by noting premature non insect related post pollination fruit drop or diminishing fruit set with increasing distance from pollinator trees or off-type genotypes within the orchard.

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Of 18 commonly used adjuvants evaluated on pecan [Carya illinoinensis (Wangenh) K. Koch], a few exhibited potential for substantially suppressing net photosynthesis (A) and the conductance of foliage to water vapor (g sw ) when used within their recommended concentration range; however, most provided no evidence of adversely influencing A or g sw . Suppression of gas exchange by certain adjuvants persisted at least 14 days after a single application. The recently developed organosilicone-based surfactants generally exhibited the greatest potential for suppression. These data indicate that orchard managers should consider the potential adverse influence of certain adjuvants when developing orchard management strategies.

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Pecan [Carya illinoinensis (Wangenh.) K. Koch] nursery transplants performed best on establishment in nonirrigated orchards when using large trees planted early in the dormant season. After 6 years, growth and survival of bare-root transplants were equal to that of containerized transplants when established during the dormant season. Reducing transplant trunk height by ≤75% at planting did not affect subsequent tree survival, although rate of height growth and tree vigor increased such that there was no difference between pruned and nonpruned trees after 3 years, except that pruned trees appeared to possess greater vigor. There also were no differences in growth or survival between augured and subsoil + augured planting sites within 6 years of transplanting, and there were no differences between root pruned (severe tap or lateral root pruning) and nonpruned trees.

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Soil drench applications of paclobutrazol (0, 0.5, 1, 2, 4, 8, 16, and 32 mg a.i·pot-1) to greenhouse grown pecan seedlings reduced plant height, plant dry weight, organ dry weight, in ter node length, leaf thickness, leaf area, and chlorophyll content. Carbohydrate levels (mg·g dry weight-1 and mg·g plant-1) in treated plants increased. Total plant carbohydrate levels were unchanged at levels ≤ 2m g a.i.·pot-1, but plants of reduced size showed increased levels of carbohydrates per mg of tissue. Seedlings treated with high levels of paclobutrazol had a slight tendency for increased net photosynthesis.

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Exposure of 13-year-old trees of several pecan [Carya illinoensis (Wangenh.) C. Koch] cultivars to severe cold in the winters of 1983-84 and 1984-85 resulted in the death of several healthy bearing trees of alternate-bearing cultivars (‘Chickasaw’, ‘Cheyenne’, ‘Cherokee’, and ‘Shoshoni’), while less tree death occurred in moderately bearing and relatively minor alternate-bearing cultivars (‘Cape Fear’ and ‘Desirable’). ‘Chickasaw’ trees entering winter after bearing a heavy nut crop the previous season experienced greater tree death and reduced midwinter trunk tissue levels of starch, sugars, and K than did trees with a light nut crop the previous season. The increased susceptibility of heavily bearing trees, especially of alternate-bearing cultivars, to extreme winter cold may be due to the effect of heavy fruiting on tree reserves and subsequent cold acclimation.

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Abstract

The problems of excessive vegetative growth and tree-size control of young pecan [Carya illinoensis (Wangenh.) C. Koch] trees prompted the evaluation of three tirazole analogs [paclobutrazol (PBZ), uniconazole (UCZ), and flurprimidol (FPD)] for their growth suppression efficacy and horticultural usefulness on pecan. A one-time soil application of these growth regulators at 132, 264, and 588 µmol·cm–2 trunk cross-sectional area suppressed shoot elongation by 50% to 90% for up to 3 years after treatment. Growth suppression greater than about 60% reduced nut yield; presumably due to drastically reduced leaf area and internal shading among leaves within compacted shoots. Relative efficacy in terms of shoot growth was UCZ > PBZ > FPD; however, all three chemicals exhibit commercial potential for controlling tree size. Chemical names used: β-[(4-chlorophenyl)methyl]-α-(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol); (E)-(p-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)-1-penten-3-ol (uniconazol, name pending); α-(1-methylethyl)-α-[4-(trifluoromethoxy)phenyl]-5-pyrimidinemethanol (flurprimidol).

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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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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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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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