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  • Author or Editor: Bruce W. Wood x
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Allometric equations were developed for orchard-grown pecan [Carya illinoinensis (Wangenh.) C. Koch] trees. Trees, ranging in size from 22 to 33 cm in trunk diameter 1.4 m above the ground, were destructively harvested from two sites. The entire aboveground portion of each tree was harvested and then divided into leaves, current season's shoots, and branches ≥1 year old plus trunk. Roots were sampled by digging a trench beginning beneath the trunk and extending to one-half the distance to an adjacent tree, then separating the roots from the soil. Roots were then divided into those less than 1 cm in diameter and those ≥1 cm in diameter. Equations in the form Y = eaXb were developed to estimate dry biomass of most tree components and the whole tree, where Y is the dry weight, e is the base of the natural logarithm, X is the trunk diameter at 1.4 m above the ground, and a and b are coefficients. A linear equation provided the best fit for estimating the weight of the current season's growth. Power equations were also developed to estimate the weights of inner bark and wood for different size trunks or branches.

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We report the composition of the rare-earth (REE) metallome component of the foliar ionomes of pecan (Carya illinoinensis) and other North American Carya and how accumulation of specific REEs relate to ploidy level and to accumulation of essential divalent nutrient elements. REE accumulation within the foliar ionomes of 12 Carya species, growing on a common site and soil, indicates that REEs accumulate according to the Oddo-Harkins rule with Ce, La, Nd, and Y (Ce > La > Nd > Y) being the dominant REEs with accumulated concentration typically being La > Ce > Nd > Y > Gd > Pr > Sm > Dy > Er > Yb > Ho > Tb >Tm > Sc >Lu. Carya species quantitatively differ in accumulation of REEs with all but C. aquatica accumulating at much greater concentrations than non-Carya tree species and with tetraploid Carya accumulating to approximately twice the concentration as diploid Carya. Carya tomentosa was an especially heavy accumulator of REEs at 859 μg·g−1 dry weight, whereas C. aquatica was especially light at 84 μg·g−1. Accumulation of REEs was such that any one element within this elemental class was tightly linked (generally r ≥ 0.94, but 0.81 for Ce) to all others. Accumulation of REEs is negatively correlated with Ca accumulation and positively correlated with Mn and Cu accumulation in diploid Carya. In tetraploid Carya, accumulated Mg, Ca, and Fe is positively associated with foliar concentration of REEs. Total concentration of REEs in pecan's foliar ionome was 190 μg·g−1, about equivalent to that of Mn. Circumstantial evidence suggests that one or more of the physiochemically similar REEs increases physiological plasticity and subsequent adaptive fitness to certain Carya species, especially tetraploids. Because all tetraploid Carya are high REE accumulators and are native to more xeric habitats than diploids, which typically occupy mesic habitats, it appears that REEs might play a role in Carya speciation and adaptation to certain site-limiting environmental stresses. REEs appear to play an unknown metabolic/physiological role in pecan and most Carya species, especially tetraploids; thus, their nutritional physiology merits further investigation.

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An ever increasing cost:price squeeze on the profitability of pecan (Carya illinoinensis) farming is driving a search for alternate husbandry approaches. `Wichita' and `Western' trees maintained at relatively high tree population density, by mechanized hedge pruning and topping, produced greater nut yield than an orchard treatment in which tree population density was reduced by tree thinning (144% for `Wichita' and 113% for `Western Schley'). Evaluation of three different hedge pruning strategies, over a 20-year period, identified a discrete canopy hedge pruning and topping strategy using a 2-year cycle, as being superior to that of a discrete canopy hedge pruning and topping strategy using an 8-year cycle, but not as good as a continuous canopy hedge pruning and topping strategy using a 1-year cycle. An evaluation of 21 commercial cultivars indicated that nut yields of essentially all cultivars can be relatively high if properly hedge pruned [annual in-shell nut yields of 2200 to 3626 lb/acre (2465.8 to 4064.1 kg·ha-1), depending on cultivar]. Comparative alternate bearing intensity and nut quality characteristics are reported for 21 cultivars. These evaluations indicate that pecan orchards can be highly productive, with substantially reduced alternate bearing, when managed via a hedge-row-like pruning strategy giving narrow canopies [3403 lb/acre (3814.2 kg·ha-1) for `Wichita' and 3472 lb/acre (3891.5 kg·ha-1) for `Western Schley']. North-south-oriented (N-S) hedgerows produced higher yields that did east-west (E-W) hedgerows (yield for N-S `Wichita' was 158% that of E-W trees and N-S `Western Schley' was 174% that of E-W trees).

These data indicate that mechanized hedge pruning and topping offers an attractive alternative to the conventional husbandry paradigm.

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Genetic variation among pecan [Carya illinoinensis (Wangenh.) C. Koch] cultivars was studied using randomly amplified polymorphic DNA (RAPD) markers. Using a combination of primers, a unique fingerprint is presented for each of the pecan genotypes studied. The genetic relatedness between 43 cultivars was estimated using 100 RAPD markers. Genetic distances, based on the similarity coefficient of Nei & Li, varied from 0.91 to 0.46, with an average value of 0.66 among all cultivars. The phenetic dendrogram developed from cluster analysis showed relatively weak grouping association. However, cultivars with known pedigrees usually grouped with at least one of the parents and genetic similarity estimates appear to agree with known genetic relationships.

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This study examines the relationship between foliar nitrogen:potassium (N:K) ratio and in-shell yield of pecan [Carya illinoinensis (Wangenh.) K.Koch]. Regression analysis of linear and curvilinear relationships between leaflet N:K ratio and in-shell yield identified associations relevant to orchard nutrition management. Analysis revealed that ON (heavy crop) year N:K ratio correlates with ON year yield (r2 = –0.69), OFF (light crop) year yield (r2 = +0.34), 2-year average yield (r2 = −0.52), and difference between ON and OFF year yields (r2 = –0.69) below the optimum yield level (less than 1800 kg·ha−1) for southeastern U.S. pecan orchards. Pecan yield therefore appears to be associated with N:K ratio. This study suggests that a decline in pecan yield is associated with high N:K ratios in the ON year, thus meriting further investigation into the relationships of N and K to yield. It is suggested that pecan orchards be managed such that foliage contains a N concentration of 2.5% to 2.9% and a K concentration of 1.3% to 1.5% while maintaining the N:K ratio at ≈2:1 for maximization of pecan yields in the southeastern United States over the long term.

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Observations of net assimilation rates (`A') by pecan sun and shade leaves in relation to various levels of solar irradiation, the light adaptation characteristics of these leaf types, the role of clouds in suppressing the penetration of solar irradiation, and the abundance of cloud cover in the southeastern U.S. during the growing season, suggest that nut production throughout the U.S. pecan belt is being limited by insufficient sunlight with the southeastern U.S. (comprising about 2/3 of the commercial U.S. pecan production) being especially impacted. In support of this hypothesis, regression analysis showed cultivar-type nut production for Georgia from 1977-1989 to be significantly (P<.0001, R2 = 0.79) associated with sunlight levels ≥ 3000 Wh m-2d-1 from mid August to early October for the same year. This is taken as evidence that the amount of sunlight reaching the canopy seems to be a major factor that should be considered in relation to orchard site selection and canopy management techniques.

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Propiconazole, a fungicide, suppressed leaf area of a wide variety of young pecan [Carya illinoinensis (Wangenh.) K. Koch] seedling genotypes but did not reduce leaf area of orchard trees. Leaf area declined linearly as dosage increased from 0.16 to 1.25 mL·L–1. Suppression of leaf area by propiconazole was inversely proportional to leaf age. No reduction of leaf area was detected in orchards where `Cheyenne', `Desirable', and `Pawnee' were treated with three applications (14-day intervals) of fungicide (either propiconazole, fentin hydroxide, or fenbuconazole) from budbreak to early May. Spring application of the three fungicides alone or in combination with zinc sulfate did not influence fruit set. Control of pecan scab [Cladosporium caryigenum (Ell. et Lang) Gottwald] was achieved with either fentin hydroxide or fenbuconazole for the full season, or with early season use of dodine, then propiconazole, and then followed by fentin hydroxide for late-season disease control. Fungicide treatments had no effect on nut weight. These data indicate that fungicides applied to pecan during pollination at commercially recommended dosages and intervals, with or without zinc sulfate, do not adversely influence leaf area or fruit set of orchard trees. Chemical names used: n-dodecylguanidine acetate (dodine); triphenyltin hydroxide (fentin hydroxide); 1-[[2-(2,4-dichlorophenyl)-4-propyl-1,3-dioxolan-2-yl] methyl]-1H-1,2,4-triazole (propiconazole); α-[2-(4-chlorophenyl)ethyl]-α-phenyl-1H-1,2,4-triazole-1-propanenitrile (fenbuconazole).

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Ground applications of ZnO to large mature pecan [Carya illinoinensis (Wangenh.) K. Koch] trees in orchards possessing an acidic soil, but with a culturally induced slightly alkaline soil surface zone, were at least as effective as was ZnSO4 for rapidly correcting severe foliar Zn deficiency, improving in-shell nut production, and maintaining kernel quality. Under such soil conditions, light disking of Zn applied at 160 kg·ha-1 from ZnO elevated foliar Zn above the sufficiency level by the second growing season after application; whereas an absence of disking delayed substantial uptake from ZnO until the fourth growing season. ZnO, usually a lower priced Zn source, was as effective as was ZnSO4 for correcting Zn deficiencies via broadcast ground application; however, same season correction of Zn deficiency was best accomplished by the standard practice of using foliar sprays of ZnSO4 rather than by heavy soil applications of either Zn source.

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Self-pollination was estimated in three Georgia pecan [Carya illinoinensis (Wangenh.) K. Koch] orchards. Selfing in two large orchards lacking an interplanted complementary pollinizer (one orchard being comprised of `Curtis' and the other `Moneymaker') was estimated to be at least 3% and 49%, respectively. A `Cheyenne' orchard containing `Stuart' as a complementary pollinizer at 5% density was estimated to have had at least 14% and 42% of ripened nuts derived from selfing in two consecutive years. These estimates suggest self-pollination may reduce yield in pecan orchards in the southeastern United States.

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This study reports on sudden death (or decline) of mature and apparently healthy pecan trees [Carya illinoinensis (Wangenh.) K. Koch]. Observations suggest that death and damage is due to winter cold injury (although the season's low was only -5 °C). The severity of this cold injury-like form of sudden death is closely associated with nut crop load (i.e., grams of kernels per square centimeter of trunk cross-sectional area) and premature defoliation. Both dead and declining trees not only produced relatively heavy crops, but also exhibited substantial premature pest-induced defoliation the previous autumn. The near absence of sugars and starch in roots and shoots of dead or declining trees at budbreak and the relatively high levels in healthy trees indicates that diminished assimilate reserves during the dormant season were the key factor causing death or decline. The diminished assimilate reserves prevented the accumulation of assimilate reserves necessary for maintaining live roots throughout the dormancy and prevented proper cold acclimation of shoot tissues. Distinct symptoms of sudden tree death or decline compared to typical cold damage are: a) a distinct top-to-bottom gradation of tree damage, with an increased proportion of dead shoots and shoots supporting abnormally small foliage being near the base of the canopy; b) dessicated and tan appearance of inner bark and phloem of the main trunk rather than brown coloration so typical of classical cold injury; c) death of roots by time of budbreak; and d) absence of resprouting from the trunk or root collar. These observations indicate that pecan trees can suddenly die due to being overly stressed for assimilates and that economic losses previously attributed to injury by severe winter cold sometimes may be due to depleted assimilate reserves during the dormant season as a result of overcropping and premature defoliation.

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