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  • Author or Editor: Bruce W. Wood x
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Genetic variation among pecan [Carya illinoinensis (Wangenh.) K. Koch] cultivars was studied using randomly amplified polymorphic DNA (RAPD) markers. Using a combination of primers, a unique fingerprint was produced for each of the pecan genotypes studied. The genetic relatedness between 44 cultivars was estimated using more than 100 RAPD markers. Genetic distances based on the simple matching coefficient varied from 0.91 to 0.59. The phenetic dendogram 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. Using RAPD information in determining genetic relationships among pecan cultivars with unknown or questionable pedigrees and the integration of that knowledge into the breeding program is discussed.

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Peach tree short life (PTSL) is associated with the presence of ring nematode, Mesocriconema xenoplax, and poor orchard management practices. The ability of postplant nickel (Ni) foliar application to suppress M. xenoplax population density and thereby prolong survival of peach trees on a PTSL site infested with M. xenoplax was investigated from 2004 to 2011. For this study, the site was divided into plots, which received the following treatments: 1) Ni (foliar-applied); 2) methyl bromide fumigation (MBr); and 3) an untreated control. Peach trees were planted into all plots in Mar. 2005 and the foliar Ni treatment was applied three times in 2005 and 2006. Nickel did not detectably suppress M. xenoplax populations as compared with MBr fumigation. The protective effect of MBr fumigation in suppressing M. xenoplax population density persisted for 27 months after orchard establishment. Trees receiving multiple foliar Ni applications at 0.45 g·L−1 over 2 years, while exposed to M. xenoplax, exhibited greater PTSL mortality than trees growing in untreated or MBr-fumigated plots. These results suggest that foliar applications of Ni to peach trees, growing on a PTSL site, should be used with caution in commercial orchards because these treatments can deleteriously disrupt tree metabolic/physiological processes sufficient to increase the incidence of PTSL tree mortality.

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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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The host-parasite interaction between the black pecan aphid (BPA) [Melanocallis caryaefoliae (Davis)] and pecan [Carya illinoensis (Wangenh.) K. Koch] was investigated. Three years of field observations of the ability of BPA populations to induce chlorotic blotches, or visual damage, on 32 pecan cultivars revealed considerable variation in cultivar susceptibility to BPA damage. Among the most commonly grown cultivars, `Sioux', `Cape Fear', `Farley', `Cowley', `Grabohls', and `Barton' exhibited the least damage, whereas `Choctaw', `Oconee', and `Sumner' exhibited the greatest, with `Sioux' and `Choctaw' exhibiting the greatest extremes in susceptibility. Subsequent evaluation indicated that the foliage of pecan genotypes can exhibit an antibiotic-like effect, resulting in the suppression of resident BPA populations. However, the relationship between the degree of this antibiotic effect and the degree of damage exhibited by trees, or field tolerance, was negligible (r = -0.10). For example, while `Choctaw' foliage greatly suppressed BPA population growth, this population was able to inflict relatively severe damage to leaves. An evaluation of feeding preference indicated that BPA alate viviparae (winged females) preferentially feed upon host cultivars on which they have been previously feeding. This feeding preference was eliminated by rinsing leaves with distilled water; hence, a water soluble factor(s) appears to be involved in host preference.

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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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Water stage fruit split (WS) is an erratic and complex problem often causing major crop losses to susceptible pecan [Carya illinoinensis (Wangenh.) K. Koch] cultivars. This study identified two episodes of WS for `Wichita' pecan—a highly susceptible cultivar. The previously recognized precipitation-induced fruit splitting is the major episode; however, a previously unrecognized precipitation-independent, minor episode can also occured before the major episode. This minor episode was associated with the low solar irradiance and high relative humidity—conditions commonly associated with August rains. The crop characteristics of affected trees also influenced WS in that WS increased as crop load per tree increased. Fruits were also more likely to exhibit WS if located within the lower tree canopy. Treatment of foliage with an antitranspirant immediately before split-inducing conditions increased WS. Maintenance of moist soils for ≈2 weeks before WS-inducing conditions substantially reduced WS-related crop losses. These findings help to explain the erratic nature of WS and indicate that maintenance of trees in a well-watered state for ≈2 weeks before the initiation of shell hardening may substantially reduce WS-related crop losses in certain years.

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Water-stage fruit-split (WSFS) is a relatively common and often major problem of certain pecan [Carya illinoinensis (Wangenh.) K. Koch] cultivars. This study evaluates the possibility that the malady can be influenced by improving tree micronutrient nutrition. Foliar sprays of boron (B) and nickel (Ni) to WSFS-susceptible fruit of ‘Cape Fear’ and ‘Sumner’ are evaluated based on the possibility that either B or Ni potentially affects the severity of WSFS exhibited by trees. Although the incidence of WSFS on ‘Cape Fear’ was unaffected by micronutrient sprays, the severity of WSFS was substantially reduced in each of the 3 study years by foliar B application and in 2005 by foliar Ni application. Repeated foliar sprays of Ni also reduced WSFS of ‘Sumner’ fruit. These data indicate that improving either B or Ni nutrition can potentially reduce crop loss resulting from WSFS in certain orchard situations and provides evidence that insufficient availability of B or Ni to developing ovary tissues potentially predisposes developing fruit to WSFS when environmental triggers occur.

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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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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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